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Optimal management of hypothyroidism, hypothyroxinaemia and euthyroid TPO antibody positivity preconception and in pregnancy

Shiao Chan

Centre for Women's & Children's Health, School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK

Centre for Endocrinology, Diabetes and Metabolism, School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK

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Kristien Boelaert,

Corresponding Author

Kristien Boelaert

Centre for Endocrinology, Diabetes and Metabolism, School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK

Correspondence: Dr Kristien Boelaert, Centre for Endocrinology, Diabetes and Metabolism, Level 2, Institute for biomedical Research, College of Medical and Dental Sciences, University of Birmingham, B15 2TT, UK. Tel.: +44 121 4147400; Fax: +44 121 4158712; E-mail: [email protected]Search for more papers by this author

Shiao Chan,

Shiao Chan

Centre for Women's & Children's Health, School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK

Centre for Endocrinology, Diabetes and Metabolism, School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK

Search for more papers by this author

Kristien Boelaert,

Corresponding Author

Kristien Boelaert

Centre for Endocrinology, Diabetes and Metabolism, School of Clinical and Experimental Medicine, College of Medical and Dental Sciences, University of Birmingham, Birmingham, UK

First published: 09 September 2014

https://doi.org/10.1111/cen.12605

Citations: 97

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Summary

Normal physiological changes of pregnancy warrant the need to employ gestation specific reference ranges for the interpretation of thyroid function tests. Thyroid hormones play crucial roles in foetal growth and neurodevelopment which are dependent on adequate supply of maternal thyroid hormones from early gestation onwards. The prevention of significant adverse obstetric and neurodevelopmental outcomes from hypothyroidism requires a strategy of empirical levothyroxine dose increases and predictive dose adjustments in pregnancy combined with regular thyroid function testing, starting before pregnancy and until the postpartum period. Subclinical hypothyroidism has been associated with an increased risk of pregnancy loss and neurocognitive deficits in children, especially when diagnosed before or during early pregnancy. Whilst trials of levothyroxine replacement for mild hypothyroidism in pregnancy have not indicated definite evidence of improvements in these outcomes, professional guidelines recommend treatment, especially if evidence of underlying thyroid autoimmunity is present. Studies of isolated hypothyroxinaemia in pregnancy have shown conflicting evidence with regards to adverse obstetric and neurodevelopmental outcomes and no causative relationships have been determined. Treatment of this condition in pregnancy may be considered in those with underlying thyroid autoimmunity. Whilst the evidence for a link between the presence of anti-TPO antibodies and increased risks of pregnancy loss and infertility is compelling, the results of ongoing randomized trials of levothyroxine in euthyroid women with underlying autoimmunity are currently awaited. Further studies to define the selection of women who require levothyroxine replacement and to determine the benefits of a predictive dose adjustment strategy are required.

Introduction

Thyroid dysfunction is relatively common in pregnancy. The optimal management of maternal thyroid disorders requires an understanding of normal physiological changes as well as an appreciation of the fact that pregnancy, uteroplacental and foetal development are highly dynamic processes. Gestational windows for key developmental events are time-dependent, transient and may not be revisited at a later stage of pregnancy. Both overt and subclinical hypothyroidism has been associated with adverse pregnancy outcomes as well as with neurodevelopmental deficits in the offspring, although some studies have indicated conflicting results. The association between the finding of isolated hypothyroxinaemia and obstetric risk as well as child neurocognitive impairment is also not consistently reported. The presence of antibodies to thyroperoxidase (TPO) is highly prevalent in women of reproductive age and especially in those with recurrent miscarriage.

Management of hypothyroidism in pregnancy requires a coordinated approach, ideally starting preconception and continued until the postpartum period. Whilst the evidence for improved outcomes following treatment for subclinical hypothyroidism shows conflicting results, several international guidelines advocate levothyroxine replacement for this condition in pregnancy. The benefits of this treatment in isolated hypothyroxinaemia and in TPO antibody positive women are yet to be determined. This review aims to provide a summary of the evidence relating to the diagnosis, management and consequences of thyroid hypofunction in pregnancy and to describe a pragmatic approach to optimising thyroid hormone replacement before, during and after pregnancy.

Normal physiological changes in maternal thyroid function

The normal physiological changes occurring to the thyroid axis in both the mother and the foetus during gestation have been described in reviews and clinical guidelines1, 2 and are summarized in Fig. 1.

Details are in the caption following the image — **Figure 1**
Open in figure viewer PowerPoint

Maternal thyroid physiology. Placental HCG peaks around 10 weeks of gestation and then declines to lower levels. HCG increases maternal oestrogen levels, which stimulates the hepatic production of thyroxine binding globulin (TBG). Circulating TBG rises during the first 20 weeks of gestation to reach a steady concentration for the rest of pregnancy. HCG, which has weak thyrotrophic activity, can also directly stimulate the maternal thyroid gland to produce thyroid hormones. There is a clear reciprocal relationship between HCG and maternal TSH, with a HCG peak coinciding with a trough in TSH concentrations.1 The placenta, which is rich in deiodinase type 3 enzymes (D3), metabolizes maternal thyroid hormones. Despite this, the foetus receives physiologically relevant levels of maternal thyroid hormones by transplacental supply at every stage of gestation13 from as early as 5 weeks onwards.9 This is particularly critical at the beginning of pregnancy, prior to endogenous foetal thyroid hormone production, which begins around 18 weeks of gestation in humans.12

As a result of these changes, the correct interpretation of thyroid function tests in pregnancy requires the use of gestation specific reference ranges, which are assay and population dependent.3, 4 Maternal TSH declines in the first trimester and returns gradually towards pre-pregnancy levels by the end of gestation, whilst maternal free T4 (fT4) concentrations decline slowly with gestation. In the absence of assay specific ranges, the American Endocrine Society (AES)5 and American Thyroid Association (ATA)2 guidelines – which are endorsed by most other endocrine societies internationally6 – have recommended TSH reference ranges of 0·1–2·5, 0·2–3·0 and 0·3–3·0 mU/l in the first, second and third trimesters respectively. The vast majority of centres will only have access to immunomediated fT4 assays, which are greatly affected by changes in the serum protein milieu during pregnancy resulting in marked interassay and interindividual variability. Hence the use of total T4 measurements by extraction/liquid chromatography/tandem mass spectometry has been recommended2 but this is not widely available. However, if suitable reference ranges are used, fT4 immunoassays are sufficiently reliable for clinical practice.

