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T3 Alternative Names
triiodothyronine, T3 radioimmunoassay
Definition
This test measures the amount of T3, which is a form of thyroid hormone, in the blood. The test is done to evaluate thyroid function, especially when a person is thought to have an overactive or underactive thyroid.
Thyroid hormone, which is also called thyroxin or T4, seems to be converted to T3 within cells. Thyroid hormone plays a large role in controlling the energy needed to keep the body functioning at rest.
How is the test performed?
To measure the amount of T3 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 T3 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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.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?
The amount of free T3 in blood normally ranges from 2.5 to 5.5 pmols/L (picomoles pre litre).
Abnormally high levels of T3 may indicate the following:. - hyperthyroidism, or an overactive thyroid
- T3 thyrotoxicosis, which is the exposure of tissues to excessive amounts of T3
- thyroiditis, which is an inflammation of the thyroid
- thyroid cancer
Abnormally low levels of free T3 may indicate the following: - Hashimoto's disease, which is an autoimmune disease. An autoimmune disease relates to an immune response against the body's own tissues.
- hypothyroidism, or an underactive thyroid
- chronic illness
- starvation
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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