Yup, either CRF (chronic renal failure) or hyperthyroidism are the usual suspects in older cats. Or, it could just be the way the kitty is aging.

For the hyperthyroid, there is a blood test most vets don't make. They routinely test TSH levels; but that can be normal, where a T4 / T3 test will show an issue.

(Sorry I forget if it is the T3 or the T4, but the vet will know which one.)