Test library
Free Thyroxine Test
A free T4 test measures the unbound portion of thyroxine, the thyroid hormone that helps set the body's metabolic rate. It is used with other thyroid markers to assess an overactive or underactive thyroid.
Order Free T4 Levels Test
Order online and visit a local collection site, or choose an at-home kit where available. No referral and no insurance required.
Order Free T4 Levels Test- Typical turnaround
- 1-3 business days
- Starting price
- From $39
- Results
- Confidential, online
Quick answer
The free T4 test measures the amount of thyroxine in the blood that is not attached to carrier proteins. Thyroxine is the main hormone produced by the thyroid gland and is converted in tissues to the more active T3. Measuring the free fraction helps clinicians assess thyroid function, and the result is usually read alongside TSH and free T3.
What this test measures
The thyroid gland produces thyroxine, known as T4, and a smaller amount of triiodothyronine, known as T3. Most T4 circulates bound to proteins, and only the free portion is available to tissues. Free T4 is therefore a useful marker of how much active hormone the thyroid is making and releasing.
The test uses a blood sample collected from a vein, and no special preparation is usually needed. Results are reported against a reference range. Levels can be influenced by illness, pregnancy, and certain medications, so clinicians consider the broader context rather than the number alone.
Free T4 is commonly ordered when someone reports fatigue, weight change, temperature sensitivity, or palpitations, or when a TSH result is abnormal. It is also used to monitor thyroid treatment over time. A qualified healthcare provider interprets the result with the rest of the thyroid panel.
- Free thyroxine (FT4)
- Total thyroxine (T4)
- Thyroid-stimulating hormone (TSH)
- Free triiodothyronine (FT3)
- Total triiodothyronine (T3)
- Thyroglobulin antibodies when included
- Thyroid peroxidase antibodies when included
- Thyroxine-binding globulin when reported
Why people order this test
- Evaluate an overactive or underactive thyroid
- Follow up on an abnormal TSH result
- Monitor thyroid hormone treatment
- Investigate fatigue, weight change, or temperature sensitivity
- Check thyroid function during pregnancy when a provider requests it
- Support a complete thyroid panel
How to prepare
No fasting is required for a free T4 test in most cases. Tell the ordering provider about any medications or supplements that may affect thyroid levels. Follow the specific timing instructions the laboratory provides.
Understanding your results
Results are typically available within one to three business days. The report shows the free T4 value and its reference range. An abnormal result is interpreted with TSH, free T3, and symptoms by a qualified healthcare provider.
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Frequently Asked Questions
What is free T4?
Free T4 is the portion of thyroxine not bound to carrier proteins. It reflects the amount of thyroid hormone available to body tissues.
Why is TSH tested with free T4?
TSH is the pituitary signal that controls thyroid hormone production. Reading it together with free T4 helps show whether a problem starts in the thyroid or the pituitary.
Do I need to fast before a free T4 test?
Fasting is not usually required. Follow any instructions provided by the laboratory, especially if other tests are included.
Can pregnancy affect free T4 results?
Yes. Pregnancy changes thyroid hormone levels and reference ranges, so clinicians use ranges suited to pregnancy when interpreting results.
Does an abnormal free T4 mean I have a thyroid disorder?
Not on its own. A result outside the range is evaluated with other thyroid tests and the person's symptoms by a qualified healthcare provider.
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