Reverse triiodothyronine (rT₃) is an isomer of triiodothyronine (T₃) with no demonstrated biological activity.^1,2 The majority of rT₃is produced through peripheral enzymatic monodeiodination of T₄ at the 5 position of the inner ring of the iodothyronine nucleus of thyroxine (T₄). A lesser amount of rT₃ is secreted directly by the thyroid gland. Reverse T₃ is biologically inactive and does not stimulate thyroid hormone receptors.