Separation of iodothyronines from iodinated metabolites in serum from reverse T sub(3) tracer kinetic studies in man and dog: A comparison of two methods

Iodothyronine separation from free iodide and iodoalbumin in serial serum samples obtained from 7 human and 5 dog studies following intravenous injection of radiolabeled reverse triiodothyronine (reverse T sub(3)) was compared using acidified ammonium acetate/tetrahydrofuran (THF) elution from C-18 SEP-PAK cartridges or ethyl acetate/butanol (EAB) extraction. Both methods excluded > 98% free iodide and > 99% iodoalbumin from the iodothyronine fraction. Recovery of labeled reverse T sub(3) was higher for the THF/SEP-PAK than for the EAB method, and intra-assay coefficients of variation were lower; HPLC analysis of iodothyronine fractions revealed a single peak co-migrating with injected tracer. Serum disappearance curves derived from these two methods were highly correlated in all cases, as were fits of data to sums of exponentials and calculated serum kinetic parameters.