Reverse triiodothyronine

Reverse triiodothyronine (3,3’,5’-triiodothyronine, reverse T₃, or rT₃) is an isomer of triiodothyronine (3,5,3’ triiodothyronine, T₃).

Reverse T₃ is the third-most common iodothyronine the thyroid gland releases into the bloodstream, of which 0.9% is rT₃; tetraiodothyronine (levothyroxine, T₄) constitutes 90% and T₃ is 9%. However, 95% of rT₃ in human blood is made elsewhere in the body, as enzymes remove a particular iodine atom from T₄.