Isolation of radioactive iodothyronines for kinetic studies: a comparison of two methods

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Abstract
A method based on the principle of gel separation followed by antibody extraction (GSAE) was developed for isolation of radioactive thyroxine (T4), 3,5, 3''-triiodothyronine (T3), 3,3'',5''-triiodothyronine (rT3), 3,3''-diiodothyronine (3'',3''-T2) 3'',5''-diiodothyronine (3'',5''-T2) and 3''-monoiodothyronine (3''-T1) in [human] serum. This method was used for the estimation of the metabolic clearance rate (MCR) of the iodothyronines using the single injection, non-compartmental approach, and was compared to the conventional trichloroacetic acid precipitation/ethanol extraction (TCA-E) technique. The GSAE method excluded the co-determination of radioactive I and iodoproteins, whereas the co-determination of radiolabeled daughter iodothyronines was negligible. The relative difference of duplicate estimations of MCR was approximately 10%. Using the TCA-E method for isolation of tracer, the MCR of T4, T3 and rT3 was underestimated to a minor degree (20%), whereas the MCR of 3,3''-T2, 3'',5''-T2 and 3''-T1 were 20-40% of those estimated by the GSAE method. The GSAE method was suitable for kinetic studies of iodothyronines, whereas the TCA-E method cannot be used for turnover studies of 3,3''-T2, 3'',5''-T2 or 3''-T1.

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