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Reverse triiodothyronine, also known as reverse T3 or rT3, is one of the hormones produced by the thyroid gland. The thyroid gland's function is to regulate metabolism, and it mainly secretes the hormones T4, or thyroxine, and T3, or triiodothyronine, but reverse T3 is also produced in small amounts. Triiodothyronine is the active thyroid hormone and, when circulating thyroxine reaches the body's tissues, a proportion of it is converted into triiodothyronine. Reverse triiodothyronine, which is inactive, is a byproduct of this conversion process, and most of it is produced in this way. Increased levels of reverse triiodothyronine are sometimes found in patients who have serious illnesses.
The reverse triiodothyronine molecule is an example of what is known as an isomer of triiodothyronine, which means it is a molecule containing all the same atoms as triiodothyronine but has those atoms arranged in a different structure. Around 95 percent of reverse triiodothyronine in the circulation is created during the conversion of T4 to T3 by deiodinase enzymes. The remaining five percent is produced by the thyroid gland.
There are many different situations in which reverse triiodothyronine levels may be raised. It is known that, in the fetus, larger amounts of reverse T3 are created, together with lower levels of triiodothyronine. Levels of reverse T3 drop several weeks after birth to match those of normal adults. Fasting initially causes a fall in T3 and an increase in reverse T3, while longer-term starvation sees reverse triiodothyronine levels returning to normal. Overeating has the opposite effect, with a rise in T3 and a decrease in rT3.
Certain drugs can affect deiodinase enzymes, inhibiting their actions and leading to a decrease in the conversion of T4 to T3 in the body's tissues, and a deficiency of selenium can have a similar effect. In what is referred to as euthyroid sick syndrome, serious illnesses such as cancer, kidney and liver failure, heart attacks, infections and burns suppress deiodinase enzymes. This means that it can be difficult to assess thyroid function in people who are gravely ill, as the levels of thyroid hormones may be abnormal in the absence of thyroid disease. In euthyroid sick syndrome, even though the amount of T3 produced by the thyroid gland remains the same, decreased conversion in the tissues causes a reduction in the overall level of T3. Less reverse T3 is cleared from the body, with the result that reverse triiodothyronine levels increase.