Free T3 (Triiodothyronine) Test Price in Laxmisagar

Test Description

Free T3 (FT3) measures the unbound, biologically active fraction of triiodothyronine (T3) in blood. T3 is the physiologically active thyroid hormone with approximately four times the biological potency of T4. Only about 20% of circulating T3 is directly secreted by the thyroid gland; the remaining 80% is produced by peripheral deiodination of T4 in liver, kidney, and other tissues by type 1 and type 2 deiodinase enzymes. Like T4, T3 is predominantly protein-bound (99.7%), and free T3 measurement avoids the confounding effects of binding protein changes. Normal free T3: 2.3–4.2 pg/mL (3.5–6.5 pmol/L). FT3 is primarily used to confirm hyperthyroidism (particularly T3-toxicosis, where T3 is elevated with normal or borderline T4 and suppressed TSH), monitor antithyroid drug therapy, and evaluate patients on levothyroxine therapy with low T4 but normal or elevated T3. Low T3 syndrome (euthyroid sick syndrome) is common in systemic illness where peripheral T4-to-T3 conversion is reduced as a protective adaptive mechanism. FT3 is less useful for diagnosing hypothyroidism as TSH and FT4 are superior markers.