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Alternative Names 
triiodothyronine, T3 radioimmunoassay

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 After the needle is withdrawn, the puncture site is covered for a short time to prevent bleeding.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
Potential conflict of interest information for reviewers available on request

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