Free T3 (Free Triiodothyronine)
Also called: Free T3, FT3, 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.
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What does Free T3 (Free Triiodothyronine) measure?
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 does a high Free T3 (Free Triiodothyronine) level 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 does a low Free T3 (Free Triiodothyronine) level 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.