Thyroxine: Free T4

Thyroxine (T4) is a hormone synthesized by Thyroid Stimulating Hormone (TSH) which circulates in the blood both in the free and bound form. More than 99% of the T4 hormone circulates in the blood bound to carrier proteins, leaving less than 1% unbound. It is this level of unbound or free hormone that correlates with the functional thyroid state in most individuals. Free T4 regulates normal growth and development by maintaining body temperature and stimulating calorigenesis, and also affects aspects of carbohydrate, lipid, and vitamin metabolism. Measurement of free T4 levels along with other thyroid tests and clinical findings can establish borderline hyperthyroid and hypothyroid diagnoses.