This isolation Reverse T3 (RT3) test measures the inactive form of the hormone T3, or Triiodothyronine. Normally, the body converts T4 into both T3 and RT3 in specific ratios. Elevated RT3 levels are often found in individuals with hypothyroidism. Measuring reverse T3 can help differentiate between thyroid-related conditions and non-thyroidal illnesses. This test is typically used when results from other thyroid tests—such as total T3, total T4, free T4, and TSH—are within expected ranges, but further investigation is needed. Fasting is not required for this test.

A provider will approve your labs, and your Lab Requisition Form will be emailed to you within 24 business hours. Labcorp will also have this on file. Please bring a valid photo ID to your nearest Labcorp location. Results will be securely emailed as soon as they are available.