The role of thyroid hormones in foetal and placental development (Table 1) has been clearly demonstrated in animal models and is supported by human data confirming that from the first trimester the foetal brain is capable of responding to thyroid hormones.7, 8 From as early as week 5 of gestation, the human foetus is exposed to physiologically relevant concentrations of maternal thyroid hormones.9 In mammalian models, thyroid hormones have been shown to regulate foetal size and to promote the maturation of multiple tissue types in preparation for extrauterine life.10 Thyroid hormones themselves can also directly affect uteroplacental development,11 hence, they are able to influence pregnancy outcome.

Table 1. The role of thyroid hormones in foetal and placental development

Species from which evidence derived	Gestational stage	Tissue type	Thyroid hormone action
Rodent, Human	Early to late gestation	Brain	Neurogenesis, neuronal migration, dendritic branching, synaptogenesis, myelination
Sheep, Pig, Rabbit, Rodent, Nonhuman primates, Human	Mid to late gestation	Auditory apparatus, Pulmonary, Cardiovascular, Hepatic, Renal, Adipose tissue, Skeletal muscle, Bone, Autonomic nervous system	Cellular differentiation, functional maturation and foetal size Regulate function of other hormones and growth factors: renin-angiotensin system, catecholamines, glucocorticoids, insulin-like growth factors, growth hormone, leptin, prostaglandins.
Human	Early to mid gestation	Placental trophoblasts	Proliferation, apoptosis, invasive capability
Human	Early to mid gestation	Maternal decidua	Cytokine and angiogenic growth factor secretion

The foetus is entirely dependent on transplacental maternal thyroid hormone supply until the onset of endogenous foetal thyroid hormone production around 18 weeks of gestation.12 Even subsequently, maternal thyroid hormone transfer to the foetus continues right up to delivery.13 This highlights the importance of maintaining euthyroidism from conception and throughout gestation.

Overt hypothyroidism

Overt hypothyroidism is defined as elevated serum TSH accompanied by below normal fT4 concentrations. In iodine replete areas, hypothyroidism is most commonly a primary autoimmune condition, Hashimoto's thyroiditis, but world-wide, hypothyroidism is mostly secondary to iodine deficiency. It may also be secondary to thyroidectomy or radio-active iodine treatment, thyroiditis and other rare disorders.

Using gestational age defined reference ranges, the prevalence of overt hypothyroidism in the first trimester of pregnancy in iodine replete populations is 0·2–1%.14-16 Untreated overt hypothyroidism is associated with spontaneous miscarriage, perinatal death, pregnancy-induced hypertension, pre-eclampsia, preterm birth, low birth weight (mostly attributable to preterm delivery), anaemia and post-partum haemorrhage.17, 18 Generally, the risk of each of these obstetric complications is two to three fold higher compared to euthyroid controls. An increased incidence of neurodevelopmental deficits in children whose mothers were hypothyroid during pregnancy is also observed.19, 20

Levothyroxine treatment in overt hypothyroidism

Retrospective evidence indicates that treatment with levothyroxine may improve obstetric and neonatal outcomes. Three cohort studies reported that women who were inadequately treated (TSH concentrations >4·0 or 4·5 mU/l) at the start of pregnancy were more likely to suffer a pregnancy loss, predominantly miscarriages, compared to those who were euthyroid on levothyroxine treatment [1 pooled OR 3·36 (95% CI: 1·16–9·69); P = 0·025].21-23 Although some studies have reported equitable obstetric outcomes when comparing thyroxine treated mothers with euthyroid controls,24, 25 others have identified residual risks. One study reported increased pre-eclampsia rates with treated hypothyroidism despite correction for maternal age and chronic hypertension (OR 1·7, 95% CI: 1·0–2·8)26 whilst a separate study found no association between treated hypothyroidism and hypertensive disorders in pregnancy after correction for confounders.27 Two studies reported increased caesarean section delivery rates [relative risk (RR) 1·6–1·7] but both concluded that this was due to “medicalization” of patients or care-giver bias.27, 28 Whether levothyroxine treatment was optimal in all cases studied could not be verified and the euthyroid controls in these studies included undiagnosed hypothyroid cases since women were not all screened for thyroid dysfunction. Furthermore, the timing of normalization of thyroid function in pregnancy may also be a factor contributing to the observed variability. The sooner the correction of thyroid dysfunction, the lower the risk of pregnancy complications.18, 22

In terms of neurodevelopmental outcomes, two small studies have shown that despite inadequate levothyroxine treatment of overt hypothyroidism in early pregnancy, childhood neurodevelopment was similar to control.19, 29 In a small case series of overt hypothyroidism diagnosed in early pregnancy where euthyroidism was not achieved until late pregnancy, child neurodevelopment was reported as normal or even advanced,30 although this study was inadequately powered to show a difference.

Management of levothyroxine therapy for overt hypothyroidism

The aim of treatment should be to prevent hypothyroidism during pregnancy and replicate normal physiology. To achieve this, patient counselling prepregnancy or as early as possible in pregnancy, encouraging treatment compliance, and using a predictive dose adjustment strategy is proposed. This strategy has five key elements (Table 2).

Table 2. Management of levothyroxine therapy in primary hypothyroidism: a predictive dose adjustment strategy aimed at preventing hypothyroidism during pregnancy and replicating normal physiology

The five key elements
1	Adequate preconception replacement	Target TSH of 2·5 mU/l or less
2	An empirical dose increase in early pregnancy	Double up dose of levothyroxine on 2 days each week as soon as she has a positive pregnancy test
3	Regular thyroid function monitoring	Monthly thyroid function testing in the first 20 weeks of pregnancy, at start of third trimester and once again in the mid-third trimester. Additional testing at 4 weeks after a dose change if not due for a routine test
4	Predictive dose adjustments	Levothyroxine dose increase as the TSH approaches the upper part of the normal trimester specific reference range (above 2 mU/l) Levothyroxine dose decrease if the TSH falls below the lower limit of the reference range
5	Postnatal advice	Reduce levothyroxine to prepregnancy dose 2 weeks postpartum and recheck thyroid function 6–8 weeks postnatally

Adequate preconception replacement

offers some protection from early pregnancy hypothyroidism.31, 32 The ATA and AES suggest a preconception target TSH of 2·5 mU/l or less.2, 5 However, a recent study in Argentina suggested a preconception target of 1·2 mU/l or less since fewer women with such TSH concentrations required a dose increase in pregnancy (17%) compared to those with a preconception TSH of 1·2–2·5 mU/l (50%),31 although the relatively low percentage of women requiring a dose increase in this study is unusual.

