Reverse T3 is a non-functioning form of the active hormone triiodothyronine (T3). In the tissues, T4 normally converts to T3. However, other hormones such as cortisol may inhibit this conversion, producing reverse T3 (rT3). Having high levels of reverse T3 is known as thyroid resistance and produces symptoms of low thyroid function (hypothyroidism). If you suffer from symptoms such as fatigue, weight gain, dry skin or hair loss and traditional thyroid lab results are inconclusive, you might consider checking reverse T3 level. Fasting is not required for this test. Take all medications as prescribed.