Reverse T3

This test measures the ratio of active thyroid hormone (T3) to reverse T3 (rT3), reflecting thyroid hormone metabolism efficiency. T3 is the active form that powers metabolism, while reverse T3 is inactive and can block T3 action. An unfavorable ratio (high rT3 relative to T3) suggests poor conversion, seen in stress, illness, or metabolic dysfunction. This ratio helps assess thyroid function beyond standard TSH testing and guides treatment decisions.

Reverse T3 (rT3) is an inactive thyroid hormone produced alongside active T3. Your body converts thyroid hormones (T4 and T3) into either active T3 or inactive rT3, depending on your metabolic needs and stress. High reverse T3 can indicate that your thyroid hormones are being shunted away from active forms, potentially causing fatigue and metabolic slowdown despite normal TSH and T4 levels. This test is useful for investigating persistent thyroid symptoms and understanding how your body is utilizing thyroid hormones during stress, illness, or metabolic imbalance.