Homogeneous enzyme immunoassay for triiodothyronine in serum.

Background: The concentration of triiodothyronine (T3) in human serum is extremely low and can be determined only by very sensitive methods. We developed a homogeneous enzyme immunoassay for T3 analysis in unextracted serum.

Methods: A T(3) derivative was conjugated to the -SH groups of glycogen phosphorylase b (GPb) from rabbit muscle. Conjugation caused inhibition of enzyme activity, and the enzyme conjugate was reactivated upon binding of anti-T3 antibody. Activation was blocked by the presence of non-antibody-bound T3; this was the basis for the development of the homogeneous enzyme immunoassay for T3 by determining GPb activity fluorometrically.

Results: We used furosemide to block the interaction of T3 with serum proteins with T3-binding sites, avoiding any serum treatment step. T3 was measured in the range 0.3-8 microg/L. T3 values obtained by this assay correlated well with those obtained by a RIA (y = 0.97x - 0.07 microg/L; r = 0.96; n = 92). Within- and between-run imprecision (CV) was 5-9% for normal and high concentrations and 16-20% for low concentrations.

Conclusions: Chemical modification of GPb with a T3 derivative allows the development of a simple homogeneous enzyme immunoassay for T3 in unextracted serum.