The reverse T3 blood test is used to help determine if an individual has sick euthyroid syndrome. Reverse T3 (rT3) is a byproduct of thyroid hormone metabolism. Reverse T3 is biologically inactive and does not stimulate thyroid hormone receptors. If you suspect of thyroid dysfunction usually the thyroid tests that are completed are TSH, T3, and T4. The reverse T3 is added to gather more information regarding your thyroid health.