fT3 (Cвободный Т3)

Triiodothyronine (T3) testing assists in the assessment of thyroid function and in diagnosing hyperthyroid states, including T3 thyrotoxicosis where T3 is elevated while thyroxine (T4) remains within reference limits. The measurement is useful in Graves disease and in toxic adenoma or multinodular thyrotoxicosis, and it may rise during therapy with synthetic T3. It also supports confirmation of typical hyperthyroidism in which both T3 and T4 are increased and is indicated when clinical features suggest hyperthyroidism but the standard profile (TSH and free T4) is normal or borderline. T3 concentrations are reported to be normal to mildly increased in familial dysalbuminemic hyperthyroxinemia. Clinicians may order total T3 (T3, total) or free triiodothyronine (FT3) in scenarios such as supraventricular tachycardia, unexplained fatigue with weight loss, or proximal myopathy when T4 concentrations are not elevated. The assay is also used to monitor response to treatment for hyperthyroidism.

Triiodothyronine is the bioactive thyroid hormone, with a potency approximately three- to fivefold greater than thyroxine (T4). Although synthesized in the thyroid, most circulating T3 originates from peripheral deiodination of T4 in organs such as the liver and kidneys. The majority of T3 binds to plasma proteins; only about 0.3% circulates as the free fraction, which is biologically active. T3 enhances tissue oxygen consumption and protein synthesis, stimulates lipolysis, accelerates cholesterol catabolism with biliary excretion, and promotes gluconeogenesis and glycogenolysis, thereby raising blood glucose and potentiating the actions of insulin and growth hormone. It supports bone growth, vitamin A synthesis, intestinal absorption of vitamin B12, gastrointestinal motility, and sex steroid synthesis, and it is essential for central nervous system development in children. Measured T3 is influenced by physiologic and nonthyroidal conditions. Concentrations can be reduced in chronic nonthyroidal illness and decline during fasting; nutritional status also affects results. Variability in thyroxine‑binding globulin and other binding proteins alters total T3 measurements, with increases commonly observed during pregnancy and with oral contraceptive use or other binding protein abnormalities. T3 may remain normal in thyroxine‑mediated thyrotoxicosis, and total T3 is not informative for evaluating hypothyroidism. Fasting can lower both T3 and TSH.