Measuring feline blood pressure is becoming routine. The highest pressure, called systole, occurs when the heart contracts. The lowest pressure, called diastole, occurs when the heart relaxes. Hypertension (high blood pressure) can affect older cats with kidney disease or hyperthyroidism (an overactive thyroid gland). If the heart sustains long periods of high pressure, it can weaken. Hyperthyroidism can lead to overstimulation of the heart muscle, which might wear out prematurely and eventually fail to control blood pressure in cats. The underlying disease must first be controlled. If no underlying disease is found, drugs may be used.