Why is renal failure potentially worsened by treating hyperthyroidism?
Blood flow to the kidneys is increased in hyperthyroid patients and this promotes the function of the kidneys. Because renal insufficiency is seen in older felines the increase in blood flow secondary to the hyperthyroidism essentially improves the function at least for the short term. When these patients are treated for their hyperthyroidism and the blood flow to the kidneys decreases or normalizes, the function of the kidney worsens. This if often reflected in an increase in both blood urea nitrogen (BUN) and creatinine and a decrease in urine specific gravity. Generally this decrease in function is not clinically a problem, however, a small percentage of cats become clinical for their renal disease.
