Hyperthyroidism is one of the most common endocrine disorder in middle aged to older cats. Renal insufficiency may be masked by the presence of hyperthyroidism and subsequent 1-131 therapy of hyperthyroid cats with abnormal renal function may precipitate renal failure. The purpose of this study was to investigate the potential association between renal protein excretion and glomerular filtration rate (GFR), as determined by nuclear scintigraphy, in detecting renal dysfunction in naturally occurring feline hyperthyroidism.
Seventeen hyperthyroid cat were prospectively evaluated prior to 1-131 therapy over an 18 month period. The diagnostic evaluation for all cats consisted of: physical examination, complete blood count, serum chemistry,urinalysis, total T4, thoracic radiographs, indirect blood pressure via Doppler manometry, single strand electrocardiography, thyroid scintigraphy with sodium pertechnetate (99mTc0 4-), and determination of GFR via 99mTc Diethyltriaminepentacetic acid nuclear scintigraphy (99mTcDTPA). Nuclear studies were performed with the cats under sedation with midazolam/ketamine or butorphanol based on the preference of the radiologist.Additional testing (echocardiography) was performed as judged necessary by the attending clinician . Urine protein excretion was determined by one of two method. A urine protein:creatinine (U P:C) ratio was measured if overt proteinuria was detected on routine urinalysis. Alternatively, urine was tested for the presence of microalbumin (E .R.D. – Health Screen, Heska) routine urinalysis failed to detect overt proteinuria . The GFR was considered the gold standard for determining renal function in each cat.
Seven cats had overt proteinuria while five cats were positive for microalbuminuria. The remaining five cats had no detectable urine protein. Mean U P:C was 0.47 (range of 0.2-0.6). Microalbuminuria was a quantitative measurement and results ranged from negative to positive. The mean GFR for all cat was 1.81 mL/kg/minute (range of 0.25- 3.39 mLI kg/minute).
Statistical analysis of the data indicated an overall negative association of positive U P:C and presence of microalbuminuria with decreasing GFR. However, neither association reached statistical significance. Correlation analysis performed on U P:C data determined a p value = 0.68 with a correlation of – 0.22. A similar result was seen with microalbuminuria. Kendall’s Tau (non-parametric) analysis resulted in a p value = 0.14 with a correlation of – 0.39.
Small sample sizes limited the power of this pilot data to detect statistically significant differences with urine protein determination and GFR. This study is presently ongoing and will continue to investigate the potential association between urine protein excretion and abnormal renal function in hyperthyroid cats.