Measurement of serum 3,3',5'-(reverse) T3, with comments on its derivation.

A radioimmunoassay for the measurement of l-3,3',5'-triiodothyronine (reverse TO, rT3) has been developed for use with unextracted serum. The highly specific antiserum showed no cross-reactivity with l-3,3'5-triiodothyronine (T3) or tetradiodothyroacetic acid (T4A) and cross-reaction with L-thyroxine (T4) was low enough to be discounted for routine assay purposes. If a normal amount of rT3 was added to serum T4 cross-reactivity decreased considerably. Serial dilutions of hyperthyroid sera gave dose-response curves which were parallel to the rT3 standard curve. Serum concentrations of rT3 (mean+/- SEM) were 0-68 +/- 0-02 nmo1/1 in sixty-seven normal subjects, 0-19 +/- 0-02 nmo1/1 in twelve hypothyroid patients and 1-18 +/- 0-12 nmo1/1 in seventeen hyperthyroid patients. In sixteen patients with TO-toxicosis rTO was 0-42 +/- 0-04 nmo1/1 and eighteen patients with high circulating TBG had a mean rT3 of 0-54 +/- 0-03 nmo1/1.