Free T3 represents the unbound, biologically active form of triiodothyronine, the most potent thyroid hormone. It reflects both thyroid gland function and peripheral conversion of T4 to T3, providing insight into cellular thyroid hormone activity. T3 is primarily produced through peripheral deiodination of T4 by deiodinase enzymes, with only about 20% directly secreted by the thyroid gland. Free T3 enters cells to bind nuclear receptors, regulating metabolism, cardiovascular function, and numerous other physiological processes.
Free triiodothyronine (T3) is the most metabolically active thyroid hormone. While T4 is produced in greater quantities by the thyroid gland, approximately 80% of circulating T3 is generated by peripheral conversion of T4 via deiodinase enzymes. Free T3 directly drives basal metabolic rate, thermogenesis, and cellular oxygen consumption.
Low free T3 is associated with fatigue, cold intolerance, weight gain, constipation, and cognitive slowing. It can occur even when TSH remains normal, particularly in non-thyroidal illness or caloric restriction. Elevated free T3 is consistent with hyperthyroidism or excessive thyroid supplementation.
Caloric restriction and prolonged endurance training are associated with reduced peripheral T4-to-T3 conversion — a physiological adaptation to conserve energy. This can lower metabolic rate and impair recovery even when TSH and T4 remain within reference ranges. Free T3 is often the first thyroid value to decline under energy deficit conditions.
Turnaround Time
3 days (up to 12 days)
Fasting Required
No
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