Effect of salt feeding on thyroid metabolism of I131 in the dog

Six adult male dogs were fed on 5% NaCl added to the same diet on which a group of six dogs were maintained as a control group. At 12 weeks 24-hour I131 uptake, PBI131, serum and urine sodium and potassium were measured. Two weeks later, 5-hour Na24 and I131thyroxine dilution curves were determined from serial blood samples in the anesthetized animals. The salt fed dogs had elevated 24-hour I131 uptake and PBI131, decreased serum sodium and increased serum potassium, and both total urine sodium and potassium were increased with salt feeding at 12 weeks. In the salt-fed dog, both Na24 and I131-thyroxine dilution volumes were increased at 5 hours over the values for the control group, indicating expansion of extravascular volume. It was suggested that alterations of I131 metabolism were influenced by extrarenal factors of expanded fluid volume and tissue concentration of electrolytes in addition to the dominant factor of renal electrolyte regulation with salt loading.