Free T3 (Free Triiodothyronine)
Free T3 is the active thyroid hormone — three to four times more biologically potent than T4. Useful for confirming hyperthyroidism and tracking specific edge cases of thyroid disease.
What it measures
The thyroid secretes mostly T4, with most T3 produced by peripheral conversion of T4 (catalysed by deiodinase enzymes). Free T3 — the small unbound fraction — is the metabolically active form that actually binds nuclear receptors and drives metabolism. In ordinary practice TSH and Free T4 answer most diagnostic questions; Free T3 is reserved for: (a) confirming hyperthyroidism when TSH is suppressed but Free T4 is normal (T3-toxicosis), (b) early Graves' disease, (c) monitoring patients on T3-containing thyroid replacement, and (d) sick euthyroid syndrome.
What a high value can mean
- Hyperthyroidism / T3-toxicosis — Graves' disease, toxic nodule; Free T3 can be the first to rise.
- Excess T3-containing replacement — Cytomel, desiccated thyroid extracts.
- Acute psychiatric illness, pregnancy — modest elevations.
What a low value can mean
- Severe non-thyroidal illness syndrome (sick euthyroid) — critically ill patients shift T4 conversion away from T3; this is adaptation, not thyroid disease.
- Severe primary hypothyroidism — late finding; Free T4 usually drops first.
- Starvation, anorexia nervosa — metabolic adaptation.
When to discuss with a doctor
Free T3 is rarely the primary diagnostic test. If your TSH is suppressed (low) but Free T4 is normal, an elevated Free T3 confirms T3-toxicosis and supports treatment. In a critically ill patient a low Free T3 alone is usually adaptive, not a reason to replace thyroid hormone. Mediora.AI shows the full thyroid panel together; treatment decisions belong with a clinician.