The biologically active thyroid hormone — what your tissues actually use. Symptoms correlate better with Free T3 than TSH.
T3 is 4× more biologically active than T4 — it's the molecule that actually binds nuclear thyroid hormone receptors and drives metabolic effects. About 80% of circulating T3 comes from peripheral T4-to-T3 conversion (mostly in liver and kidney) via deiodinase enzymes; the thyroid produces only ~20% directly.
Low Free T3 with normal T4 and TSH points to peripheral conversion problems — often called "low T3 syndrome" or "non-thyroidal illness syndrome." Common in chronic stress, severe caloric restriction, chronic disease, and significant inflammation.
Free T3 is influenced by: T4-to-T3 conversion efficiency (selenium, zinc, iron status), liver function, kidney function, caloric intake (low calories → less T3), chronic illness, cortisol levels (high cortisol shunts T4 to Reverse T3 instead), and pharmacologic agents — beta blockers reduce conversion; cytomel/T3 supplements raise it directly; HGH and tesamorelin enhance T4→T3 conversion.
Track Free T3 alongside Free T4 + Reverse T3 to assess conversion. Sustained low Free T3 in dieting or training-stressed users suggests reducing intensity or increasing calories. Some optimization clinicians add low-dose T3 (Cytomel) when conversion is poor. On GH peptides, expect Free T3 to rise modestly — symptom improvement (energy, body temp) often tracks with this.
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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.