Definition This test measures the amount of T4, which is also called thyroxine. T4 is a form of thyroid hormone in the blood. The 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? To measure the amount of T4 in the blood, 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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.
What is involved in preparation for the test? A person should request specific instructions from his or her doctor.
What do the test results mean? Normal levels of free T4 range from 10 - 20 pmols/L (picomoles per litre). Abnormally high levels of free T4 may be associated with Graves' disease, a disorder of the thyroid with symptoms of:
heat intolerance and increased sweating.
hand and tongue tremors or shaking.
Some people with Grave's disease have bulging eyes.
Abnormally high levels of T4 may also be associated with the following:
Plummer's disease, a toxic goitre, or enlargement of the thyroid gland, with bumps that can be detected by touch
acute thyroiditis, or inflammation of the thyroid
Abnormally low T4 levels may indicate the following:
Hashimoto's disease, an autoimmune disorder. An autoimmune disorder relates to an immune response against the body's own tissues.
Author: David T. Moran, MD Reviewer: eknowhow Medical Review Panel Editor: Dr John Hearne Last Updated: 21/03/2005 Contributors Potential conflict of interest information for reviewers available on request
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