CHANGES IN RENAL-FUNCTION IN CATS FOLLOWING TREATMENT OF HYPERTHYROIDISM USING I-131

Changes in renal function of twenty-two cats treated for hyperthyroidi sm using radioiodine were evaluated, Serum thyroxine (T-4), serum crea tinine, blood urea nitrogen (BUN) and urine specific gravity were meas ured before treatment and 6 and 30 days after treatment, Twenty-two ca ts had pretreatment and 21 cats had 6 day posttreatment measurement of glomerular filtration rate (GFR) using nuclear medicine imaging techn iques, There were significant declines in serum T-4 at 6 days followin g treatment, but the changes in GFR, serum creatinine and BUN were not significant, At 30 days following treatment, there were significant i ncreases in BUN and serum creatinine and further significant declines in serum T-4. Nine cats were in renal failure prior to treatment and 1 3 cats were in renal failure 30 days following treatment, Renal failur e was defined as BUN greater than 30 mg/dl and/or serum creatinine gre ater than 1.8 mg/dl with concurrent urine specific gravity less than 1 .035, These 13 cats included eight of 9 cats in renal failure prior to treatment and 5 cats not previously in renal failure, Follow up infor mation beyond 30 days following treatment on 9 of these 13 cats indica ted that all remained in renal failure, Based on receiver operating cu rve analysis of pretreatment glomerular filtration rate (GFR) in predi cting posttreatment renal failure, a value of 2.25 ml/kg/min as a poin t of maximum sensitivity (100%) and specificity (78%) was derived, Fif teen of 22 cats had pretreatment GFR measurements of less than 2.25 ml /kg/min. These 15 cats included all 9 cats in renal failure and 5 cats with normal renal clinicopathologic values prior to treatment, At 30 days following treatment, 13 of these 15 cats were in renal failure, T he 2 cats not in renal failure had persistently increased serum T-4 va lues, Seven of 22 cats had pretreatment GFR measurements greater than 2.25 ml/kg/min, None of these 7 cats was in renal failure at 30 days f ollowing treatment, all cats having normal BUN, serum creatinine, and urine specific gravity values, It was concluded that significant decli nes in renal function occur after treatment of hyperthyroidism and thi s decline is clinically important in cats with renal disease, Pretreat ment measurement of GFR is valuable in detecting subclinical renal dis ease and in predicting which cats may have clinically important declin es in renal function following treatment.