Hypertension and Hyperthyroidism in Cats: What's the Correlation?

My patient is a 15-year old, FS, DSH cat who was successfully treated with radioiodine a year ago.

This cat is very fractious and extremely difficult to do a complete examination. At the time of her initial appointment (when the diagnosis of hyperthyroidism was established), her systolic blood pressure reading was high at 210 mmHg. But again, she was very upset and we were lucky to even get this blood pressure reading without being eaten!  So even though this blood pressure reading was high, we felt that it was likely due to "white coat hypertension" rather than true hypertension requiring treatment.

The following day, she was even more fractious but we did a fundic exam which was normal. Her repeat blood pressure reading was 185 mmHg, so I was not convinced that she was truly hypertensive based on these results. Her renal function tests and urinalysis were all normal. She was treated with the radioiodine and discharged without any therapy.

Four months later, the client had her examined because of decreased vision. Her systolic blood pressure now was very high (260 mmHg) and her fundic examination revealed bilateral detached retinas. Laboratory testing revealed a high serum urea nitrogen concentration (45 mg/dl) but a normal serum creatinine (1.3 mg/dl). Her urine specific gravity was only 1.016.

So my questions:

• What are your thoughts on the 4-month lapse of time between when I treated her and when she detached her retinas? 
• Should I have treated her for hypertension at time of the diagnosis of her hyperthyroidism?  In my experience with other cases that have been hypertensive due to hyperthyroidism, once their thyroid has normalized, they don't require continued treatment with amlodipine.
•  Or was her hypertension due to her subsequent chronic renal disease?

I'd love any comments you may have that would help me with this unfortunate situation.

My Response:

Hyperthyroidism does not lead to hypertension in human patients
In human patients, hyperthyroidism does not cause hypertension (1). Rather, hyperthyroidism tends to cause a small decrease in mean arterial blood pressure (mild hypotension). This decrease is attributable to a consequence of peripheral vasodilation and a 40–60% reduction in total peripheral resistance.  So the bottom line is this — hypertension is not characteristic of thyrotoxicosis in humans with the disorder.

So why do hyperthyroid cats develop hypertension?
Early feline studies suggested that systolic hypertension was a very common finding in hyperthyroidism (2,3). However, the case numbers in these studies were small and the criteria used for the diagnosis of hypertension were variable. In addition, it's likely that many of those cats probably had stress-induced or white coat hypertension (2-5).

Based on more recent studies, the prevalence of systemic hypertension in newly diagnosed hyperthyroid cats ranges from 9% to 23%, suggesting that systemic hypertension may be less prevalent in the hyperthyroid cat population than first reported (6,7).

Large case studies of hypertensive cats with retinopathy all find relatively few cats with hyperthyroidism as an underlying cause of the eye disease (8,9,10). Similarly, large retrospective studies of cats with hyperthyroidism have not found a high prevalence of either hypertensive retinopathy or choroidopathy (6,7,11).

Bottom line: In other words, hyperthyroidism is not a common cause of feline hypertension. And when hypertension does develop, these hyperthyroid cats tend not to develop any ocular changes.

So why do hyperthyroid cats develop hypertension?

Even though true hypertension appears to be relatively uncommon in untreated cats with hyperthyroidism, approximately 20% of these cats may develop systemic hypertension after treatment (5-7).

One study (7) reported that the median time from treatment to subsequent development of hypertension was 5.3 months. Of those cats that developed hypertension, 35% overtly azotemic (serum creatinine >2 mg/dl), whereas two-thirds had normal kidney function (7) This suggests that hypertension after treatment of hyperthyroidism does not always develop as a consequence of unmasking of underlying renal insufficiency. 

In cats without obvious renal disease, the mechanism of this post-treatment hypertension is unclear, this also highlights the importance of longitudinal assessment of blood pressure in cats diagnosed with and treated for hyperthyroidism.

Bottom line
I think you made the correct decision by not treating her for hypertension at the time of initial diagnosis of the cat's hyperthyroidism. Ideally, she would have had a follow-up blood pressure done 1-3 months after treatment because she may have had severe hypertension by that time.

Nevertheless, this is a very uncommon scenario, and it is extremely difficult to accurately diagnose hypertension in cats, especially when they are stressed, fractious, or difficult to handle.

References:

1. Saito I, Saruta T. Hypertension in thyroid disorders. Endocrinology and Metabolism Clinics of North America 1994;23:379-386.
2. Kobayashi DL, Peterson ME, Graves TK, et al. Hypertension in cats with chronic renal failure or hyperthyroidism. Journal of Veterinary Internal Medicine 1990;4:58–62.
3. Stiles J, Polzin DJ, Bistner SI. The prevalence of retinopathy in cats with systemic hypertension and chronic renal failure or hyperthyroidism. Journal of the American Veterinary Medical Association 1994;30:564–572.
4. Stepien RL. Feline systemic hypertension: Diagnosis and management. Journal of Feline Medicine and Surgery 2011;13:35-43. 
5. Jepson RE. Feline systemic hypertension: Classification and pathogenesis. Journal of Feline Medicine and Surgery 2011;13:25-34.
6. Syme HM, Elliott J. The prevalence of hypertension in hyperthyroid cats at diagnosis and following treatment [abstract]. Journal of Veterinary Internal Medicine 2003;17:754.
7. Morrow LD, Adams VJ, Elliott J, et al. Hypertension in hyperthyroid cats: prevalence, incidence and predictors of its development [abstract]. Journal of Veterinary Internal Medicine 2009;23:699.
8. Maggio F, Defrancesco TC, Atkins CE, et al. Ocular lesions associated with systemic hypertension in cats: 69 cases (1985–1998). Journal of the American Veterinary Medical Association 2000;217:695–702.
9. Littman MP. Spontaneous systemic hypertension in 24 cats. Journal of Veterinary Internal Medicine 1994;8:79–86.
10. Jepson RE, Brodbelt D, Elliott J, et al. Evaluation of the effects of control of systolic blood pressure on survival in cats with systemic hypertension. Journal of Veterinary Internal Medicine 2007;21:402–409.
11. van der Woerdt A, Peterson ME. Prevalence of ocular abnormalities in cats with hyperthyroidism. Journal of Veterinary Internal Medicine 2000;14:202-203.