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Also known as: Free T4; T4
Formal name: Thyroxine
Related tests: TSH; FT3; thyroid antibodies

At a Glance

Why Get Tested?

To diagnose hypothyroidism or hyperthyroidism in adults

When to Get Tested?

FT4 may be is measured in response to an abnormal TSH test result

Sample Required?

A blood sample drawn from a vein in the arm.

Test Preparation Needed?

None but tell your doctor what medications you are taking as some may affect the test results.

The Test Sample

What is being tested?

This test measures the amount of free thyroxine, or FT4, in your blood. T4 is one of two major hormones produced by the thyroid gland (the other is called triiodothyronine, or T3).

T4 makes up nearly all of what we call thyroid hormone, while T3 makes up less than 10%. Thyroid hormones help regulate the body’s metabolism (that is, how the body functions).

Most T4 in blood is attached to a protein; less than 1% is unattached. The blood test measures the amount of free (unattached) T4 hormone in your blood since this is the biologically relevant fraction.

Scientists believe that free hormone is responsible for all the effects of thyroid hormone.

There is a feedback system between the pituitary gland which produces TSH and the thyroid. Normally TSH produced in the pituitary drives thyroid T3 and T4 production. If thyroid hormone production falls, TSH rises and if T3 and T4 become too high, TSH levels fall.

How is the sample collected for testing?

A blood sample is obtained from a needle placed in a vein in your arm.

Is any test preparation needed to ensure the quality of the sample?

No test preparation is needed.

The Test

Common Questions

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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.