Definition 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 After the needle is withdrawn; the puncture site is covered for a short time to prevent bleeding.
What is involved in preparation for the test? No preparation is generally needed.
What do the test results mean? Thyroid function tests can tell the doctor if there is a normal amount of thyroid hormone in the body. The general results of the test show if the overall thyroid hormone level is low, normal, or high.
Thyroid function tests can also help determine the cause of an abnormal thyroid level. For example, the test measures thyroid hormones and a pituitary hormone that stimulates the thyroid gland. If the thyroid hormone level is low, it may be due to a disease within the thyroid gland. It may also be due to a disease in the pituitary gland. This is because the thyroid gland will not secrete thyroid hormone unless the pituitary gland stimulates it. If a person is already taking thyroid hormone replacement pills, an abnormal level may mean that the dose needs to be adjusted.
The exact levels of the different components in thyroid function tests vary. The following can change what are considered normal values in the thyroid function tests:
age
gender
pregnancy
certain medications
other illnesses
The results of the thyroid function test should be discussed with the doctor who ordered the test.
Author: Adam Brochert, MD Reviewer: eknowhow Medical Review Panel Editor: Dr John Hearne Last Updated: 26/05/2005 Contributors Potential conflict of interest information for reviewers available on request
