Disparate Effects of Heparin on Free Thyroxine as Measured by Two Radioimmunoassays: Concise Communication

Free thyroxine (FT4) has been measured in two groups of patients, before and after heparin, using equilibrium dialysis (ED) and two radioimmunoassays, RIA-I and RIA-II. In Group A, nine patients were tested before and after 12-24 hr of intravenous heparin, FT4 (ED) rose from 1.9 +/- 0.5 to 2.8 +/- 3.0 ng/dl, and RIA-I from 1.37 +/- 0.37 to 1.89 +/- 1.21 ng/dl, whereas RIA-II results fell from 0.97 +/- 0.38 to 0.66 +/- 0.32 ng/dl. In Group B, ten patients were tested before and after 15 min of intravenous heparin. FT4 (ED) rose from 1.7 +/- 0.7 to 3.2 +/- 1.6 ng/dl (p less than 0.02), and RIA-I rose from 1.3 +/- 0.46 to 2.02 +/- 0.01 ng/dl (P less than 0.05), whereas RIA-II results fell from 1.07 +/- 0.38 to 0.63 +/- 0.31 ng/dl (P less than 0.02). The correlation coefficient between FT4 (ED) and RIA-I in 38 paired results was 0.96, but there was no correlation between FT4 (ED) and RIA-II results.