The renal clearances of iodide and L-diiodotyrosine measured simultaneously.

After the intravenous administration of Il'1labeled L-diiodotyrosine (DIT), an increasing fraction of the labeled iodine of the serum becomes inorganic iodide as the DIT undergoes deiodination. Most of the labeled iodine which appears in the urine is inorganic iodide (1). If one makes an estimate of the renal clearance of inorganic iodide, based on the concentrations of labeled inorganic iodide in serum and urine after the administration of DIT, then values considerably in excess of those reported for iodide (2-5) are obtained. This could occur if DIT undergoes further deiodination after it enters the kidney. The studies reported here are concerned with simultaneous measurements of the renal clearance of iodide and DIT, using a double-labeling technique with I133-labeled iodide and I131-labeled DIT. It is shown that the apparent renal clearance of I'31-labeled DIT is considerably less than the true clearance and, thus, that intrarenal deiodination of DIT does indeed occur.