An empirical dose increase in early pregnancy

In addition to the normal physiological changes in pregnancy, gastrointestinal absorption of levothyroxine declines in pregnancy, thus increased dose requirements are expected. A US study reported serum TSH concentrations >5·0 mU/l in 30% of treated-hypothyroid women with preconception TSH <5 mU/l (90% <2·5 mU/l), by a median gestation of 5·5 weeks.32 In the UK, 70% of a group with preconception TSH <2·5 mU/l had serum TSH >2·5 mU/l in the first trimester of pregnancy.33 In a mildly iodine deficient area of Italy, 97% of women required a dose increase at some point during pregnancy despite a mean preconception TSH of 2·1 (±0·7).34 Requirement for the first dose increase usually occurs early in pregnancy, sometimes as early as 4 weeks of gestation, with the number of cases peaking at 8 weeks, and in almost all cases by 20 weeks.34, 35 The timing is independent of the aetiology of hypothyroidism and the initial thyroxine dose.34 These studies all report the requirement of a mean total increase of around 30–50% of the prepregnancy dose, however, there is much interindividual variability. Some studies have reported greater absolute dose increases in women who have little or no residual thyroid function36 whilst others have found mildly hypothyroid women to have the highest percentage dose increases.34 Ideally, maintaining euthyroidism in early pregnancy is best achieved by giving preconception advice to women to double their dose of thyroxine on 2 days each week as soon as they have a positive pregnancy test32 since a routine antenatal visit usually does not occur until 10–12 weeks of gestation. Another way is to advise an empirical dose increase of 25 μg daily for women taking 100 μg or less and 50 μg daily for those on more than 100 μg levothyroxine.

Regular thyroid function monitoring

Despite an empirical dose increase, 40–70% require further dose adjustments during pregnancy32, 34 to maintain euthyroidism. This is not surprising given the highly dynamic physiological changes occurring particularly in the first half of pregnancy. Monthly thyroid function testing in the first 20 weeks of pregnancy and once again in the third trimester can detect 92% of abnormal TSH values compared with 73% in a 6-weekly testing protocol.32 In addition, thyroid function tests should also be performed 4 weeks after each dose change. Although the clinical implications of an elevated TSH in late pregnancy are unknown, the authors find it useful to routinely evaluate thyroid function in the early third trimester as well as in the mid-third trimester to detect TSH elevations due to decreased thyroxine absorption secondary to the simultaneous ingestion of iron supplementation37 commenced for iron deficiency following a routine haemoglobin test at 28 weeks (in the UK) and to the start of antacid intake for gastrooesophageal reflux, which occurs more commonly in late pregnancy.

Predictive dose adjustments

Levothyroxine dose titration using TSH is recommended as this is the best marker of thyroid sufficiency demonstrating much less interindividual variation than fT4 concentrations in pregnancy. If the first thyroid function in pregnancy indicates serum TSH above the trimester specific reference range or below 0·1 mU/l, the magnitude of the initial empirical dose increase should be adjusted accordingly. During subsequent monitoring, the authors advocate levothyroxine dose increases to prevent hypothyroidism as the TSH approaches the upper part of the normal trimester specific reference range. In practice a TSH concentration above 2·0 mU/l should trigger a dose increase to bring TSH values towards the median for gestation,38 whilst a dose decrease is instituted if the TSH falls below the lower limit of the reference range. However, some have argued that titration should be performed using fT4 concentrations instead as this may be the leading determinant of thyroid hormone supply to the foetal brain to optimise development.12 In women with secondary hypothyroidism, titration has to be done by fT4 concentrations, aiming for the upper third of trimester specific reference ranges.

Benefits and safety of predictive dose adjustments: In terms of maintaining euthyroidism, the safety of this strategy of empirical preemptive dose increases and frequent monitoring, has been demonstrated.32 However, the potential benefits in terms of obstetric and neurodevelopmental outcomes of this approach compared with previous practice of reactive dose adjustments to abnormal thyroid function tests, less frequent thyroid function monitoring and more liberal acceptance of mildly elevated TSH concentrations, have not yet been established.

With a predictive dose adjustment strategy, the theoretical risk of over-replacement and elevated maternal fT4 concentrations resulting in an adverse pregnancy outcome is feared. This was initially based on observations in cases of maternal thyroid hormone resistance syndrome where foetuses who did not share the maternal mutation were more likely to be miscarried and growth restricted compared to foetuses who shared the mutation and thus were able to tolerate high maternal fT4 concentrations.39 Lately, concerns were raised following observations in euthyroid women with no thyroid pathology, that maternal fT4 concentrations at the upper end of the normal range during early pregnancy40 and at 28 weeks of gestation41 were associated with decreased infant birthweight independently of prematurity. In fact, such fears have not been realised in treatment practices of women with hypothyroidism that aimed to maintain maternal TSH concentrations between 0·1–2·028 or 0·1–1·0 mU/l,42 which run the inevitable risk of over replacement. Even if there was excess replacement, this would be transient and corrected with monitoring, and cannot be directly compared with the situation of a persistently elevated maternal fT4. Certainly subclinical hyperthyroidism (low TSH with normal fT4 concentrations) is not thought to be harmful in pregnancy43 and is generally well-tolerated by patients. Further studies are required to fully evaluate the overall benefits of a predictive dose adjustment strategy on all aspects of pregnancy and offspring outcomes but there is sufficient evidence to merit adopting this practice currently.

Obstetric management for overt hypothyroidism in pregnancy: If overt hypothyroidism is found in pregnancy, normalization of thyroid function should be achieved as soon as possible. However, the adverse effects on uteroplacental development would have already occurred in early pregnancy and may not be reversible. Hence, despite the absence of evidence specifically in hypothyroidism, using evidence drawn from other high risk medical conditions in pregnancy, we advocate commencement of low dose aspirin (if diagnosed prior to 16 weeks gestation) as pre-eclampsia prophylaxis, uterine artery Doppler studies in mid-gestation as a screening test for malplacentation, serial ultrasound scans for foetal growth and more frequent blood pressure and urine dipstick monitoring for pre-eclampsia in the third trimester.

