Renal structural and functional changes and sodium balance in hypothyroid rats

Significant retardation in body and kidney growth and in renal glomerular and proximal tubular development was observed in hypothyroid rats in comparison with euthyroid controls. Body mass was found to increase by 40% but kidney mass only by 3.5% over a period of 3 weeks following thyroidectomy in contrast to an average increment of 178 and 113%, respectively, in normal animals. Dissociation was found also in the development of glomeruli and proximal tubules: glomerular size correlated with body weight while tubular size with kidney weight. Single nephron glomerular filtration rate was adapted not to the glomerular as normally but rather to the tubular volume thereby compensating functionally for the dimensional glomerulotubular imbalance. However, despite the decreased filtered and excreted amount of sodium relative to the body weight sodium balance was maintained by depressed tubular reabsorption which was uniformly observed in thyroid deficiency. Because of the uncertainty regarding the direct action of thyroid hormones on the tubular function it is assumed that decreased tubular sodium reabsorption is principally caused by the mechanism subserving the maintenance of sodium balance also in salt loaded normal and sodium sensitive Dahl's rats. This seems to be supported by the exaggerated natriuresis observed both in hypothyroid and sodium sensitive Dahl's rats after saline infusion. Justification of the proposed mechanism needs further investigation.