Epidemiology: Hyperthyroidism is frequently cited as an important cause of systemic hypertension in cats. However, in large retrospective studies of cats with hyperthyroidism, clinical signs associated with hypertension were not reported, suggesting that extreme elevation of blood pressure may be relatively infrequent [1]. Similarly, ocular examinations performed in a large series of hyperthyroid cats did not identify changes consistent with hypertensive/retinopathy [2].

We have recently completed an epidemiological study of 100, sequentially diagnosed, hyperthyroid cats. Of these cats, only 9 were hypertensive (5/9 had ocular lesions) at the time the hyperthyroidism was diagnosed. In addition, 3 cats were receiving amlodipine for previously diagnosed hypertension. These results indicate that hypertension is less common in cats with hyperthyroidism than has been previously supposed.

Interestingly, a proportion of cats actually seem to develop hypertension following treatment for hyperthyroidism. Initial indications are that this occurs in about 20-25% of cases. This finding needs to be substantiated by following the blood pressure of a larger number of cats during treatment for hyperthyroidism. It is unclear whether this change is associated with the decline in renal function that occurs as euthyroidism is achieved.

In humans with hyperthyroidism diastolic hypertension is uncommon, but elevation of systolic blood pressure does occur, and is more marked (compared with euthyroid controls) in younger age groups [3]. Following treatment for hyperthyroidism hypertension is more likely to resolve in younger patients and to persist in elderly patients who presumably have co-incident essential hypertension. Paradoxically, hypothyroidism is also associated with the development of hypertension in humans. This is primarily associated with an increase of total peripheral resistance in the hypothyroid state.

Mechanisms of hypertension in hyperthyroidism: The pathogenesis of systolic hypertension in cats with hyperthyroidism has not been studied directly. Experimentally, rats treated with thyroid hormones have increased cardiac output and decreased peripheral resistance, as well as a tendency to retain sodium due to activation of the renin-angiotensin-aldosterone system (RAAS) [4]. Thus the tendency for the blood pressure to increase due to an increase in cardiac output is, at least partially, offset by the decrease in total peripheral resistance; the balance of these processes determines whether hypertension develops.

Glomerular hypertension: The perfusion pressure of certain vascular beds is regulated independently of the systemic blood pressure, at least within certain limits. One area where autoregulation is especially well developed is in the kidney. However, autoregulation may fail, resulting in the development of glomerular hypertension, when there is a reduction in the number of functioning nephrons (resulting in hyperfiltration of the remaining nephrons), in the presence of severe systemic hypertension, or when differential regulation of afferent and efferent arteriolar tone is disrupted. Glomerular filtration rate is increased in hyperthyroid cats [5]. Whether this is associated with the development of glomerular hypertension has not been demonstrated directly but this has been postulated as a mechanism for renal injury in cats with hyperthyroidism. Indirect evidence for glomerular hypertension in hyperthyroid cats is provided by the observation that urinary protein excretion is increased in many cats at diagnosis, but this resolves rapidly with treatment [6].

References

1. Peterson ME, Kintzer PP, Cavanagh PG, Fox PR, Ferguson DC, Johnson GF, Becker DV. Feline hyperthyroidism: pretreatment clinical and laboratory evaluation of 131 cases. J Am Vet Med Assoc 1983;183:103-10.

2. van der Woerdt A, Peterson ME. Prevalence of ocular abnormalities in cats with hyperthyroidism. J Vet Intern Med 2000;14(2):202-3.

3. Saito I, Ito K, Saruta, T. The effect of age on blood pressure in hyperthyroidism. J Am Geriar Soc 1985;33:19-22.

4. Garcia del Rio C, Moreno MRR, Osuna A, Luna J, Garcia-Estan J, Vargas F. Role of the renin-angiotensin system in the development of thyroxine-induced hypertension. European Journal of endocrinology 1997;136:656-660.

5. Becker TJ, Graves TK, Kruger JM, Braselton WE, Nachreiner RF. Effects of methimazole on renal function in cats with hyperthyroidism. J Am Anim Hosp Assoc 2000;36(3):215-23.

6. Syme HM, Elliott J. Prevalence and significance of proteinuria in cats with hyperthyroidism. BSAVA Congress 2003 Scientific proceedings, p533.