This test was once used to help calculate the Free Thyroxine Index (FTI), an estimation of the free T4 concentration. It is determined from the total T4 test and some estimation of the level of thyroid hormone binding proteins. The T3 uptake test was the original test for estimating the level of binding proteins, and later versions were called T-uptake methods. These are rarely used now that there are methods available to measure free T4 and free T3 directly.
Reverse T3 (RT3 or REVT3) is a biologically inactive form of T3. Normally, when the liver converts T4 to T3, it also produces a certain percentage of RT3. When the body is under stress, such as during a serious illness, it tries to prevent many tissues that depend on T3 from being metabolically active by producing more RT3 than T3. This is believed to be a way of conserving energy until the stress is relieved and it causes a syndrome called non-thyroidal illness (NTI). RT3 may also be elevated in hyperthyroidism. Use of the RT3 test remains controversial and it is not widely requested.
This article was last reviewed on November 3, 2014. | This article was last modified on March 11, 2015.
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