Morrison J, Jergens A, Deitz K, et al.
Hyperthyroidism is one of the most common endocrine disorders in middle aged to older cats. Renal disease may be masked by hyperthyroidism and subsequent I-131 therapy of hyperthyroid cats with subclinical renal dysfunction may precipitate progression to renal failure. The purpose of this study was to prospectively investigate potential prognostic factors for predicting the development of renal disease in spontaneous feline hyperthyroidism. Fifty-two hyperthyroid cats were evaluated prior to I-131 therapy over a 48 month period. The diagnostic evaluation for all cats included: physical examination, CBC, serum biochemistry proﬁle, urinalysis, total T4, thoracic radiographs, indirect blood pressure via Doppler manometry, single strand electrocardiography, thyroid scintigraphy with sodium pertechnetate (99mTcO4-), and determination of GFR via 99mTc-Diethyltriaminepentacetic acid nuclear scintigraphy (99mTc-DTPA). Nuclear studies were performed in cats sedated with either midazolam/ketamine or butorphanol. Urine protein excretion was determined by either a urine protein:creatinine (U P:C) ratio if pathologic proteinuria was suspected on routine urinalysis, or urine was tested for the presence of microalbumin (E.R.D. – Health Screen, Heska) if proteinuria was not initially detected. Speciﬁc physical examination parameters recorded in all cats included age, weight, breed, gender, and heart rate. Repeat diagnostic testing occurred no sooner than 4 weeks post I-131 therapy whereby all cats minimally had a serum creatinine measurement determined. A diagnosis of renal disease was made on the basis of increased serum creatinine levels (> 1.6 mg/dl) coupled with urine speciﬁc gravity when available. Forty-six cats completed I-131 therapy with 13/46 (28%) determined to have renal disease at the time of follow up. Pre-treatment serum creatinine levels were signiﬁcantly (p < 0.02) associated with the development of renal disease as determined by logistic regression analysis. Mean pre-treatment creatinine concentration for all cats was 0.96 mg/dl (range: 0.2–2.2 mg/dl) and mean post-treatment creatinine was 1.65 mg/dl (range: 0.9–4.1 mg/dl). Pre-treatment GFR was less signiﬁcantly (p o 0.08) associated with the development of renal disease. Mean GFR for all cats before treatment was 2.65 ml/min/kg (range: 1.03–7.41 ml/min/kg). The predicted chance for renal disease in a cat with a pre-treatment creatinine of 1.6 mg/dl and GFR of 2.25 ml/min/kg was 67%. No other patient factors were signiﬁcantly associated with the development of renal disease post I-131 therapy. The data of the present study conﬁrms a 30% incidence of renal disease following radioactive iodine therapy of feline hyperthyroidism. Pre-treatment serum creatinine concentration was the variable most highly correlated with a negative outcome of renal disease.