Free Thyroxine (T4)
Disclaimer: Information on this page is for educational purposes. Consult a physician to interpret your test results. Lissa Health helps track biomarker trends but does not replace medical advice.
What This Test Measures
Free T4 is the biologically active unbound thyroxine fraction reflecting thyroid hormone production.
In primary hypothyroidism T4 is low and TSH is high. In hyperthyroidism the pattern is reversed.
Total T4 is less reliable due to binding proteins; free T4 is preferred.
Normal Ranges
| Group | Reference Range |
|---|---|
| Men | 0.8–1.8 ng/dL (9–19 pmol/L) |
| Women | 0.8–1.8 ng/dL |
| Children | 0.9–1.7 ng/dL |
Reference ranges may vary by laboratory and assay method.
Causes of High Levels
- Hyperthyroidism
- Thyroiditis (destructive phase)
- Excess levothyroxine
Causes of Low Levels
- Hypothyroidism
- Severe illness (euthyroid sick syndrome)
- Drugs (carbamazepine, phenytoin)
Test Preparation
- Morning draw
- Before levothyroxine dose
- Report biotin (may interfere with assays)
Related Biomarkers
| Biomarker | Relationship |
|---|---|
| Thyroid Stimulating Hormone (TSH) | Primary screening |
| Free Triiodothyronine (T3) | Conversion to active T3 |
| Prolactin | Pituitary axis |
FAQ
How often should I take this test?
With TSH in thyroid disease — every 6–12 months.
What should I do if my result is abnormal?
Low T4 + high TSH — start levothyroxine; high T4 + low TSH — hyperthyroidism workup.
Last updated: June 2026