T4, also known as Levothyroxine, is a synthetic form of the thyroid hormone thyroxine (T4). It is available in solution form with a concentration of 400 micrograms (mcg) per milliliter. Levothyroxine is used primarily in the treatment of hypothyroidism, a condition characterized by an underactive thyroid gland, which can lead to symptoms such as fatigue, weight gain, and cold intolerance. By replacing or supplementing the body’s natural thyroid hormone, T4 helps to restore normal thyroid function and alleviate symptoms of hypothyroidism.