Reverse Triiodothyronine (RT3)

Reverse T3 (rT3) is an inactive form of thyroid hormone that your body produces as a byproduct of processing the main thyroid hormone, T4. When your body is under significant stress—such as during a serious illness, major surgery, or prolonged dieting—it tends to make more rT3 instead of the active thyroid hormone your cells need. This is generally considered a protective response rather than a thyroid disease. Most thyroid specialists do not routinely test rT3 because standard thyroid tests like TSH and free T3/T4 provide more reliable information about thyroid health.

When elevated: Elevated rT3 may indicate non-thyroidal illness syndrome, significant physiological stress, or impaired peripheral T4-to-T3 conversion. It is associated with critical illness, sepsis, major trauma, and prolonged caloric restriction. Certain medications can also drive rT3 elevation. Isolated elevated rT3 in an otherwise healthy individual should be interpreted cautiously, as clinical significance outside of acute illness contexts is not well established. When low: Low rT3 levels are generally not considered clinically significant in isolation. They may be observed in hyperthyroidism, where accelerated T4 metabolism favors active T3 production, or in conditions with reduced T4 substrate availability such as hypothyroidism. Low rT3 is not typically associated with adverse clinical outcomes.

RT3 may reflect metabolic adaptation to high training load, caloric deficit, or overtraining—conditions that shift T4 metabolism away from active T3. Elevated RT3 with low-normal T3 can signal insufficient recovery, poor nutrition, or excessive stress, potentially impacting energy availability and performance. However, RT3 is not routinely monitored in sports medicine; it's most useful when athletes experience persistent fatigue despite normal TSH and free T3 levels.