Thyroid hormones in tissues from fetal and adult rats

Concentrations of T4 and T3 were recently measured in rat fetal tissues, and the reported values were found to be more than 10-fold higher than those found by us. The differences have been explained by the assumption that previous analytical procedures, neither avoid deiodination during autopsy of the animals or during extraction and purification, because phloretin [(3'),4',4,6-(tetra)trihydroxyaurone], a potent inhibitor of 5'-iodothyronine deiodinase activity in vitro, had not been used to prevent such problems. We here show that perfusion with phloretin during autopsy does not affect 5'-iodothyronine activity or T4 and T3 concentrations in liver, kidney, or brain. Evidence is also provided that the addition of phloretin during the homogenization process is superfluous, as the use of 80% ethanol and 0.02 M NaOH for this step results in undetectable deiodinase activity. Data are presented showing that during the final sample drying, no losses or degradation of T4 and T3 occur, confirming the adequacy of the individual recovery corrections using radiolabeled iodothyronines as internal tracers. We also present quantitative information on the intralaboratory variability of the T4 and T3 concentrations found in tissues from normal fetuses and their mothers as well as in adult males and nonpregnant females. Results are comparable to those obtained by others using entirely different analytical procedures.