Eltroxin 75mcg Tablet belongs to 'thyroid agents', primarily used to treat underactive thyroid (hypothyroidism). Hypothyroidism is a chronic disorder, which happens when the thyroid gland (located in the front lower part of the neck), is unable to make enough thyroid hormone. The thyroid gland is responsible for releasing hormones, which are composed of tri-iodothyronine (T3) and thyroxine (T4), which helps the body to regulate and use energy. The thyroid controls functions of the heart and digestive system, without the right amount of these hormones, the body is unable to function properly.