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Reverse T3 Testing

Meaning

Reverse T3 (rT3) Testing is a specialized endocrine laboratory test that measures the circulating level of reverse triiodothyronine, an inactive metabolite of the primary thyroid hormone, T4. Reverse T3 is produced when the body shunts T4 away from the active T3 pathway, often in response to physiological stress, caloric restriction, or acute illness, a state known as euthyroid sick syndrome. Elevated rT3, particularly in relation to T3, can indicate a compensatory mechanism where the body attempts to slow metabolism to conserve energy. This test is a critical diagnostic tool for understanding subtle thyroid dysfunction and metabolic adaptation.