Free T3
The biologically active thyroid hormone - what your tissues actually use. Symptoms correlate better with Free T3 than TSH.
How Free T3 ranges relate
The standard lab range vs the optimization-focused target. Illustrative trajectory shows what a 12-week improvement pattern looks like — not real user data.
What Free T3 Measures
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.
What Affects This Biomarker
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.
In the Context of Peptide Protocols
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.
Peptides That Commonly Move Free T3
Related Reading
Conditions That Track Free T3
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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.