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T3/rT3 Ratio

Meaning

The T3/rT3 Ratio is a clinical laboratory index that compares the circulating concentration of the biologically active thyroid hormone, triiodothyronine (T3), to the concentration of its metabolically inactive isomer, reverse triiodothyronine (rT3). This ratio serves as a functional marker of peripheral thyroid hormone utilization and the body’s metabolic state. A low T3/rT3 ratio often suggests a state of allostatic load or physiological stress, where the body is preferentially shunting T4 metabolism toward the inactive rT3 pathway to conserve energy, a phenomenon often termed non-thyroidal illness syndrome.