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Also known as: Free T3; T3
Formal name: Triiodothyronine
Related tests: TSH; FT4; thyroid antibodies

At a Glance

Why Get Tested?

To help diagnose hyperthyroidism

When to Get Tested?

If you get an abnormal TSH or FT4 result

Sample Required?

A blood sample taken from a vein in the arm

The Test Sample

What is being tested?

The test measures the amount of free triiodothyronine, or FT3, in your blood.

T3 is one of two major hormones produced by the thyroid gland (the other hormone is called thyroxine, or T4). T3 makes up less than 10% of what we call thyroid hormone, while T4 makes up the rest. T3, however, is about four times as strong as T4, and is thought to cause most, if not all, the effects of thyroid hormones.

Many of your body’s cells can turn T4 into T3; T4 may be mainly a "reservoir" used to make T3 available.

Thyroid hormones help regulate the body’s metabolism (how the body functions).

About 99.7% of the T3 in blood is attached to a protein, and the rest is unattached. The blood test can measure either the total (both bound and unattached) or free (unattached) T3 hormone in the blood.

There is a feedback system between the pituitary gland, which produce 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?

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

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.