Liothyronine (CYTOMEL®) is a T3 thyroid hormone normally synthesized and secreted by the thyroid gland in much smaller quantities than its prohormone thyroxine (T4). Most T3 is derived from peripheral monodeiodination of T4 at the 5' position of the outer ring of the iodothyronine nucleus. The hormone finally delivered and used by the tissues is mainly T3. The mechanisms by which thyroid hormones exert their physiologic action are not well understood. These hormones enhance oxygen consumption by most tissues of the body, increase the basal metabolic rate and the metabolism of carbohydrates, lipids, and proteins. Thus, they exert a profound influence on every organ system in the body and are of particular importance in the development of the central nervous system. Thyroid hormone drugs are indicated: as the replacement or supplemental therapy in patients with hypothyroidism of any etiology; as pituitary thyroid-stimulating hormone (TSH) suppressants, in the treatment or prevention of various types of euthyroid goiters; as diagnostic agents in suppression tests to differentiate suspected mild hyperthyroidism or thyroid gland autonomy.

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.

A radioconjugate of synthetic active thyroid hormone, liothyronine (T3), labeled with Iodine 131. Liothyronine involves many important metabolic functions and is essential to the proper development and differentiation of all cells. I131 liothyronine may be used in radiotherapy in thyroid cancers.

3,3',5,5'-Tetraiodothyroacetic acid (Tetrac) is a deaminated analog of L-thyroxine (T4) that blocks the proangiogenesis actions of T4 and 3, 5, 3’-triiodo-L-thyronine as well as other growth factors at the cell surface receptor for thyroid hormone on integrin αvβ3. 3,3',5,5'-Tetraiodothyroacetic acid blocks the transcriptional activities directed by L-thyroxine (T4) and 3,5,3’-triiodo-L-thyronine (T3) at αvβ3, but, independently of T4 and T3, 3,3',5,5'-Tetraiodothyroacetic acid modulates transcription of cancer cell genes that are important to cell survival pathways, control of the cell cycle, angiogenesis (VEGFA, FGF), apoptosis, cell export of chemotherapeutic agents, and repair of double-strand DNA breaks. 3,3',5,5'-Tetraiodothyroacetic acid was found to perturb the angiogenesis process stimulated by VEGF (Vascular Endothelial Growth Factor) or FGF (Fibroblast Growth Factor) without influencing the preexisting blood vessels.

3,5-Diiodo-L-thyronine (3,5-T2), a potential metabolite of 3,3',5-triiodothyronine (T3), is an active thyroid hormone. It acts as an alternative ligand for thyroid hormone receptor beta. 3,5-Diiodo-L-thyronine has the capacity to stimulate hepatic lipid catabolism but acts by different from T3 molecular mechanisms to achieve this effect.

Diiodotyrosine (DIT) is a regulatory ligand for thyroid peroxidase, enzyme, which is involved in the production of thyroxine (T4) and triiodothyronine (T3), the thyroid hormones. Diiodotyrosine was proposed to be a new marker of leukocyte phagocytic activity in sepsis and severe infections.

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.