Free T4 (thyroxine) is the main hormone made by the thyroid. 'Free' means the part not bound to proteins, which is available to your cells.
- Normal range (adults)
- 0.8–1.8 ng/dL (10.3–23 pmol/L)
- Sample
- Blood
- Fasting
- Not usually needed
- Also called
- FT4, Free Thyroxine, T4 Free, Thyroxine Free, Free T4 Index, T4, Thyroxine, T4 Total
Free T4 normal range
| Group | ng/dLConventional units: US, South Asia | pmol/LSI units: UK, Europe, Australia, international |
|---|---|---|
| Adults | 0.8–1.8 | 10.3–23 |
Your report may use either unit system; labs in the Gulf often print both. Ranges also vary slightly between laboratories, so the range printed on your own report is the one to go by.
Why is the Free T4 test done?
It is measured with TSH to confirm whether the thyroid is under- or overactive and to adjust treatment.
What does a high Free T4 level mean?
A high Free T4 usually means an overactive thyroid (hyperthyroidism), or too high a dose of thyroid medicine.
Common causes
- Graves' disease
- Toxic thyroid nodules
- Thyroiditis
- Excess levothyroxine
- Biotin supplements can interfere with the test
Diet and lifestyle tips
- Avoid iodine supplements and seaweed
- Limit caffeine
- Include calcium and vitamin D
- Stop biotin supplements 2 to 3 days before testing
What does a low Free T4 level mean?
A low Free T4 means an underactive thyroid (hypothyroidism), especially when TSH is high.
Common causes
- Hashimoto's thyroiditis
- Iodine deficiency
- After thyroid surgery or radioactive iodine
- Pituitary problems (if TSH is also low)
Diet and lifestyle tips
- Iodised salt, dairy, eggs and seafood
- Selenium sources such as fish and eggs
- Take thyroid medicine on an empty stomach, away from calcium and iron
When should you see a doctor?
Abnormal Free T4 should be reviewed with your TSH by a doctor; treatment is usually simple and very effective.
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This page is for general education and is not a diagnosis. Normal ranges differ between laboratories, so always compare your result with the reference range printed on your own report, and discuss abnormal results with your doctor.