Reverse T3 (rT3) is an inactive isomer of triiodothyronine that is produced when thyroxine (T4) is metabolized through an alternative pathway. This test measures rT3 levels in the blood to evaluate thyroid hormone metabolism and diagnose non-thyroidal illness syndrome. Elevated rT3 levels typically indicate that the body is converting T4 to inactive rT3 instead of active T3, which commonly occurs during acute illness, stress, or chronic disease. This pattern is characteristic of the 'euthyroid sick syndrome' where patients have abnormal thyroid hormone levels despite normal thyroid gland function. High rT3 levels have been associated with increased mortality in critically ill patients and may indicate the severity of illness. This test is particularly valuable for distinguishing between true thyroid disease and temporary thyroid hormone changes due to non-thyroidal illness.