Blood Pressure in Hyperthyroid Cats Before and After Radioiodine Treatment

Abstract: Background— Feline hyperthyroidism is commonly associated with systemic hypertension, with reported prevalence of 9% to 48%. Although hypertension might be expected to resolve once methimazole restores euthyroidism, it can persist or only first develop after treatment. Further, the effect of radioiodine treatment or iatrogenic hypothyroidism on resolution or development of hypertension has not been investigated.
Objectives— To determine proportion of hyperthyroid cats with hypertension (systolic blood pressure [SBP]>160 mmHg); persistence or first development of hypertension after successful 131I treatment, and correlation of post-treatment hypertension with azotemia or iatrogenic hypothyroidism.
Animals— 407 hyperthyroid cats, 257 rechecked post-treatment.
Methods— Hyperthyroid cats had SBP measured by Doppler; 257 had SBP rechecked 6 months after successful 131I treatment.
Results— Of untreated hyperthyroid cats, 111/407 (27%) and 88/257 (34%) were hypertensive. A higher proportion of hypertensive cats were nervous/excited compared with normotensive cats (45% vs 14%; P< 0.001). After 131I treatment, 44/257 (17%) previously hypertensive cats normalized SBP, whereas 44/257 remained hypertensive; 16 (6%) other cats first developed hypertension after treament. Of these 60 cats, only 5 (8%) were azotemic and 6 (10%) were hypothryroid. A higher proportion of cats remaining hypertensive had nervous/excited demeanor than did normotensive cats (41% vs 19%; P=0.002).
Conclusions/clinical importance— Hyperthyroid cats infrequently (27%) had SBP >160 mmHg, half of which normalized after 131I treatment. Cats rarely developed post-treatment hypertension. Persistent or newly diagnosed hypertension in these cats was unrelated to azotemia or iatrogenic hypothyroidism. Nervousness/anxiety in hypertensive cats suggests that many of these cats might have “white coat” situational hypertension.