Managing the hyperthyroid cat with renal disease (Proceedings)
Hypertension in feline hyperthyroidism
Recognizing hypertension in cats with hyperthyroidism is not always simple. In normal cats, measurement of blood pressure is fairly reliable, whether using oscillometry or Doppler ultrasonography.1 Both correlate well with intra-arterial measurements.2 The "white coat effect", however, is not always recognized in feline medicine, and this effect may well be more pronounced in cats with hyperthyroidism than in less stressed normal cats. Initially, the prevalence of hypertension in cats with hyperthyroidism was considered very high. An early publication showed a prevalence of 87%.3 In that study the definition of hypertension may have been unrealistically low, but the study was well-controlled, with hyperthyroid cats being compared to groups of both normal cats and cats with chronic renal failure...
Later reports showed lower prevalence rates (between 5% and 20%) of hypertension in cats with hyperthyroidism.4,5 Stepien et al. showed a significant "white coat effect" in cats with hyperthyroidism, with no decrease in blood pressure after treatment. Syme and Elliot, showed a significant increase in the occurrence of hypertension in cats after treatment for hyperthyroidism. Hypertension occurs rarely in human hyperthyroidism, and on rare occasions when hypertension is associated with thyrotoxicosis, the hypertension is usually systolic only. Thyroid hormone causes a pronounced decrease in peripheral vascular resistance. Hemodynamic effects of thyrotoxicosis include increased heart rate and increased stroke volume. It has been proposed that increased heart rate causes a summation of pressure in peripheral arteries with the pressure from systole, resulting in overall systolic hypertension.6 Whether or not this also occurs in cats with hyperthyroidism is unknown, but these cats have also been reported to have diastolic hypertension. This may be related to underlying kidney disease. Furthermore, beta adrenergic blockers are effective in treatment of systolic hypertension when it occurs in hyperthyroid human patients, but the effect of atenolol on hypertension is cats with hyperthyroidism is inconsistent.7 It is difficult to tell if hyperthyroidism really does cause hypertension in cats. There is an association between the two, but a cause and effect has not been established. If hyperthyroidism is a significant cause of hypertension, it may not be as common a cause as some clinicians might believe. In one study, only 5 of 30 cats with hypertension were diagnosed with hyperthyroidism.8 In a study of cats with hypertensive retinopathy, only 5 of 69 cats were hyperthyroid.9Regardless of the debate over hypertension in cats with hyperthyroidism, treatment of hypertension is recommended because of the risk of organ damage.2 Drugs used to treat hypertension in cats fall include angiotensin-converting enzyme (ACE) inhibitors, calcium-channel antagonists, and beta adrenergic antagonists. Treatment of hypertension in cats is reviewed elsewhere.2