Radioimmunoassay for triiodothyronine: use of polyethylene glycol to precipitate immune complexes

1. A rapid radioimmunoassay (RIA) for determining serum triiodothyronine (T3) concentration has been developed. The reaction of T3 with its antibody was performed at 37 degrees C for 3 hours; precipitation of antibody bound T3 by 15% polyethylene glycol (PEG) was done at ice-bath temperatures. 2. The effects of pH on the interaction of T3 with its antibody and on the precipitation of antibody gamman globulin by PEG are discussed. 3. PEG precipitates and supernatants were characterized by immunoelectrophoresis and polyacrylamide gel electrophoresis. 4. The RIA has a between run precision of 9.3% at a mean T3 concentration of 179 ng/dl. The recovery of 50 or 100 ng/dl T3 was 95-102% and the normal range estimate was 66-215 ng/dl (N equal to 49).