About Free T4
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.