Tiratricol (also known as TRIAC or triiodothyroacetic acid) is a thyroid hormone analogue, which has been studied since the 1950s. Tiratricol is used as a dietary supplement for thyroid problems including thyroid cancer. It is also used for increasing metabolic rate for weight loss, and reducing cellulite. In the US, the Food and Drug Administration (FDA) has determined that the product Triax (TRIAC, tiratricol) is not a dietary supplement but an unapproved new drug containing a powerful thyroid hormone, which may cause serious health consequences. The State of Missouri embargoed the product at its distributor (Syntrax) and the Utah-based manufacturer (Pharmatech) has agreed to stop distributing any product containing the ingredient TRIAC. The FDA has issued recalls for other tiratricol-containing products, including Tricana Metabolic Hormone Analogue, Tria-Cutz Thyroid Stimulator Dietary Supplement Capsules, and Sci-Fi-Tri-Cuts Dietary Supplement Capsules. Tiratricol is a prescription drug in France used for therapy of thyroid hormone resistance and therapy of thyroid cancer.

Levothyroxine (T4) is a synthetically prepared levo isomer of thyroxine, the major hormone secreted from the thyroid gland. Thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. Thyroxine is peripherally deiodinated to form triiodothyronine (T3) which exerts a broad spectrum of stimulatory effects on cell metabolism. Thyroid hormone increases the metabolic rate of cells of all tissues in the body. In the fetus and newborn, thyroid hormone is important for the growth and development of all tissues including bones and the brain. In adults, thyroid hormone helps to maintain brain function, food metabolism, and body temperature, among other effects. The symptoms of thyroid deficiency relieved by levothyroxine include slow speech, lack of energy, weight gain, hair loss, dry thick skin and unusual sensitivity to cold. Levothyroxine acts like the endogenous thyroid hormone thyroxine (T4, a tetra-iodinated tyrosine derivative). In the liver and kidney, T4 is converted to T3, the active metabolite. In order to increase solubility, the thyroid hormones attach to thyroid hormone binding proteins, thyroxin-binding globulin, and thyroxin-binding prealbumin (transthyretin). Transport and binding to thyroid hormone receptors in the cytoplasm and nucleus then takes place. Thus by acting as a replacement for natural thyroxine, symptoms of thyroxine deficiency are relieved. Levothyroxine is used for use alone or in combination with antithyroid agents to treat hypothyroidism, goiter, chronic lymphocytic thyroiditis, myxedema coma, and stupor.