Free T4 (Thyroxine)
Free T4 (free thyroxine) is the unbound form of the primary hormone produced by the thyroid gland. T4 serves as a reservoir hormone that is converted into the active form T3 as needed by peripheral tissues. About 80% of circulating thyroid hormone is T4.
Free T4 is preferred over total T4 because it measures only the biologically available hormone, unaffected by changes in binding proteins (which can fluctuate with pregnancy, oral contraceptives, or liver disease).
Free T4 is most useful when interpreted alongside TSH and free T3. Low free T4 with elevated TSH confirms hypothyroidism. Normal free T4 with elevated TSH suggests subclinical hypothyroidism. Low free T4 with low TSH may indicate central (pituitary) hypothyroidism.
What It Measures
Free T4 measures the unbound, available thyroxine in the blood. It reflects the thyroid gland's direct hormonal output before peripheral conversion to active T3.
Reference Ranges
| Status | Range (ng/dL) |
|---|---|
| Optimal | 1.1–1.5 ng/dL |
| Normal | 0.8–1.8 ng/dL |
| Borderline | 0.6–0.8 ng/dL |
| Out of Range | < 0.6 ng/dL or > 2.0 ng/dL |
Optimal Ranges
| Group | Range | Unit | Source |
|---|---|---|---|
| Standard | 0.82–1.77 | ng/dL | Standard reference |
| Optimal (functional) | 1.1–1.5 | ng/dL | Functional range |
When Values Are High
Elevated Free T4 with low TSH indicates hyperthyroidism. Elevated Free T4 with normal/high TSH may indicate a TSH-secreting pituitary adenoma or thyroid hormone resistance.
When Values Are Low
Low Free T4 with high TSH confirms hypothyroidism. Low Free T4 with normal TSH may indicate central hypothyroidism (pituitary dysfunction).
What To Do
Lifestyle
- •Same foundational support as TSH and Free T3
- •Avoid excessive soy consumption which may interfere with thyroid hormone absorption
Supplements
- •If on levothyroxine, take on empty stomach 30–60 min before food
- •Avoid calcium and iron supplements within 4 hours of thyroid medication
When to Retest
Retest 6–8 weeks after medication adjustment; always test alongside TSH and Free T3
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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