What is a T3 (triiodothyronine) test?

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.

What is a T3 (triiodothyronine) and what does it do?

Triiodothyronine, also known as T3, is one of the two main hormones your thyroid gland releases into your bloodstream. Your thyroid also produces thyroxine, also known as T4 and tetraiodothyronine. T4 and T3 work together and are commonly referred to as “thyroid hormone.”

Most of the T3 (approximately 80%) in your blood is from your body’s conversion of T4 into T3 outside of your thyroid gland. The rest of the T3 in your bloodstream is produced by your thyroid gland.

Hormones are chemicals that coordinate different functions in your body by carrying messages through your blood to your organs, muscles and other tissues. These signals tell your body what to do and when to do it.

T3 is the active form of thyroid hormone, meaning it impacts cells in your body, whereas T4 is the inactive form of thyroid hormone. Your liver and kidneys convert most of the T4 your thyroid releases into T3.

Together, T4 and T3 play vital roles in regulating your body’s:

• Metabolic rate (the rate at which your body transforms the food you eat into energy).
• Heart and digestive functions.
• Muscle control.
• Brain development.
• Bone maintenance.

Why do I need a T3 (triiodothyronine) test?

Healthcare providers most often order T3 tests to help diagnose hyperthyroidism, a condition in which your thyroid makes too much thyroid hormone, or to determine the severity of hyperthyroidism.

Your provider may order regularT3 tests to monitor your T3 levels if you’re taking thyroid hormone replacement therapy (medication) for a thyroid condition.