A new radioimmunoassay procedurefor triiodothyronine (T3) is described. The methoduses unextracted serum at a 1:4 dilution. The novel feature of the process is that preincubation and incubation with isotope take place at 60 C. At this temperature the binding capacity of thyroxine-binding globulin (TBG) is virtually eliminated and the antigen-antibody reaction is greatly accelerated.Separation of antibody-bound hormone from freehormone is effected by polyethylene glycol solution. By plotting the standard curve on Probability × logarithmic paper, the curve is linear. Logit × log plots give the same results and facilitate calculation of data by a simple computer program. Sera of 31 normal individuals showed a meanvalue (±sd) of 189 ± 30 ng per 100 ml. Fourteen clinically thyrotoxic individuals had T3 values of 838 ± 398 ng per 100 ml and 19 clinically myxedematous patients had T3 values of 47 ± 39 ng per100 ml. These values were confirmed by using several different antisera and by performing the radioimmunoassay on previously extracted samples.
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Sterling et al. (1974) studied this question.