Postnatal advice

Since TBG only decline to prepregnancy levels over a period of 4–6 weeks following delivery,44, 45 the authors recommend that levothyroxine should be returned to the prepregnancy dose 2 weeks after delivery and thyroid function re-evaluated about 6–8 weeks postdelivery. Mothers should also be reminded to have their thyroid function checked before trying to conceive again and to empirically increase the levothyroxine dosage at the beginning of the next pregnancy.

Subclinical hypothyroidism

Subclinical hypothyroidism is defined as elevated TSH concentration accompanied by normal fT4 concentration. This is the most common form of thyroid dysfunction in pregnancy. Reported prevalences in pregnancy vary widely and are influenced by the iodine status of the population and the definition of reference ranges (Table 3A). Studies using gestation-specific, assay-specific and population-specific reference ranges avoid over-diagnosis and probably reveal more accurate prevalences compared with the use of generic reference ranges recommended by the ATA and AES which tend to show higher rates.

Table 3. Prevalences of maternal (A) subclinical hypothyroidism and (B) isolated hypothyroxinaemia during the first 20 weeks of pregnancy in different populations using various definitions for reference ranges

	Condition	Definition of reference range	Iodine status	Prevalences at lower end (%)	Prevalences at higher end (%)
A	Subclinical hypothyrodisim (Elevated TSH, normal fT4)	Gestational age, assay and population specific	Sufficient	2·2–2·514, 15	4–1016, 86, 87
		Gestational age, assay and population specific	Mildly to Moderately deficient	2·342
		Trimester, assay and population-specific	Sufficient	4–6·888, 89	15·590
		Trimester, assay and population-specific	Mildly to Moderately deficient	1·791	10–11·869, 92
		Generic – recommended by ATA and AES2, 5	Sufficient	11·3–15·877, 93	28–3088
		Generic – recommended by ATA and AES2, 5	Mildly to Moderately deficient	891	16·658
B	Isolated hypothyroxinaemia (Normal TSH, low fT4)	Gestational age, assay and population specific	Sufficient	1·3–2·315, 94	3·786
B	Isolated hypothyroxinaemia (Normal TSH, low fT4)	Gestational age, assay and population specific	Mildly to Moderately deficient	1·7–3·242, 69, 91, 92	895

Excluding women with inadequately treated overt hypothyroidism, subclinical hypothyroidism can be indicative of early thyroid insufficiency, which has come to light given the increased thyroid demand in pregnancy. This postulation is supported by studies of longer term follow-up of these women postpartum. In those with subclinical hypothyroidism at 28 weeks gestation, 75% were euthyroid, 20% remained subclinically hypothyroid and 5% were on levothyroxine treatment when reassessed 5 years post-partum.46 Both TPO antibody positivity and TSH concentrations more than 5 mU/l in pregnancy were predictive of later persistent hypothyroidism. The natural history of subclinical hypothyroidism diagnosed in pregnancy is similar to subclinical hypothyroidism diagnosed outside pregnancy with studies reporting progression to overt hypothyroidism in 2–5% and reversal to normal in up to 62% after 5 years of follow-up.47, 48

Pregnancy outcomes with subclinical hypothyroidism

Observational studies of pregnancy outcome have largely been retrospective. Our meta-analysis of studies (Table 4A and B), which includes subclinically hypothyroid populations defined by different TSH and fT4 cut-offs, has shown a significantly increased risk of pregnancy loss (including miscarriages, stillbirths and perinatal deaths), preterm delivery, placental abruption and breech presentation at term. When assessing pregnancy loss, a retrospective study which had screened and diagnosed subclinically hypothyroid women prior to conception reported a very high pregnancy loss rate, mainly first trimester miscarriages, in up to 70% of pregnancies when not treated.22 Studies which screened and diagnosed women only during the first half of pregnancy (as in Table 4A) demonstrated a pregnancy loss rate of lesser magnitude, suggesting that most pregnancy losses are first trimester miscarriages, the majority of which would not have been included in studies which involved screening in pregnancy. Even after the first trimester, pregnancy/perinatal loss may still be higher in women with an elevated TSH concentration. In fact another study has shown a positive correlation between TSH at around 13 weeks of gestation and later pregnancy/child loss with an adjusted OR of 1·8 (95% CI: 1·07–3·03) for every doubling of TSH but no correlation with fT4 concentrations.49 Studies defining the cohort by the outcome of interest have reported a significantly higher incidence of an elevated TSH (>97·5 centile) and/or low fT4 (<2·5th centile) at 11–13 weeks of gestation in women who subsequently suffered a pregnancy loss50 or late-onset pre-eclampsia51 compared to women who were unaffected. This is in agreement with a published meta-analysis showing increased perinatal mortality (OR 2·7; 95% CI: 1·6–4·7) and pre-eclampsia (OR 1·7; 95% CI: 1·1–2·6) risk with subclinical hypothyroidism,52 although the latter is in contrast to our meta-analysis which included more studies (Table 4B).

Table 4. Pregnancy outcomes with subclinical hypothyroidism (SCH; elevated TSH accompanied by normal fT4). A: Summary of case–control studies included in the meta-analysis. B: Meta-analysis of pregnancy outcomes. Pregnancy loss includes miscarriages, stillbirths and perinatal losses up till the first week of life. Pooled odds ratio (OR) compared with euthyroidism using random effects model, given the clinical and statistical heterogeneity as well as threshold variations. * indicates statistical significance

