The study led by senior investigator and endocrinologist Victor Bernet said the supplements contain two different kinds of thyroid hormones T3, or triiodothyronine, and T4, or thyroxine, which can cause heart irregularities and palpitations, nervousness, and diarrhea.
"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 added.
The researchers studied 10 commercially available thyroid supplements and separated them using high-pressure liquid chromatography to identify the chemical components of T3 and T4.
The study found that nine of the 10 supplements studied had animal hormone and the amount of hormones in the products varied significantly.