[Clinical studies on regulatory system of thyroid hormone secretion and serum triiodothyronine. Part II. Development of rapid radioimmunoassay for triiodothyronine (author's transl)].

A rapid and simple radioimmunoassay method for serum T3 was developed using home made potent rabbit anti-T3 serum: 1) The incubation mixture which contained 0.1 ml of 1:2,000 diluted anti-T3 serum (finally 1:20,000), 250 mug of 8-anilino-1-naphthalene sulphonic acid and 0.1 ml of serum was run at 37 degrees C for 1 hour. Dextran-coated charcoal was added for separation of free and bound T3. No significant undersirable effects due to high incubation temperature and short incubation time was observed. The lowest detectable amount of T3 was 20 pg per assay though this antiserum showed a 0.24% cross reaction with T4 the purity of which was 99%. The dilution curve of serum from Graves' patients was parallel to the standard. The recovery of added T4 over the range 39-625 pg was 98.1%. Inter and intra-assay variations of the estimated T3 values at the euthyroid level were 14.3% and 16.9%, respectively. 2) Serum T3 concentrations in 21 normal subjects, 23 patients with Graves' disease, 9 primary hypothyroidism, 19 Hashimoto's thyroiditis were estimated as 131 +/- 29 ng/100 ml (mean +/- s.d.), 546 +/- 274 ng/100 ml, 77 +/- 27ng/100 ml and 154 +/- 30 ng/100 ml respectively. Calculated T3/T4% in 10 normal subjects was 1.48 +/- 0.44, in 21 Graves' patients 2.84 +/- 1.59, in 9 primary hypothyroidism 3.88 +/- 1.36 and 19 Hashimoto's thyroiditis 2.41 +/- 0.86. 3) T3 levels in 131I treated Graves' patients corresponded well with their thyroid functional status. On the other hand their T3/T4% was distributed over a wide range and had no significant relationship with the clinical status. A hyperbolic inverse correlation between serum T3 and TSH levels (r= --0.21) was found in 88 cases with various thyroid disease, 25 cases of which showed high TSH with normal T3 levels. A significant increase in serum T3 at 90 and 120 minutes after intravenous administration of 500 mug TRH was observed, and the average of the absolute maximum increment was 42 +/- 24 ng/100 ml in 9 normal subjects. It is considered feasible to evaluate both thyroid and pituitary reserve quantitatively by simultaneous estimation of serum T3 and TSH levels after TRH administration.