Reference	Country	Time of SCH diagnosis (weeks of gestation)	TSH range for SCH	Free T4 range for SCH	Untreated SCH cases (n)	Euthyroid cases (N)
(A)
Allan et al.96	USA	15–18	>6 mU/l	Not defined	209	9194
Pop et al.71	Netherlands	32	>2 mU/l	8·7–19·6 pmol/l	21	183
Casey et al.94	USA	Up to 20	>3 mU/l	0·86–1·9 ng/dl	598	16 011
Cleary-Goldman et al.15	USA	10–13	>97·5th percentile	2·5th to 97·5th percentile	240	10 021
Mannisto et al.16	Finland	Up to 20 (mean 11)	>95th percentile	5th to 95th percentile	224	4719
Hamm et al.97	Canada	15–16	>4·0 mU/l	Not defined	15	756
Kooistra et al.98	Netherlands	35–38	>95th percentile (2·7 mU/l)	9·6–18·2 pmol/l	77	1112
Kuppens et al.99	Netherlands	36	>95th percentile (2·89 mU/l)	7·2–24·6 pmol/l	59	999
Negro et al.58,100,101	Italy	Up to 11	>2·5 mU/l (TPO Ab +ve and -ve)	Not defined	676	3741
Sahu et al.102	India	13–26	>5·5 mU/l	Not defined	41	552
Su et al.103	China	Up to 20	>95th percentile	5th to 95th percentile	41	845
Wang et al.104	China	Up to 12	≥2·5 mIU/l	12–23·34 pmol/l	168	542
Korevaar et al.84	Netherlands	9·6–17·6 (Median 13·2)	>97·5th percentile	2·5th to 97·5th percentile	188	4970
Breathnach et al. 105	Ireland	Mean 14 (SD ± 2·7)	>98th percentile	2th to 98th percentile	16	870

Pregnancy outcome	Pooled odds ratio (95% CI) compared with euthyroidism	P value	Studies used for meta-analysis
(B)
Pregnancy loss	1·93 (1·40–2·64)	<0·0001*	15, 16, 58, 94, 96, 100-104
Pre-eclampsia	1·41 (0·89–2·25)	0·147	15, 58, 94, 100, 101
Gestational hypertension	0·94 (0·77–1·15)	0·527	15, 58, 94, 96, 100-102, 104, 105
Preterm delivery before 37 weeks gestation	1·30 (1·05–1·60)	0·015*	15, 16, 58, 84, 94, 97, 100-105
Placental abruption	2·16 (1·15–4·06)	0·017*	14, 15, 96, 105
Gestational diabetes	1·38 (0·97–1·96)	0·074	15, 58, 94, 100-102, 105
Breech presentation at term	2·30 (1·50–3·51)	0·0001*	71, 98, 99
Caesarean delivery	1·07 (0·96–1·21)	0·232	16, 58, 94, 96, 100-102, 105
Foetal macrosomia >4 kg	0·80 (0·55–1·16)	0·246	15, 16, 58, 94, 100, 101, 103, 105
Low birthweight <2·5 kg	1·02 (0·79–1·32)	0·894	15, 16, 58, 94, 100, 101, 103, 104
Small for gestational age	0·98 (0·47–2·05)	0·955	16, 97, 102, 103, 105
Low Apgar score	1·54 (0·97–2·46)	0·069	16, 58, 94, 96, 100-102, 105
Admission to neonatal unit	1·30 (0·71–2·37)	0·397	58, 94, 100, 101, 105

Our meta-analysis showed a significantly increased risk of prematurity (OR 1·3; 95% CI: 1·05–1·60) and placental abruption (OR 2·16; 95% CI: 1·15–4·06), which could be factors mediating the poor perinatal outcomes associated with subclinical hypothyroidism. Circulating TSH concentrations >95th centile at 36 weeks have also been associated with breech presentation (Table 4B) and a reduced external cephalic version success rate.53

Neurodevelopmental risks

An increased incidence of neurodevelopmental deficits has also been reported in children whose mothers had subclinical hypothyroidism early in pregnancy.19, 54 Untreated mild hypothyroidism, including overt and subclinical forms, was associated with a 7 point lower mean IQ score compared with controls when assessed at 7–9 years of age.19 An inverse correlation between maternal TSH and child IQ may suggest that maternal hypothyroidism has a causative role in impaired child neurodevelopment.55 It has been estimated in the US that subclinical hypothyroidism at 16 weeks gestation is associated with a 3-fold increased predisposition for offspring having learning difficulties and accounts for 1·5% of all children with IQs more than 2 SD below the mean.18

Levothyroxine treatment of subclinical hypothyroidism

Evidence for the efficacy of levothyroxine treatment in reducing risks in pregnancies complicated by subclinical hypothyroidism is not robust.

Impact on obstetric risk

A meta-analysis of three small prospective randomized control trials of levothyroxine treatment in subclinically hypothyroid women undergoing assisted conception found that treatment commencing prior to ovulation induction increased live birth rates (n = 220, pooled RR of 2·76, 95% CI: 1·20–6·44).56 Outside the context of assisted conception, there are suggestions from small nonrandomized prospective and retrospective studies22, 57, 58 that levothyroxine treatment starting preconception or in early pregnancy could reduce the risk of miscarriage (total n = 393, 1pooled OR 0·15, 95% CI: 0·03–0·74) and preterm delivery (total n = 298, 1pooled OR 0·3, 95% CI: 0·10–0·90).22, 57, 58 A prospective nonrandomized Polish study of women (n = 258) who had just suffered a first trimester miscarriage found that it was cost-effective to screen and treat women with overt and subclinical hypothyroidism with improvements in the livebirth rate.59 However, a US study of 286 women with a history of recurrent miscarriage showed no difference in the live birth rate when comparing subclinically hypothyroid and euthyroid women, and when comparing treated and untreated women with subclinical hypothyroidism.60 In the CATS (Controlled Antenatal Thyroid Screening) study, the largest prospective randomized control trial of screening and levothyroxine treatment (starting from mean gestation of 13 weeks and 3 days) of maternal subclinical hypothyroidism and isolated hypothyroxinaemia (n = 1050 totally), there was no difference in the mean gestational age at delivery nor in infant birthweight with levothyroxine treatment.42 The results of another large multicentred randomized control trial “Randomized Trial of Thyroxine Therapy for Subclinical Hypothyroidism or Hypothyroxinaemia Diagnosed During Pregnancy” by the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD), which screened and treated subclinically hypothyroid and hypothyroxinaemic women by 20 weeks of gestation in the US are due to be released in 2015 (http://www.clinicaltrials.gov/ct2/show/NCT00388297).

Thus the efficacy of levothyroxine treatment in reducing obstetric risk may vary by medical history and prior risk factors as well as by the timing of commencement of therapy.

Impact on neurodevelopmental risk

The CATS study showed no beneficial effect of levothyroxine treatment upon offspring mean IQ scores at age 3.42 Subgroup analysis, albeit underpowered, found no differences in those whose treatment commenced at earlier gestations compared to later, nor between subclinically hypothyroid and hypothyroxinaemic women. Further studies will be required to address speculations that initiation of treatment is required prior to 12 weeks, when foetal brain development is critically dependent on maternal thyroid hormones, or whether using more sensitive neurodevelopmental tests and at a later age could uncover potential benefits of treatment.

