Surprisingly, nine out of ten supplements showed detectable amounts of T3, ranging from 1.3 mcg to 25.4 mcg per tablet.
Taken at the recommended dosage, five of the ten supplements delivered T3 quantities in excess of 10 mcg/day, and four delivered T4 quantities ranging from 8.57 to 91.6 mcg/day.
Among the five thyroid supplements labeled as containing bovine (cow) thyroid tissue, extract or concentrate, one had no level of T3 or T4 detectable; two contained T3 only, and two showed detectable amounts of both T3 and T4.
The researchers concluded that most of the OTC thyroid supplements that were studied contained clinically-significant amounts of T4 and T3, in some cases doses that exceed typical prescribed amounts being used for thyroid hormone replacement treatement.
According to the research abstract, "this potentially exposes patients to risk of iatrogenic hyperthyroidism by taking easily accessible OTC supplements not requiring prescription. The current study results emphasize the importance of patient and provider education regarding the use of OTC herbal supplements and highlights the need for greater regulation of OTC supplements potentially dangerous to the public."
The message for patients? You'll want to be discuss use of over-the-counter "thyroid support" supplements with your doctor, and make sure that you don't end up accidentally overmedicated, by combining your thyroid hormone replacement medication with over-the-counter supplements that contain active thyroid hormone.
Source: Kang, G.Y. "Thyroxine and Triiodothyronine Content in Commercially Available Thyroid Health Supplements," Abstracts of the American Thyroid Association Annual Meeting, 2011