Levothyroxine/Liothyronine Sodium Capsules

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.

Liothyronine (L-triiodothyronine or L-T3) is a synthetic sodium salt of the endogenous thyroid hormone triiodothyronine (T3). The oral tablet is indicated for use as replacement or supplemental therapy in the treatment of hypothyroidism of any etiology, except transient hypothyroidism during the recovery phase of subacute thyroiditis; as a pituitary thyroid-stimulating hormone (TSH) suppressant in the treatment or prevention of various types of euthyroid goiters; and as a diagnostic agent in T3 suppression tests. Liothyronine injection is indicated for intravenous use in the treatment of myxedema coma/precoma. Either form of liothyronine may be used for patients who are allergic to desiccated thyroid or thyroid extract derived from pork or beef. Supraphysiologic thyroid hormone concentrations may occur following orally administered liothyronine, but not after intravenous administration. Liothyronine is potentially more cardiotoxic than levothyroxine. However, due to the faster onset of action and the need to peripherally convert levothyroxine (T4) to the biologically active T3, liothyronine has been recommended for treatment of myxedema coma. There are no comparative studies available. Levothyroxine is generally considered the most appropriate of the thyroid replacement agents for long-term treatment of hypothyroidism. However, a small randomized clinical trial demonstrated improvement in mood and neuropsychological function of hypothyroid patients with partial substitution of liothyronine for levothyroxine. Liothyronine received approval for use by the FDA in 1954.