Risks of levothyroxine treatment

The potential risk of levothyroxine treatment includes over-treatment leading to hyperthyroidism which can be minimized through regular monitoring of thyroid function as discussed earlier. A study of an international network of 12 birth defect registries (n = 18 131 cases) has reported an association between exposure to levothyroxine in the first trimester with unilateral renal agenesis,61 a defect that stems from an anomalous developmental step occurring at 5 weeks of gestation and affects 1 in 1000 births. It remains to be determined if this association is causative or due to confounding factors.

Recommendations by professional bodies

Even though evidence of a definite beneficial effect of levothyroxine treatment in large scale prospective studies is not available, many endocrine professional bodies have advocated levothyroxine treatment of subclinical hypothyroidism in pregnancy, ideally commencing preconception, given the highly favourable potential benefit to risk ratio of therapy.5, 38, 62-65 Recent ATA guidelines support treatment of subclinical hypothyroidism only in TPO antibody positive women but remain in equipoise for treatment of TPO antibody negative subclinical hypothyroidism.2 This recommendation is based on one prospective study which demonstrated, in a subgroup analysis, a composite obstetric risk reduction in levothyroxine treated TPO positive subclinically hypothyroid women compared to those who were untreated. By contrast the American Congress of Obstetricians and Gynaecologists has not endorsed these recommendations of treating subclinical hypothyroidism in pregnancy.66

Screening for thyroid dysfunction

In the absence of strong evidence for treatment efficacy in subclinical hypothyroidism, the most common thyroid dysfunction found in pregnancy, the issue of whether there should be universal screening of all women for subclinical thyroid dysfunction preconception or in early pregnancy remains highly controversial. This is compounded by the lack of understanding of the mechanisms by which adverse obstetric events arise, of exact critical gestational windows and of the interaction of other medical and obstetric risk factors with thyroid dysfunction. Overall there is agreement that targeted screening of at risk women is indicated although the criteria by which this is defined varies considerably between professional bodies. Furthermore case-finding will miss a significant proportion of women with thyroid dysfunction.67 The issue of universal screening vs aggressive case-finding is addressed in recent reviews67, 68 and guidelines2, 5, 37 and will not be discussed further.

A large scale prospective study of preconception and early pregnancy levothyroxine treatment in subclinical hypothyroidism is needed to help define more clearly the population of women who would benefit from levothyroxine treatment and whether universal screening is cost effective. Meanwhile, the risks and potential benefits of treatment should be presented to the patient. The authors offer levothyroxine treatment preconception and prior to 12 weeks of pregnancy in all women found to have subclinical hypothyroidism. If they are not treated initially, we monitor thyroid function in pregnancy and offer treatment if significant deterioration occurs.

Isolated hypothyroxinaemia

Isolated hypothyroxinaemia is defined as circulating TSH concentrations within the normal range accompanied by fT4 concentrations below the reference range. This could be associated with interference of the specific fT4 assay by the individual's protein milieu. However, it can be indicative of relative iodine deficiency or the early stages of thyroid insufficiency or rarely secondary hypothyroidism, which should be excluded. If conception is spontaneous, implying a functional gonadotrophic axis, the risk of significant hypopituitarism is negligible. Like subclinical hypothyroidism, reported prevalence rates of isolated hypothyroxinaemia vary widely, with poor iodine status being a main factor associated with higher prevalences (Table 3B). Furthermore, in mildly to moderately iodine deficient areas increasing rates of isolated hypothyroxinaemia with advancing gestation has been reported, reaching 25·4% in the third trimester in one study.69

Obstetric outcomes with isolated hypothyroxinaemia

There have been several observational studies (Table 5A) examining the obstetric risks associated with isolated hypothyroxinaemia with variable results. Our meta-analysis (Table 5B) showed only a statistically significant increase risk in placental abruption (OR 2·3; 95% CI: 1·10–4·80) compared with euthyroid controls. There was a higher trend towards both extremes of birthweight and preterm birth, which was not statistically significant.

Table 5. Pregnancy outcomes with untreated isolated hypothyroxinaemia (IH; normal TSH accompanied by low fT4) diagnosed before 20 weeks of gestation. A: Summary of case–control studies included in the meta-analysis. B: Meta-analysis of pregnancy outcomes. Pregnancy loss includes miscarriages, stillbirths and perinatal losses up till the first week of life. Pooled odds ratio (OR) compared with euthyroidism using random effects model, given the clinical and statistical heterogeneity as well as threshold variations. * indicates statistical significance

Reference	Country	Time of IH diagnosis (weeks of gestation)	TSH range for IH	Free T4 range for IH	IH cases (n)	Euthyroid cases (n)
(A)
Casey et al.94	USA	Up to 20	0·08–2·99 mU/l	<0·86 ng/dl (2·5th percentile)	233	16 011
Cleary-Goldman et al.15	USA	10–13	2·5th to 97·5th percentile	<2·5th percentile	232	10 021
Hamm et al.97	Canada	15–16	0·15–4·0 mU/l	<8·5 pmol/l (10th percentile)	89	756
Su et al.103	China	Up to 20	5th to 95th percentile	<5th percentile	43	845
Korevaar et al.84	Netherlands	9·6–17·6 (Median 13·2)	2·5th to 97·5th percentile	<2·5th percentile	145	4970
Breathnach et al.105	Ireland	Mean 14 (SD ± 2·7)	2th to 98th percentile	<2th percentile	18	870

Pregnancy outcome	Pooled OR (95% CI) compared with euthyroidism	P value	Studies used for meta-analysis
(B)
Pregnancy loss	0·40 (0·10–1·62)	0·200	15, 94, 103
Pre-eclampsia	0·90 (0·51–1·56)	0·697	15, 94
Gestational hypertension	0·83 (0·57–1·21)	0·338	15, 94, 105
Preterm delivery before 37 weeks gestation	1·41 (0·98–2·04)	0·066	15, 84, 94, 97, 103, 105
Placental abruption	2·30 (1·10–4·80)	0·026*	14, 15, 105
Gestational diabetes	2·16 (0·70–6·71)	0·182	14, 15, 105
Caesarean delivery	1·04 (0·78–1·38)	0·796	94, 105
Foetal macrosomia >4 kg	1·65 (0·99–2·76)	0·055	15, 94, 103, 105
Low birthweight <2·5 kg	0·59 (0·34–1·001)	0·0503	15, 94, 103
Small for gestational age	1·24 (0·17–9·20)	0·831	97, 103, 105
Low Apgar score	1·37 (0·05–40·77)	0·855	94, 105
Admission to neonatal unit	0·71 (0·26–1·94)	0·508	94, 105

