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.

Low thyroid hormone levels can occur naturally or when the thyroid gland is injured by radiation/medications or removed by surgery. Levothyroxine is also known as L-thyroxine or T4.

Thyroid medication is frequently used by bodybuilders getting ready for a competition to lose excess fat by speeding up their basal metabolic rate and help them to produce maximum muscle definition. Although the thyroid hormone T3 is more popular for this purpose, T4 may lead to increased muscle retention and improve the effects of growth hormone in weightlifters who are also taking growth hormone. Having excess thyroid hormones in your body can help produce more short-term weight loss than by dieting alone, but once the thyroid supplements are stopped, the excess weight loss is usually regained.