Effect of oral intake of thyroxine on results of thyroid function tests in patients receiving thyroid replacement therapy

To investigate the effect of orally administered thyroxine on thyroid function tests, serial thyroid function tests were performed in 29 patients receiving maintenance thyroxine replacement therapy. The results showed that plasma thyroxine levels rose significantly in most subjects after an oral dose of thyroxine. The patients who had the largest rise had high basal levels and were taking a higher mean replacement dose of thyroxine. This increase may move a patient's thyroxine level from within the reference range to the abnormal range if blood is taken soon after thyroxine ingestion.