Hyperthyroidism or an over active thyroid is a common condition of older cats. A small and normally benign growth on the thyroid gland causes it to release too much hormone. This results in a metabolism stuck in 5th gear when it should be ticking along in 2nd gear! Your cat may have an elevated heart rate and blood pressure, may be starving hungry but lose weight despite eating everything in sight, may have increased thirst, be passing increased amount of urine or have diarrhoea or may vomit frequently. These cats tend to be vocal and yowl-y and will often be quite active for an older cat. Not all cats will have all of these signs.