Blood pressure in hyperthyroid cats before and after radioiodine treatment

Hyperthyroid cats commonly have systemic hypertension, with a reported prevalence of 7% to 48%. Although hypertension might be expected to resolve once treatment restores euthyroidism, it can persist or only first develop after treatment.

To determine the proportion of hyperthyroid cats with hypertension (systolic blood pressure [SBP] ≥160 mm Hg), persistence or first development of hypertension after successful radioiodine treatment, and correlation of post-treatment hypertension with azotemia or hypothyroidism.

Four hundred one hyperthyroid nonazotemic cats were included in the study.

Prospective, cross-sectional and before-and-after studies. All hyperthyroid cats had SBP measured by Doppler; 255 had SBP rechecked 6 months after successful radioiodine (¹³¹I) treatment.

Of untreated hyperthyroid cats, 108/401 (27%) were hypertensive. A higher proportion of hypertensive cats were nervous/excited compared with normotensive cats (47% vs 12%; P < .001). Of the initially hypertensive cats, 87/108 cats were reexamined after ¹³¹I treatment; 43/87 (49%) cats normalized SBP, whereas 44/87 (51%) remained hypertensive. Of the initially normotensive cats, 16/168 (9.5%) first developed hypertension after successful ¹³¹I treatment. 7/60 (12%) of the ¹³¹I-treated hypertensive cats were azotemic and 9/60 (15%) were hypothyroid. A higher proportion of cats remaining hypertensive had nervous/excited demeanor than did normotensive cats (50% vs 17%; P < .001).

Hypertension, when present, resolves in many hyperthyroid cats after successful treatment. Hyperthyroid cats uncommonly develop new hypertension after treatment. Persistent or newly detected hypertension was unrelated to azotemia or iatrogenic hypothyroidism. More frequently perceived nervousness/anxiety in radioiodine-treated hypertensive cats suggests that many of these cats might have "situational" hypertension, as hyperthyroid-induced hypertension should resolve after treatment.