Triiodothyronine Immunoassay Using Polyethylene Glycol to Precipitate Antibody-Bound Hormone

A simple and reliable radioimmunoassay of triiodothyronine (T3) using polyethylene glycol 6000 (PEG) to separate free from bound ligands is described. With an increase in concentration of PEG added, the percentage of precipitated bound 125I-T3 rose gradually, reaching a plateau at a final concentration of 12.5%. The addition of human γ-globulin at a final concentration of 0.5mg/ml was required for the precipitation of bound ligands. Time lapse after the addition of PEG and γ-globulin did not affect the precipitation of labeled hormones. The minimal detectable quantity by this method is 12.5ng/dl. Determination of serum T3 by this method gave satisfactorily small interassay and intraassay variations. The PEG method is easy, inexpensive and reliable, especially when a large number of samples are assayed. By using this method, we studied T3 concentration of 12 patients with obesity, 15 patients with anorexia nervosa, and 4 treated patients with anorexia nervosa. The mean plasma T3 concentration in 12 normal control subjects was 129.0±9.1ng/dl, while the plasma T3 levels in patients with obesity, anorexia nervosa, and treated anorexia nervosa were 138.2±5.6, 68.5±0.3, and 81.3±16.2ng/dl, respectively. Plasma T3 in patients with anorexia nervosa was significantly lower than those in obese subjects.