Hyperthyroidism and chronic kidney failure (CKD) are common diseases in older cats. It can be difficult to evaluate kidney function in cats suffering from both conditions simultaneously. In the daily clinic serum creatinine is the analysis of choice when evaluating kidney function and secondary glomerular filtration rate (GFR). This marker does not, however, detect kidney damage until about 75 % of the function is lost. There are several other techniques available when investigating GFR but none are readily available for the small clinic.