Senior investigator Dr. Victor Bernet, an endocrinologist at Mayo Clinic in Florida, said the supplements -- hormones known as T3, or triiodothyronine, and T4, or thyroxine -- contain varying amounts of two kinds of thyroid hormones apparently derived in large part from chopped up animal thyroid glands.
They are regulated by the U.S. Food and Drug Administration and intended for use only in prescription medication because they can cause significant health issues, such as an increase in heart rate, heart irregularities and palpitations, nervousness, and diarrhea, Bernet said.
"These hormones have effects throughout the body, which is why they are controlled," Bernet said in a statement.
In addition, the amount of hormones in the products varied significantly, sometimes exceeding doses used for individual patients and comparable to levels found in prescription thyroid medication, Bernet said.
The supplements likely do not give most people the results they are seeking, such as weight loss or less fatigue, Barnet said.
"The amount of thyroid hormone a normal person would have to take to lose weight would be dangerously high and there is no evidence that use of thyroid hormone effectively treats fatigue when used in people without actual hypothyroidism," Bernet said.
The findings were presented at the annual meeting of the American Thyroid Association.