A T3 (triiodothyronine) test helps diagnose thyroid conditions, particularly hyperthyroidism (overactive thyroid).

Your thyroid is a small, butterfly-shaped gland located at the front of your neck under your skin. It’s a part of your endocrine system. Triiodothyronine, also known as T3, is one of the two main thyroid hormones. Thyroxine, or T4, is the other hormone.

Healthcare providers test T3 levels using blood tests. Triiodothyronine comes in two forms:

Free T3: This form enters your body’s tissues where it's needed.

Bound T3: This form attaches to proteins, which prevents it from entering your body’s tissues.

Because of this, there are a few different tests that measure T3 levels. A blood test that measures both free T3 and bound T3 is called a total T3 test. A different blood test measures just free T3 levels. The tests for free T3 are generally less accurate than for total T3.

Healthcare providers often order additional tests to assess thyroid function alongside a T3 test, including a T4 (thyroxine) test and a TSH (thyroid-stimulating hormone) test.

Other names for a T3 test include:

Thyroid function test.

Total triiodothyronine.

Free triiodothyronine.

FT3.