Medical Health Encyclopedia

T3

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.

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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.

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