MEASUREMENTS OF SERUM TRIIODOTHYRONINE BY RADIOIMMUNOASSAY

Abstract

ABSTRACT A radioimmunoassay of serum T 3 was developed displacing T 3 from the thyroxine binding proteins by merthiolate. Separation of the free and the antibody bound fraction was accomplished by the use of dextran-coated charcoal. The addition of 14 mg BSA was shown to diminish the adsorption of the antibody bound fraction to charcoal. In 90 controls serum T 3 was 1.33 ± sd 0.29 ng/ml. None of 94 hyperthyroid patients had serum T 3 values within normal range, whereas 12% of 34 hypothyroid patients had normal serum T 3 values. The T 4 :T 3 ratio in serum was found to be 67.0 ± sd 11.9 in normals, while the ratio was significantly ( P < 0.001) decreased as well in hyperthyroid as in hypothyroid patients. The discriminant value of serum T 3 was found to be superior to T 4 in the diagnosis of hyperthyroidism, whereas serum T 4 was found to be better with regard to hypothyroidism. However, correction of the influence of different TBG levels by calculating free T 4 index and free T 3 index showed that both parameters almost completely separated normals from patients with untreated hyperthyroidism, respectively hypothyroidism.