Normally T4 is converted to T3, the biologically active thyroid hormone. However, when the body is under stress (due to illness, starvation, stress or extreme cold), it converts T4 to reverse T3 (rT3) which inhibits the action of T3 in our cells. This is thought to be a protective mechanism to conserve energy.
What might a low result mean?
It is normal to have some reverse T3 in your blood as your body converts T4 into both T3 and rT3 in varying proportions.
What might a high result mean?
A high level of reverse T3 in your blood may indicate that your body is trying to conserve energy. This test will not tell you why you have elevated rT3 but it may indicate that an underlying condition needs more careful management, and may explain why you are experiencing symptoms of an underactive thyroid when your thyroid hormones appear normal.
Knowing your reverse T3 level is of limited value in guiding treatment of a thyroid condition as there has been very little research into the use of rT3 in the treatment of thyroid disease.