Only one study has reported on intrapartum events and found that lower fT4 concentrations at 36 weeks of gestation is associated with suboptimal labour progress with tendencies towards a nonoccipital anterior foetal head position in labour and the need for instrumental deliveries.70 One cohort study reported an association between fT4<10th percentile at 12 weeks with breech presentation at term (OR 4·7; 95% CI: 1·1–19, P = 0·03).71 However, these factors have not increased the overall risk of caesarean delivery (Table 5B).

Neurodevelopmental outcomes with isolated hypothyroxinaemia

Lower maternal fT4 concentrations in the context of mild iodine deficiency have been associated with neurodevelopmental delay before age 2.72, 73 However, even in the Netherlands which is iodine replete, isolated mild maternal hypothyroxinaemia in the late first and early second trimesters has been associated with delayed cognitive, language and motor function compared to controls up to age 2 years.74-76 There was a demonstrable dose-dependent response of lower maternal fT4 being associated with greater developmental impairment,75, 76 suggestive of causation, consistent with findings in animal models.12 These findings have been replicated in a separate study conducted in an iodine-replete part of China.54 However, this same Chinese study also found similar neurodevelopmental deficits in children born to mothers with subclinical hypothyroidism (normal fT4) and euthyroid TPO antibody positivity, which suggest possible confounders in the population and questions a causative role for fT4 in imparied neurodevelopment.

In contrast, three US studies reported no associations between maternal fT4 concentrations at the end of the first trimester,77 in the mid-2nd trimester78 and in the early 3rd trimester79 with offspring neurodevelopment. This discrepancy may suggest confounding differences between the Dutch and US populations which are of greater importance in ensuring normal neurodevelopment than maternal fT4 concentrations. However, in one Dutch study, children whose mothers initially had a low fT4 concentration at 12 weeks of gestation and then showed spontaneous recovery of the fT4 to the 50–90th percentile by 32 weeks of gestation, performed no worse than euthyroid controls74 suggesting that normalization of fT4 concentrations may have a protective effect on child neurodevelopment.

A causative relationship between isolated hypothyroxinaemia and any obstetric or neurodevelopmental complication has not been established. The CATS study remains the only levothyroxine intervention study to be performed in pregnant women with isolated hypothyroxinaemia and showed no difference in offspring neurodevelopmental outcomes at age 342 and no apparent differences in obstetric outcomes despite the normalization of fT4 concentrations by 30 weeks of gestation. Further studies are required to determine whether treatment commencing preconception or earlier in gestation can alter obstetric or neurodevelopmental risks.

Recommendations by professional bodies

Given the lack of evidence of treatment efficacy, the latest ATA Guidelines do not recommend treatment of isolated hypothyroxinaemia when identified through screening in pregnancy.2 However, some experts advocate treatment based on the strong evidence of neurodevelopmental benefits seen in rat models of maternal hypothyroxinaemia.12 Recent guidance from the European Thyroid Association65 suggest that levothyroxine replacement may be considered if isolated hypothyroxinaemia is diagnosed in the first trimester because of its association with neuropsychological impairment in the offspring. They do not advise treatment if the condition is diagnosed in the second or third trimester.

In the authors' opinion levothyroxine treatment should be considered if hypothyroxinaemia is found in the presence of TPO antibodies, which is associated with development of hypothyroidism in pregnancy and in those with a personal history of thyroid dysfunction or previous thyroid insult. If levothyroxine treatment is not commenced, there is little evidence to recommend serial thyroid function testing during gestation for development of hypothyroidism unless there is TPO antibody positivity or in an iodine deficient population.

Euthyroid Thyroid Peroxidase (TPO) antibody positivity

TPO antibodies are highly prevalent in women of the reproductive age. In unselected populations of women reported prevalences have ranged between 5·4% and 20% whilst in women with recurrent miscarriages and infertility, TPO prevalences are generally higher at 14–33%.80 Although TPO antibody positivity is associated with thyroid autoimmune disorders, the majority of these women are euthyroid, be it as a group they have higher mean TSH and lower mean fT4 concentrations, which may indicate relative thyroid insufficiency. Approximately 15–20% of TPO antibody positive women who are euthyroid outside pregnancy, will have elevated TSH by the 3rd trimester of pregnancy.81, 82 Thus, in untreated TPO antibody positive women thyroid function monitoring is warranted in each trimester of pregnancy.

TPO antibody positivity is also associated with many different nonthyroidal autoimmune diseases and may be a marker of altered immune responses globally. Thus, it is postulated that TPO antibody positive women are unable to mount an appropriate inflammatory response to sustain pregnancy.

Obstetric risks with TPO antibody positivity

The odds of miscarriage and preterm delivery in the presence of thyroid auto-antibodies were significantly increased at 3·9 (95% CI: 2·48–6·12) and 2·07 (95% CI: 1·17–3·68), respectively.83 The increased risk in preterm delivery associated with TPO antibody positivity is independent of thyroid dysfunction.84 TPO antibody positivity is also associated with subfertility (OR 1·5, 95% CI: 1·1–2·0) but not with clinical pregnancy rates following IVF treatment.52 One study has associated TPO antibody positivity (of whom only 4·2% had thyroid dysfunction) with increased perinatal mortality (OR 3·2; 95% CI: 1·4–7·1),16 which is contrary with another study that showed no correlation with perinatal mortality.49 TPO antibody positivity is also associated with both large for gestational age and low birth weight (<2500 g) infants,16 and a significantly increased risk of post-partum thyroiditis (OR 12; 95% CI: 5·6–24).52

Neurodevelopmental risks with TPO antibody positivity

Psychomotor delay has been reported in the offspring of TPO antibody positive mothers independently of thyroid dysfunction in apparently iodine replete populations,54, 85 which suggest a nonthyroid and a noniodine mediated mechanism involved in neurodevelopmental impairment.

