• "Natural" thyroid preparations, a type of bovine desiccated thyroid, are sold without prescription in "health-food" stores or by mail; they may or may not contain thyroid hormone. One such preparation was the cause of erratic thyroid test results in a patient who took it instead of thyroxine. Further study showed that the preparation contained biologically active thyroid hormone and was capable of causing hyperthyroidism. Natural thyroid preparations are biologically active yet unstandardized; because the hormonal content of a natural thyroid product is unknown, its use as a substitute for thyroxine can lead either to relapse of hypothyroidism or to hyperthyroidism.
(Arch Intern Med. 1989;149:2117-2118)