Thyroid-hormone metabolism in the kidney.

Present knowledge of the metabolism of the thyroid hormones in the kidney is confined mainly to information obtained from experiments with 231I-labelled thyroxine and tri-iodothyronine. Gross & Leblond (1951 a) found that after the administration of ['31I]thyroxine to rats or mice, the kidney concentrated radioactivity from the blood. Subsequent chromatographic analysis of butanol extracts of kidney revealed the presence of at least four 'III-labelled compounds: thyroxine, tri-iodothyronine, iodide and an unknown compound which was later shown to give rise to thyroxine on alkaline hydrolysis (Leblond & Cambron, 1952). The compound was also detected in bile and was thought to be a conjugate of thyroxine with glucuronic acid. Gross, Ford, Symchowiez & Horton (1957) found that [UlI]tri-iodothyronine when aministered to rabbits gave rise to a metaboite in the kidney, plasma and liver which yielded triiodothyronine on hydrolysis with 0-2N-hydrochloric acid. The liver also contained a second conjugated compound which they identified as tri-iodothyronine glucuronide. Roche, Michel, Jouan & Wolf (1956) have shown that the kidney can deiodinate and deaminate [13I]tri-iodothyronine, forming 3:3'-di-iodothyronine and triiodothyroacetic acid [4-(4-hydroxy-3-iodophenoxy)-3:5-di-iodophenylacetic acid]. Results obtained in experiments with synthetic thyroid hormones do not necessarily reflect the normal metabolism of endogenous thyroid hormone. A satisfactory qualitative and quantitative study ofthyroid-hormone metabolism in the kidney has not yet been carried out. For this it is essential to label the endogenous thyroid hormones with 3l3I; further, an analysis of the iodinated compounds present in the kidney must be carried out only after the administration of tracer doses of 1311, in order to avoid radiation damage to the thyroid gland sufficient to cause the release of thyroglobulin into the circulation. The kidney is primarily an excretory organ and would be expected to partake in the removal of any abnormal iodinated compounds present in the circulation. Fletcher (1957) detected a number of compounds in the urine but not in the serum of patients treated