Levothyroxine is a synthetic form of thyroxine (T4), an endogenous hormone secreted by the thyroid gland, which is converted to its active metabolite, L-triiodothyronine (T3). T4 and T3 bind to thyroid receptor proteins in the cell nucleus and cause metabolic effects through the control of DNA transcription and protein synthesis. Like its naturally secreted counterpart, levothyroxine is a chiral compound in the L-form. Levothyroxine is a drug that is used to treat underactive thyroid (hypothyroidism) and other conditions when a person's thyroid hormone production is below normal. Levothyroxine sodium replaces or provides thyroid hormone support, which is normally produced by the thyroid gland.