A radioimmunoassay of serum T3 was developed displacing T3 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 T3 was 1.33 ± sd 0.29 ng/ml. None of 94 hyperthyroid patients had serum T3 values within normal range, whereas 12% of 34 hypothyroid patients had normal serum T3 values.
The T4:T3 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 T3 was found to be superior to T4 in the diagnosis of hyperthyroidism, whereas serum T4 was found to be better with regard to hypothyroidism. However, correction of the influence of different TBG levels by calculating free T4 index and free T3 index showed that both parameters almost completely separated normals from patients with untreated hyperthyroidism, respectively hypothyroidism.