Thyroid Function in the Absence of Vasopressin1

Abstract

Thyroid function was assessed in rats with hereditary hypothalamic diabetes insipidus (DI). Vasopressin almost certainly is absent from these rats since thev appear unable to synthesize the hormone. The urinary excretion of 131I following a single injection of 131I-labeled thyroxine (T4) was significantly greater in untreated DI rats than in normal animals of the same strain. However, this increase was not observed when urine flow was reduced in both groups either by administration of vasopressin or by deprivation of water. Furthermore, urinary excretion of 131I was also greater than normal in untreated DI rats after an injection of 1311-labeled sodium iodide and, as with T4, the increase was eliminated when urine flow was reduced. Differences in urinary excretion of 131I were not observed between DI and normal rats when they were brought to iso-topic equilibrium with 131I-labeled T4, and the deiodination of T4 in vitro in liver and kidney from DI rats was no greater than that from normal animals. It was thus concluded that the difference between normal and DI rats in the urinary excretion of 131I following a single injection of 131I-labeled T4 was due primarily to a difference in the renal handling of iodide, not to a difference in the rate of deiodination. Thyroid function also appeared normal when assessed by other tests: BMR [basal metabolic rate], renal oxygen consumption, PBI [protein-bound iodine], 24-hr. thyroidal 131I uptake, and thyroid secretion rate. These studies strongly suggest that, in rats, vasopressin plays no pay-siological role in regulating activity of the pituitarythyroid axis.