Hypertension in Hyperthyroid Cats: Prevalence, Incidence, and Predictors of Its Development

Morrow L.D., Adams V.J., Elliott J., et al.

Conference Proceedings, (2009). American College of Veterinary Internal Medicine:


Hypertension and azotemic chronic kidney disease (aCKD) can be diagnosed concurrently with hyperthyroidism and have also been detected after initiation of treatment. The aims of this study were to estimate the prevalence of hypertension in cats at first diagnosis of hyperthyroidism, to estimate its incidence after treatment for hyperthyroidism and to identify predictors of the development of hypertension in hyperthyroid cats.

Hyperthyroidism was diagnosed when total plasma thyroxine concentration was >55 nmol/l. Systolic blood pressure (SBP) was measured using the indirect Doppler method and the mean of five consecutive measurements was calculated. Hypertension was diagnosed when a cat had SBP greater than 170 mm Hg on two separate occasions or when it had SBP greater than 170 mm Hg on one occasion and compatible ocular signs. Hyperthyroid cats were treated medically, surgically or with a combination of these methods. A cat was diagnosed as having aCKD if it had a plasma creatinine concentration of >2.0 mg/dl and a urine specific gravity of <1.035 on the same visit. Three hundred and twenty four cats newly diagnosed as hyperthyroid at two first opinion geriatric cat clinics in London, UK between April 1, 1999 and June 18, 2008 were included in this retrospective analysis of longitudinal data using Kaplan-Meier survival analysis and Cox regression with time-varying covariates. The time-varying data that were analysed represented a total of 2205 visits to the clinic for 215 cats.

Of 324 cats diagnosed with hyperthyroidism, 21 were already receiving amlodipine for treatment of hypertension and were excluded from further study. Thirty nine of the remaining 303 cats were hypertensive at time of diagnosis of hyperthyroidism; a prevalence of 12.9% (95% CI: 9.4-17.3). The incidence with which hypertension developed in hyperthyroid cats following treatment was 22.8% (49/215, 95% CI: 17.5-29.1). Hypertension developed a median of 5.3 months (95% CI: 3.2-9.9) after treatment began. Of the newly hypertensive cats for which renal status data were available, 15 out of 42 (35.5%, 95% CI: 22.0-52.0) had a creatinine concentration of >2.0 mg/dl. Both urine and blood samples had been taken at diagnosis of hypertension in 31 of the cats; of these, 11 (35.5%, 95% CI: 19.8-54.6) had aCKD. Overall survival did not differ between hypertensive and normotensive cats (p=0.4). Age and treatment for hyperthyroidism were predictive of the development of hypertension.

We conclude that although hypertension can present concurrently with hyperthyroidism, in many cats hypertension develops after anti-thyroid treatment. In addition, hypertension does not always develop in association with the unmasking of underlying renal disease. These findings suggest that it would be prudent to assess hypertensive status in both newly diagnosed and treated hyperthyroid cats.