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Free T3 (free triiodothyronine) is the biologically active form of thyroid hormone that enters cells and directly drives metabolism, body temperature, heart rate, mood, and energy production. It is the most functionally important thyroid marker — yet it's routinely excluded from standard thyroid panels, leaving fatigue, weight gain, cold intolerance, and depression undiagnosed. Low free T3 with normal TSH and T4 indicates a conversion problem: the thyroid is producing hormone but the body can't activate it.
Also known as: Free T3, Free Triiodothyronine, FT3, T-3, T3 Free, T3, Free, T3Free, Triiodothyronine, Free
High free T3 indicates hyperthyroidism or T3-predominant thyrotoxicosis. Low free T3 occurs with hypothyroidism, poor T4-to-T3 conversion, chronic illness, or caloric restriction.
Standard range is 2.3-4.2 pg/mL. Functional practitioners target 3.0-3.5 pg/mL. Low free T3 with normal TSH and T4 suggests a conversion problem, not primary thyroid disease.