Free T4 (Free Thyroxine): Normal Range, High & Low Causes
What Free T4 measures
Free T4 is the unbound, active form of thyroxine. It is checked with TSH to tell whether the thyroid is making the right amount of hormone.
Free T4 reference ranges
| Category | Conventional units | SI units |
|---|---|---|
| Typical adult range | 0.8 – 1.8 ng/dL | 10 – 23 pmol/L |
Reference limits vary with the lab method. Compare your result with the range printed on your own report.
Common causes of a high result
- Hyperthyroidism, including Graves disease
- Too much thyroid medicine
- Thyroiditis (inflammation releasing stored hormone)
- Biotin interference in some assays
Common causes of a low result
- Hypothyroidism
- Pituitary gland problems
- Severe illness
- Iodine deficiency
How to prepare for the test
No fasting needed. If you take levothyroxine, ask whether to take it before or after the blood draw.
How often to retest
Usually with TSH, about 6 to 8 weeks after a treatment change.
Track Free T4 over time
Upload your reports and Labgist plots every Free T4 result on one timeline, converts units and shows what changed.
Get the appFrequently asked questions
What is a normal free T4?
About 0.8–1.8 ng/dL (10–23 pmol/L), depending on the lab.
Why test free T4 as well as TSH?
TSH shows how hard the body is pushing the thyroid, while free T4 shows how much hormone it is actually producing. Together they identify the type and degree of a thyroid problem.
Should I take my thyroid pill before the blood test?
Follow your clinician’s advice. Some people delay the dose until after the draw, because taking it just before can briefly raise free T4.
Related tests
Sources
This page is general education and was last updated 2026-10-30. It is not a substitute for medical advice. See our editorial approach.