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Free T4 (free thyroxine) is the primary hormone the thyroid gland secretes, serving as the precursor to the metabolically active thyroid hormone T3. The 'free' fraction is what matters because only unbound hormone can enter cells — total T4 can be misleadingly altered by protein changes. Normal free T4 with elevated TSH confirms primary hypothyroidism; normal free T4 with persistent symptoms often points to a T4-to-T3 conversion problem that only shows on free T3 testing.
Also known as: Free T4, Free Thyroxine, FreeT4, FT4, L-thyroxine, Levo-thyroxine, Levothyroxine, T-4
High free T4 indicates hyperthyroidism, excess thyroid medication, or early thyroiditis releasing stored hormone. Low free T4 confirms hypothyroidism when TSH is elevated.
Standard range is 0.8-1.8 ng/dL. Functional practitioners target upper half of range, 1.1-1.5 ng/dL, reflecting adequate thyroid output for active T3 conversion.