ESTIMATION OF FREE THYROXINE IN SERUM - INFLUENCE OF PREDIALYSIS AND RADIOACTIVE CONTAMINANTS

It has been investigated whether radioactive contaminants in commercial [125I]thyroxine charges altered the results of free thyroxine estimations in human serum by means of equilibrium dialysis and subsequent MgCl2 precipitation. By means of paper chromatography, between 4.6 and 11.8% di-iodo- and triiodothyronine and 3-19% (R)-tri-iodothyronine could be demonstrated as contaminants. Dialysis for purification decreased the contamination, at the same time increasing the thyroxine content (85-95%). The iodothyronines investigated here were precipitable with MgCl2 showing a ratio of 1:2:4 (T2:T3:T4), whereas iodotyrosines and iodide did not precipitate. The amount of contaminating iodine compounds increased with the length of time after the synthesis of [125I]thyroxine. By purification dialysis the mean value of free thyroxine was significantly decreased by 0.004%; therefore, a routine predialysis is recommended when free thyroxine is estimated by equilibrium dialysis and MgCl2 precipitation. In euthyroidism (n:49) free thyroxine (AFT4; ng%) was 2.16 ± 0.48, in hyperthyroidism (n:35) 12.30 ± 6.64 and in hypothyroidism (n:7 0.38 ± 0.26. © 1969.