Ultrasensitive radioimmunoassay for direct determination of free triiodothyronine concentration in serum.

Free triiodothyronine (T3) in serum has been measured directly in dialysates of serum, using a wick chromatographic radioimmunoassay. Adequate sensitivity was attained by the use of [1251]T3 with very high specific activity (2,000 to 3,000 muCi/mug). Sera were dialysed against a Krebs-Ringer bicarbonate buffer modified so as to be similar to plasma water. Dialysis took place under carefully controlled circumstances. The influence on the equilibrium of total to free T3 of temperature, serum dilution, and dialysis time was studied. By the present method, free T3 in serum from groups of subjects including 20 men, 10 women taking oral contraceptives, and 20 women with normal menstrual cycles were identical, averaging 5.2 pg/ml. A chromatographic radioimmunoassay of total T3 using high specific activity [1251]T3 and very small test samples is also described.