Free T3 (Triiodothyronine)
Free T3 (free triiodothyronine) is the active form of thyroid hormone. While the thyroid primarily produces T4, the body converts T4 to T3 in peripheral tissues (liver, kidneys, muscle). T3 is approximately 4–5 times more biologically active than T4 and is the hormone that actually enters cells and drives metabolic processes.
Free T3 measures the unbound, bioavailable fraction of T3 in the blood. Low free T3 with normal TSH and T4 may indicate poor T4-to-T3 conversion, which can occur with nutrient deficiencies (selenium, zinc, iron), chronic stress, caloric restriction, or illness.
Symptoms of low free T3 include fatigue, cold intolerance, brain fog, hair loss, constipation, and difficulty losing weight — even when TSH appears "normal."
What It Measures
Free T3 measures the unbound, biologically active form of triiodothyronine in the blood. It reflects the body's actual thyroid hormone availability at the cellular level.
Reference Ranges
| Status | Range (pg/mL) |
|---|---|
| Optimal | 3.0–3.5 pg/mL |
| Normal | 2.3–4.2 pg/mL |
| Borderline | 2.0–2.3 pg/mL |
| Out of Range | < 2.0 pg/mL or > 4.5 pg/mL |
Optimal Ranges
| Group | Range | Unit | Source |
|---|---|---|---|
| Standard | 2–4.4 | pg/mL | Standard reference |
| Optimal (functional) | 3–4 | pg/mL | Functional range |
When Values Are High
Elevated Free T3 may indicate hyperthyroidism, excess T3 supplementation, or thyroiditis. Symptoms include anxiety, weight loss, tremor, and palpitations.
When Values Are Low
Low Free T3 with normal TSH suggests impaired T4→T3 conversion (often from stress, inflammation, or nutrient deficiency). This is the active thyroid hormone — low levels cause fatigue even with normal TSH.
What To Do
Lifestyle
- •Manage chronic stress (cortisol blocks T4→T3 conversion)
- •Ensure adequate caloric intake (undereating reduces T3)
- •Regular moderate exercise supports conversion
Supplements
- •Selenium 200mcg supports T4→T3 conversion enzyme
- •Zinc supports thyroid hormone receptor binding
- •Adequate iron is required for thyroid hormone synthesis
When to Retest
Retest with TSH and Free T4 as a complete thyroid panel
How GATOR Tracks This
Lab Providers
Related Biomarkers
This content is for informational purposes only and does not constitute medical advice. Reference ranges may vary by laboratory and individual factors. Consult a qualified healthcare provider for interpretation of your results and treatment decisions.
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