The bioactive thyroid hormone in storage form — gets converted to active T3 in peripheral tissues.
T4 is the primary thyroid hormone produced by the thyroid gland — about 80% of total thyroid output. It's a prohormone; tissues convert it to active T3 (triiodothyronine) via deiodinase enzymes. Free T4 measures the unbound, biologically available fraction (~0.05% of total T4).
Free T4 alongside TSH gives the basic thyroid picture: low T4 + high TSH = primary hypothyroidism; low T4 + low TSH = secondary (pituitary) hypothyroidism; high T4 + low TSH = hyperthyroidism.
Free T4 is influenced by: thyroid gland output, iodine status (deficiency lowers T4), pituitary function, autoimmune thyroid disease (Hashimoto's lowers, Graves' raises), pregnancy, and pharmacologic agents — levothyroxine raises serum T4 directly; methimazole lowers; some peptides (tesamorelin, HGH) modestly accelerate T4-to-T3 conversion, can shift T4 lower without TSH change.
Track Free T4 with Free T3, Reverse T3, and TSH for the complete picture. On levothyroxine, target Free T4 in the upper half of range. On GH protocols (tesamorelin, HGH), expect modest Free T4 decline as T3 conversion accelerates — TSH should stay stable. If Free T4 drops AND TSH rises, thyroid is being stressed.
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Start tracking →Informational only — not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.