Thyroxine (T4), Free, Serum or Plasma, Immunoassay

What Is Free T4?

Thyroxine, often called T4, is a hormone made by the thyroid gland, a small butterfly-shaped gland in the neck. Most T4 in the blood is attached to carrier proteins, but a small fraction circulates unbound as free T4. Free T4 is the form that can enter tissues and act on the body, so measuring it gives a clearer picture of thyroid activity than total T4 alone.

Why Is This Test Ordered?

Your clinician may order a free T4 test to help detect an underactive thyroid (hypothyroidism) or an overactive thyroid (hyperthyroidism), to monitor treatment for a thyroid condition, or to follow up on an abnormal thyroid stimulating hormone (TSH) result. It is often ordered together with TSH and sometimes total T4 or T3.

How the Sample Is Collected

The test uses a serum or plasma sample, usually from a routine blood draw from a vein in your arm. No special preparation is typically required unless your clinician gives other instructions. Tell your care team about any thyroid medicines, supplements containing biotin, or other medications you take, because some can affect thyroid test results.

How to Interpret Results

Results are interpreted alongside your symptoms, medical history, and other thyroid tests such as TSH. A free T4 value outside the laboratory's expected range may suggest an overactive or underactive thyroid, but results can be affected by illness, pregnancy, medications, and other factors. Your clinician will explain what your specific result means; reference ranges vary by laboratory and are not listed here.