Active Thyroid Hormone (T3) Test - Triiodothyronine (T3) Test

This test measures the concentration of triiodothyronine (T3) hormone in the blood serum. Only about 20% of T3 is produced directly by the thyroid gland, while the remaining 80% is converted to thyroxine (T4) in peripheral tissues (such as the liver and kidneys). T3 is the biologically active form of thyroid hormone, binding to cell receptors and producing physiological effects (increased basal metabolism, thermogenesis, cardiac and nervous stimulation). It is often ordered along with T4 and TSH for a comprehensive evaluation of thyroid function, particularly in cases of suspected hyperthyroidism.