Used as a synthetic form of the thyroid hormone thyroxine (T4) to treat hypothyroidism, a condition where the thyroid gland does not produce enough hormones. It helps restore normal hormone levels, improving symptoms such as fatigue, weight gain, and cold intolerance. Also used in the management of goiter and certain types of thyroid cancer by suppressing thyroid-stimulating hormone (TSH) secretion. Commonly prescribed for long-term replacement therapy in patients with underactive thyroid function.