RT3 is a metabolite (small molecule) of T4 (thyroxine). Typically T4 converts to T3 conversion - it becomes triiodothyronine (T3), the active thyroid hormone. But in some cases, the body conserves energy by converting the T4 instead into rT3, an inactive form of T3 that is incapable of delivering oxygen and energy to the cells, as T3 does. Measuring Reverse T3 or rT3 can be measured by a blood test.
Reverse T3 is kind of a hibernation hormone, in times of stress and chronic illness, it lowers your metabolism. So many people seemingly have normal thyroid levels, but if they have high Reverse T3, they're actually suffering from hypothyroidism.