Thyroid function tests measure the levels of various hormones in the blood related to the function of the thyroid gland. The thyroid gland is located in the front of the neck just below the Adam's apple. It produces thyroid hormone, which plays an important role in metabolism. The thyroid gland is stimulated to produce hormones by a gland attached to the brain called the pituitary.

Who is a candidate for the test?

Thyroid function tests are generally recommended for people with known or suspected thyroid gland problems.

How is the test performed?

To measure thyroid function tests, 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 to enlarge 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 in a syringe or vial for testing in the laboratory. After the needle is withdrawn; the puncture site is covered for a short time to prevent bleeding.