Test guide
Thyroxine (T4) Test
Thyroxine is also called T4. T4 is a hormone that your thyroid gland makes. A T4 test measures the level of T4 in a sample of your blood. Too much or too little T4 may be a sign of a thyroid problem .
What it is for
A T4 test is usually used with other thyroid tests to help diagnose and monitor thyroid disease and to gather more information about other conditions that may affect your thyroid. It may be used to: In certain cases, a T4 test may be done as part of a group of thyroid tests called a thyroid panel. Source: MedlinePlus, National Library of Medicine. Medical test information is public domain. Source identity and definition checked automatically on 2026-10-18. No individual clinical interpretation or human reviewer is asserted.
Limitations
Reference intervals and interpretation depend on the laboratory, specimen, method and clinical context. This source definition is educational information; it does not establish a diagnosis, product approval or suitability for an individual.
Also known as
- free thyroxine, free T4, total T4 concentration, Free T4 index (FTI), thyroxine screen, free T4 concentration, thyroxine test by equilibrium dialysis
Reviewed products
Curated product links for this test are still under editorial review. Meanwhile, browse the product directory or request a supplier match below.