Serum triiodothyronine concentration in the iodine‐deficient rat

Abstract A low iodine diet fed to rats resulted in decreased absolute amounts of circulating triiodothyronine and protein‐bound iodine (PBI), but an increase in the triiodothyronine: protein‐bound iodine ratio. Since baseline protein‐bound iodine values were high, it is possible the “normal” diet fed was in fact a high iodine one. In this event, the fall in protein‐bound iodine with the low iodine diet would be greater than that in thyroxine concentration. The relative increase in triiodothyronine compared to PBI concentration with the low iodine diet could result from preforential synthesis and secretion of triiodothyronine by the iodine‐deficient thyroid; or could be due to accelerated removal of thyroxine due to increased peripheral utilization. Either change could keep body metabolism normal or nearly so over an extended period of iodine want.