T4 Free

The T4 Free (FT4) test is a way to measure a specific type of thyroid hormone in the blood called thyroxine, or T4, that is not stuck to proteins that usually carry it around. This free T4 is important because it can go into body cells and do its job without being blocked by these proteins. Doctors use this test along with another test for a hormone called thyroid-stimulating hormone (TSH, test code 899) to check if someone has thyroid problems like hyperthyroidism, where there’s too much hormone, or hypothyroidism, where there’s too little. This includes both types that people are born with and types that develop later.

The FT4 test helps doctors understand how well the thyroid is working, keep an eye on thyroid disease, and see how well treatments are working. This test is more popular than an older test called the total T4 test (test code 867), which measures all the T4 in the blood, both free and stuck to proteins. The FT4 test is better because it doesn’t get messed up by the amount of protein available to carry the hormone.