Van Hoek I., Lefebvre H.P., Kooistra H.S., et al.
BACKGROUND: Glomerular filtration rate (GFR) can be measured by clearance methods of different markers showing discrepancies and different reproducibility in healthy cats. Studies comparing different methods of GFR measurement in hyperthyroid cats have not yet been performed. HYPOTHESIS: Plasma clearance of exogenous creatinine (PECCT), exo-iohexol (PexICT), and endo-iohexol (PenICT) could lead to differences in GFR measurement and the need to use the same clearance method when comparing GFR before and after radioiodine treatment in hyperthyroid cats. ANIMALS: Fifteen client-owned hyperthyroid cats. METHODS: GFR was measured 1 day before and 1, 4, 12, and 24 weeks after treatment. Intravenous injection of iohexol was followed immediately by IV injection of creatinine. Plasma creatinine was measured by an enzymatic method. Plasma endo- and exo-iohexol were measured by high-performance liquid chromatography coupled to ultraviolet detection. RESULTS: Globally, the 3 GFR methods resulted in significantly different (P < .001) GFR results. GFR results among the different methods were the same (P= .999) at all time points. All 3 techniques indicated decreasing GFR after (131)I treatment. For each GFR technique, a significant decrease in GFR was observed between time point 0 and all other time points. This decrease stabilized 4 weeks after treatment, with very little decline afterward. CONCLUSION AND CLINICAL IMPORTANCE: It is mandatory to use the same GFR technique in follow-up studies. GFR testing at 4 weeks posttreatment could allow assessment of the final renal functional loss after treatment in hyperthyroid cats.