Levothyroxine exhibits all the actions of endogenous thyroid hormone. Liothyronine (T3) is the principal hormone that exhibits these actions whereas levothyroxine (T4) is the major hormone secreted by the thyroid gland and is metabolically deiodinated to T3 in peripheral tissues. Serum concentrations of T4 and TSH are typically used as the primary monitoring parameters for determining thyroid function.
Levothyroxine (T4) is a synthetically prepared levo-isomer of thyroxine, a hormone secreted by the thyroid gland. Levothyroxine is used in the treatment of primary, secondary (pituitary), and tertiary (hypothalamic) hypothyroidism. Levothyroxine will potently suppress thyrotropin secretion in the management of goiter and chronic lymphocytic thyroiditis, and it can be used in combination with antithyroid agents to prevent the abc development of hypothyroidism or goitrogenesis during the treatment of thyrotoxicosis. Intravenous levothyroxine is primarily used to treat myxedema coma or stupor. Levothyroxine therapy is preferred over thyroid and thyroglobulin because the hormonal content of levothyroxine is standardized, and the effects of the drug are more predictable. Levothyroxine provides only T4, of which roughly 80% is deiodinated to T3 and reverse T3. Since T3 is three times as potent as T4, virtually all of the activity of T4 can be ascribed to T3. Levothyroxine has been used clinically since the 1950s. Thyroid drugs containing levothyroxine sodium were sold for years without FDA approval. For many years, there has been controversy regarding the bioequivalence of different oral levothyroxine products, which had not been reviewed by modern FDA approval processes.1 A controversial study published in Gericke KR. Possible interaction between warfarin and fluconazole. Pharmacotherapy 1993;13:508—9.7 showed that several products were bioequivalent.2 Subsequently, in August Gericke KR. Possible interaction between warfarin and fluconazole. Pharmacotherapy 1993;13:508—9.7, the FDA announced that NDAs would be required for all oral levothyroxine products by August 2003. The required NDA will ensure that FDA standards for purity, potency, stability, safety and efficacy are met. Unithroid® and Levoxyl® received an FDA approval via the new NDA requirements in August 2000 and May 2001, respectively. Levo-T® received formal FDA-approval in early 2002; followed by the FDA-approval of Synthroid®, after 47 years of marketing, in July 2002. Levolet® products were FDA-approved in 2003; Levothroid® products were FDA-approved in 2004. Tirosint™ capsules were FDA-approved in October of 2006. The NDA status of other marketed brands is not clear.