Total T3

Triiodothyronine (T3) is one of the main thyroid hormones and is the more active form compared to T4. A T3 test measures circulating T3 levels and helps assess thyroid function, particularly in evaluating hyperthyroidism, monitoring thyroid medication, and investigating fatigue or weight changes. T3 affects metabolism, energy production, and body temperature. Low T3 levels may indicate hypothyroidism or a metabolic adaptation, while high T3 suggests overactive thyroid function.

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.