A recent study found that 9 out of 10 supplements marketed for "thyroid support" and purchased at retail stores or on the Internet contained actual thyroid hormones thyroxine (T4) and/or triiodothyronine (T3), posing a risk to consumers. The research was conducted in response to several reports of people developing thyrotoxicosis after using thyroid supplements. Symptoms of thyrotoxicosis, which is an elevated metabolic rate, may include nervousness, irritability, increased perspiration, and elevated heart rate. 

The tested products were not identified by name in the study, but half were labeled to contain "raw thyroid" tissue, concentrate, or powder from a bovine (cow) source. It can be expected that these would contain thyroid hormone, although the amounts hormone were not labeled (nor could they be labeled on a supplement). The other five were herbal-based, so there would be no expectation that they would contain thyroid hormone. 

Daily doses as high as 91.6 mcg of T4 and 32.1 mcg of T3 were found in the supplements. These are significant amounts: a prescribed starting dose of T4, for example, is only 25 micrograms. Surprisingly, products with largest amounts of hormone, particularly T4, were herbal supplements - suggesting the addition of thyroid hormones, while a "raw thyroid" product had no detectable T3 or T4 - suggesting that it didn't contain actual thyroid tissue, and two "raw thyroid" products contained only T3 and not T4 -suggesting they did not contain thyroid tissue but, instead, had added thyroid hormone. 

Herbal ingredients in many products included iodine-rich substances such as kelp or bladderwrack (as iodine is needed for thyroid hormone production), or substances believed to be thyroid stimulants based on animal studies (Ashwagandha root, C. forskohlii, and guggul).