T4

Alternate Names : Thyroxine Test

A T4 blood test measures the amount of a thyroid hormone known as thyroxine, or T4.

Who is a candidate for the test?

A T4 test is usually performed to evaluate thyroid function. The levels of thyroid hormone in the blood are important to health. Thyroid hormone controls the body's rate of basal metabolism, which is the energy needed to keep the body functioning at rest.

How is the test performed?

A blood sample is taken from a vein on the forearm or hand. First, the skin over the vein is cleaned with an antiseptic. Next, a strong rubber tube, or tourniquet, is wrapped around the upper arm. This enlarges the veins in the lower arm by restricting blood flow through them. A fine needle is gently inserted into a vein, and the tourniquet is removed. Blood flows from the vein through the needle and is collected into a syringe or vial for T4 testing in the laboratory. After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.