The Reverse T3 Test helps evaluate thyroid function and metabolism.The Reverse T3 Test is also known as RT3; REVT3; Reverse Triiodothyronine; and T3, Reverse. No fasting is required for this blood test, and results will be ready within 2-3 days. Patients do not need a doctor’s order or insurance to purchase this test.Reverse T3, also known as RT3 or REVT3, is a biologically inactive form of T3, or triiodothyronine, one of two major hormones produced by the thyroid (the other is T4, or thyroxine). Under normal conditions, the body converts T4 to T3 and RT3 in specific ratios. In certain circumstances, the amount converted to RT3 rises, such as when the body is under stress, as in cases of serious or acute illness or injury. Drugs such as amiodarone and glucocorticoids can also cause increased RT3 levels. RT3 levels alone may not be indicative of a thyroid condition, since stress can cause levels to rise. A more accurate assessment may be gained by combining the Reverse T3 Test with the Free T3 Test to evaluate the ratio between RT3 and free T3 levels.RT3 levels are elevated at birth, but decline to normal levels in the first week of life. A Reverse T3 Test may be of use in the assessment of thyroid function and metabolism in newborns.