A simplified radioimmunoassay for triiodothyronine (T3) using pre-incubated labelled antigen and antibody

We discribe the development of a simplified radioimmunoassay for triiodothyronine (T₃) using pre-incubated labelled T₃ and antibody. The assay is carried out by adding 50 l of standard or sample to 0.4 ml of pre-incubated reagent dispensed in assay tubes. The reaction is allowed to proceed for about four hours and the antigen-antibody complex precipitated by the addition of 1 cm³ of 22% polyethylene glycol solution. Due to the high dissociation constant of T₃-antibody complex at 37° C (2.83·10^–4 S^–1), the labelled antigen-antibody complex dissociates and thereby the unlabelled antigen binds with the antibody. With a four hour incubation the sensitivity of this assay is comparable to an assay done by the equilibrium method using the same antibody. Sixty serum samples were analyzed using this method and compared with the equilibrium assay (Y=0.94x+0.046 ng/cm³, r=0.98).