Levothyroxine treatment of TPO antibody positive women

The presence of anti-TPO may indicate an underlying subtle alteration in thyroid reserve, which is unable to adapt to the physiological changes of pregnancy. Thus, it has been proposed that exogenous levothyroxine treatment may correct any relative thyroid hormone deficiency, and have a positive impact upon systemic immune function and the local uteroplacental environment.

There have been very few studies investigating the potential efficacy of levothyroxine treatment in reducing obstetric risk in TPO antibody positive women. A meta-analysis of two small randomized studies totalling 187 women showed a reduction in miscarriages with levothyroxine treatment (RR 0·48, 95% CI: 0·25–0·92).83 One of these also reported a reduction in preterm birth rate (RR 0·31; 95% CI: 0·11–0·90).82 Two large randomized controlled trials are currently underway to confirm these findings: TABLET trial (https://www.clinicaltrialsregister.eu/ctr-search/trial/2011-000719-19/GB) and T4life trial (http://www.trialregister.nl/trialreg/admin/rctview.asp?TC=3364).

The role of micronutrients

Apart from levothyroxine treatment, several micronutrients have been proposed as alternative or adjunctive therapies in subclinical hypothyroidism, hypothyroxinaemia and TPO antibody positivity. The role of iodine, selenium and iron in the management of these conditions in pregnancy is summarized in Table 6.

Table 6. The role of micronutrients in the management of thyroid hypofunction in pregnancy

Micro-nutrient

Physiological role

Evidence of treatment benefit and risk

Recommendations

Iodine

Essential component of thyroid hormones. Increased thyroid demand in pregnancy requires a well-functioning thyroid gland with good iodide supply

Supplementation in pregnancy does not improve maternal T4 concentrations or newborn thyroid function in mildly to moderately iodine deficient populations but does so only in severely iodine deficient populations106

The impact of supplementation in pregnancy on offspring neurodevelopment and pregnancy outcome in mildly to moderately iodine deficient populations are unclear although definite benefits are seen in severely iodine deficient populations

Iodine as TREATMENT of maternal hypothyroidism only recommended in severely iodine deficient populations

WHO and UNICEF recommend iodine SUPPLEMENTS in pregnancy if <90% of households use iodised salt and if the median urinary iodine in school children is <100 μg/l (http://www.who.int/nutrition/publications/micronutrients/WHOStatement__IDD_pregnancy.pdf)

ATA and AES recommend routine oral iodine SUPPLEMENTATION of 150 μg in all pregnant and lactating women2, 5

Selenium

Essential trace element of selenoproteins, e.g. antioxidative enzymes, iodothyronine deiodinases for thyroid hormone activation/inactivation, regulation of circulatory/local thyroid hormone concentrations

Supplementation in euthyroid TPO antibody positive pregnant women made no difference to hypothyroidism rates during pregnancy, pre-eclampsia and preterm birth rates but reduced incidence of post-partum thyroiditis.107 Long-term selenium supplementation outside pregnancy associated with increased insulin resistance and type 2 diabetes108

Selenium supplementation not recommended in pregnancy2

Iron

TPO is a heme-containing enzyme required for thyroid hormone synthesis. Iron deficiency is associated with hypothyroidism and hypothyroxinaemia

Outside of pregnancy, treatment of patients who have both subclinical hypothyroidism and iron deficiency anaemia with a combination of levothyroxine and iron was superior in achieving euthyroidism and improving haemoglobin counts compared to levothyroxine or iron on their own109

In pregnancy, poor maternal iron status was predictive of higher TSH and lower total T4 concentrations in an area of mild iodine deficiency110

Iron deficiency is common in pregnancy

Optimal treatment of maternal hypothyroidism must include concurrent treatment of iron deficiency where present

Conclusion

There is compelling evidence that the diagnosis and management of mild and overt hypothyroidism before, during and after pregnancy requires a structured and individualized approach, which is different to the management of these conditions outside this period of marked physiological changes. Studies regarding the benefits of increased levothyroxine treatment in overt hypothyroidism are convincing, whilst the evidence for beneficial effects of thyroid hormone replacement in maternal subclinical hypothyroidism, isolated hypothyroxinaemia or in euthyroid women with evidence of thyroid autoimmunity is less consistent. Nonetheless, experts recommend treatment of subclinical hypothyroidism and consideration of levothyroxine replacement in those with isolated hypothyroxinaemia especially if anti-TPO antibodies are raised. It is anticipated that ongoing and future trials will more clearly define the subgroups of pregnant women who require treatment and shed further light on the risks and benefits of structured replacement strategies which involve empirical and predictive levothyroxine dose alterations.

Acknowledgements

We thank Professor Arri Coomarasamy for his review and comments of the meta-analyses presented in this review.

Conflicts of interest

We have no financial or other conflicts of interest to declare.

Note

1 Denotes pooled with random effects model, given the clinical and statistical heterogeneity as well as threshold variations throughout manuscript.

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Citing Literature

Volume82, Issue3

March 2015

Pages 313-326

Optimal management of hypothyroidism, hypothyroxinaemia and euthyroid TPO antibody positivity preconception and in pregnancy

Summary

Introduction

Normal physiological changes in maternal thyroid function

Overt hypothyroidism

Levothyroxine treatment in overt hypothyroidism

Management of levothyroxine therapy for overt hypothyroidism

Adequate preconception replacement

An empirical dose increase in early pregnancy

Regular thyroid function monitoring

Predictive dose adjustments

Postnatal advice

Subclinical hypothyroidism

Pregnancy outcomes with subclinical hypothyroidism

Neurodevelopmental risks

Levothyroxine treatment of subclinical hypothyroidism

Impact on obstetric risk

Impact on neurodevelopmental risk

Risks of levothyroxine treatment

Recommendations by professional bodies

Screening for thyroid dysfunction

Isolated hypothyroxinaemia

Obstetric outcomes with isolated hypothyroxinaemia

Neurodevelopmental outcomes with isolated hypothyroxinaemia

Recommendations by professional bodies

Euthyroid Thyroid Peroxidase (TPO) antibody positivity

Obstetric risks with TPO antibody positivity

Neurodevelopmental risks with TPO antibody positivity

Levothyroxine treatment of TPO antibody positive women

The role of micronutrients

Conclusion

Acknowledgements

Conflicts of interest

Note

References

Citing Literature

Figures

References

Related

Information