(See also Overview of the Thyroid Gland.)
Euthyroid means the thyroid gland is functioning normally. Euthyroid sick syndrome commonly occurs in people who have a severe illness other than thyroid disease. When people are sick or undernourished or have had surgery, less thyroid hormone T4 (thyroxine, or tetraiodothyronine) is converted to the active thyroid hormone, T3 (triiodothyronine). Levels of T3 are low and large amounts of reverse T3, an inactive thyroid hormone, accumulate. Despite this abnormal conversion, the thyroid gland continues to function and to control the speed at which the body’s chemical functions proceed (metabolic rate) normally despite changes in some of the results of thyroid blood tests.
Because no problem exists with the thyroid gland, no treatment is needed. Laboratory tests return to normal once the underlying illness resolves.