Triiodothyronine; T3 radioimmunoassay
This is a test that measures the amount of T3 hormone in the blood.
How the test is performed
Blood is drawn from a vein or by capillary puncture (finger prick).
How to prepare for the test
The health care provider may advise you to withhold drugs that can affect the test (see “special considerations”).
How the test will feel
When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.
Why the test is performed
T3 is measured as part of a thyroid function evaluation. Most of the thyroid hormone made in the thyroid is in the form of T4. The body’s cells convert the T4 to T3, which is the more active hormone.
Sometimes it can be useful to measure both T4 and T3 when looking at thyroid function. For example, in some cases of hyperthyroidism, T4 may be normal but T3 will be elevated.
Most of the T4 and T3 in the body is attached to proteins in the blood. These proteins serve as carriers. The T3 test measures both the T3 that is bound to the proteins and the T3 that is “free” floating in the blood. The free fraction is the hormone that is active.
Conditions that increase the levels of the carrier proteins - such as pregnancy and liver disease - will falsely raise the T3 level. In these cases, it is useful to measure either the free T3 level or to perform the RT3U test, which gives a measure of the amount of carrier protein.
T4 and T3 are important hormones in the regulation of metabolism. The exact mechanisms are not understood, but it is known that T4 increases the concentrations of numerous enzymes involved in the production of energy in the body.
100 to 200 ng/dl (nanograms per deciliter)
What abnormal results mean
Greater-than-normal levels may indicate:
- hyperthyroidism (for example, Graves’ disease)
- T3 thyrotoxicosis (rare)
- thyroid cancer (rare)
Lower-than-normal levels may indicate:
- chronic illness
- hypothyroidism (for example, Hashimoto’s disease)
Additional conditions under which the test may be performed:
- painless (silent) thyroiditis
- thyrotoxic periodic paralysis
- toxic nodular goiter
What the risks are
The only risks of the test is those minor risks associated with having blood drawn.
Drugs that can increase T3 measurements include clofibrate, estrogens, methadone, and oral contraceptives.
Drugs that can decrease T3 measurements include anabolic steroids, androgens, antithyroid drugs (for example, propylthiouracil), lithium, phenytoin, and propranolol.
by Janet G. Derge, M.D.