Methods for (1) evaluating feline kidney function by determining ghrelin level in feline tissue or biofluid and correlating the ghrelin level directly to kidney function and (2) diagnosing kidney disease in a feline comprises determining an observed ghrelin level in a tissue or biofluid of the feline and comparing the observed ghrelin level to a reference ghrelin level indicative of normal kidney function, wherein an observed level lower than the reference level is indicative of kidney disease or susceptibility thereto.