Changes in renal function associated with treatment of hyperthyroidism in cats

We measured glomerular filtration rate (GFR) estimated by plasma disappearance of 99mTc-labeled diethylenetriaminepentaacetic acid, serum concentrations of thyroxine (T4), creatinine, and urea nitrogen, and urine specific gravity in 13 cats with naturally acquired hyperthyroidism before and 30 days after treatment by bilateral thyroidectomy, and in a group of 11 control cats. Mean (+/- SD) serum T4 concentration decreased from a pretreatment value of 120.46 (+/- 39.21) nmol/L to a posttreatment value of 12.15 (+/- 6.26) nmol/L (P < 0.0001; reference range, 10 to 48 nmol/L). Treatment of hyperthyroidism resulted in a decrease in mean (+/- SD) glomerular filtration rate, from 2.51 (+/- 0.69) ml/kg of body weight/min to a posttreatment value of 1.40 (+/- 0.41) ml/kg/min (P < 0.0001). Mean serum creatinine concentration increased from 1.26 (+/- 0.34) mg/dl to 2.05 (+/- 0.60) mg/dl (P < 0.01). Mean serum urea nitrogen concentration increased from 26.62 (+/- 6.83) mg/dl to a mean postthyroidectomy concentration of 34.92 (+/- 8.95) mg/dl (P < 0.01). All changes were significant. Two cats developed overt renal azotemia after treatment of hyperthyroidism. Our results provide further evidence that treatment of hyperthyroidism can result in impaired renal function. In addition, our results suggest that, in some instances, thyrotoxicosis might mask underlying chronic renal insufficiency.