Measures inactive thyroid hormone:
Unlike the active thyroid hormone T3, reverse T3 does not exert metabolic effects in the body, but high levels can indicate that the body is converting more T4 into the inactive form instead of the active form. 
Used when other thyroid tests are inconclusive:
If someone has symptoms of hypothyroidism (fatigue, weight gain, etc.) but standard thyroid tests like TSH, T3, and T4 are normal, a reverse T3 test may be ordered to look for potential underlying issues. 
Stress and illness marker:
Elevated reverse T3 levels can be seen in situations like severe illness, surgery, starvation, or prolonged stress, where the body prioritizes other functions over normal thyroid hormone production.