Is my Free T4 (Free Thyroxine) normal?
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What is a normal Free T4 level?
Free T4 (thyroxine) is the inactive thyroid hormone that converts to T3 (the active form) in tissues. It's produced by the thyroid gland and regulated by TSH. Testing Free T4 alongside TSH gives a more complete picture of thyroid function than TSH alone. Normal is 0.8-1.8 ng/dL, with optimal at 1.0-1.5 ng/dL.
| Range | Values |
|---|---|
| Standard Reference | 0.8–1.8 ng/dL |
| Optimal | 1–1.5 ng/dL |
| Longevity Optimal | 1.0-1.5 ng/dL |
| High Risk | > 3 ng/dL |
| Low Risk | < 0.5 ng/dL |
What causes abnormal Free T4 levels?
Low Free T4 is caused by hypothyroidism (Hashimoto's thyroiditis, iodine deficiency, thyroid surgery/radiation, pituitary disorders). High Free T4 is caused by hyperthyroidism (Graves' disease, toxic nodules, thyroiditis, excess thyroid medication). Biotin supplements can interfere with thyroid lab assays, causing falsely abnormal results.
When should you see a doctor?
See your doctor if Free T4 is below 0.8 ng/dL or above 1.8 ng/dL, especially with abnormal TSH. Also consult if you have thyroid symptoms (fatigue, weight changes, temperature sensitivity, hair loss, anxiety) even with normal Free T4, as Free T3 may need testing. Stop biotin supplements 48 hours before testing.
Free T4 and longevity
Free T4 is the reservoir hormone that converts to T3 (the metabolically active form) in peripheral tissues via deiodinase enzymes. The conversion efficiency varies by organ — the brain, liver, and muscle each have distinct T4-to-T3 conversion rates that are affected by selenium status, inflammation, caloric restriction, and stress hormones. "Sick euthyroid syndrome" (low T3 with normal TSH and T4) is common during illness or severe caloric deficit and represents the body downregulating metabolism as a survival mechanism. For longevity, the mid-range sweet spot of 1.0-1.5 ng/dL is associated with optimal metabolic rate without the tissue damage of hyperthyroidism. Biotin supplementation is a frequently overlooked confounder — it interferes with immunoassay-based thyroid tests, causing falsely abnormal results.
Where to get a Free T4 test
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Related biomarkers
These biomarkers are often tested alongside Free T4 for a more complete picture.
Frequently asked questions
What is a normal Free T4 level?
What is the difference between Free T4 and Total T4?
Should I test Free T3 too?
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Sources: Reference ranges based on AHA/ACC, ADA, and AACE clinical guidelines. Biological variation data from Ricos/Westgard database. Longevity-optimal ranges reflect targets used by preventive and functional medicine practitioners.
Last reviewed: April 2026. This tool provides general health information and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your healthcare provider about your specific results.