T3 (Triiodothyronine), Reverse - RT3

Reverse T3 (rT3) is a biologically inactive form of thyroid hormone derived from the conversion of T4 (thyroxine); unlike active T3, rT3 does not stimulate metabolism.

This test measures serum rT3 to help evaluate situations where the body may be converting more T4 into rT3 (rather than active T3) — for example during chronic illness, severe stress, or non-thyroidal illness (“euthyroid sick syndrome”).