In this video, we're going to explore one of the most common endocrine conditions affecting cats and you won't be surprised to hear that this is also a condition that's been associated with hypertension. Of course, it's hyperthyroidism. So what proportion of cats that you diagnose with hyperthyroidism do you also diagnose with hypertension?

Well, the literature would tell us that it's somewhere between 7 to 47% of cats that are diagnosed with hyperthyroidism that will be simultaneously diagnosed with systemic hypertension. This wide range depends on the study population and also the historical criteria that have been used to diagnose hypertension. But it's likely to be somewhere between at least 10 to 20% with our current criteria.

Hyperthyroidism is a condition that we see in the older cat and therefore age can certainly be a factor in identifying hyperthyroidism and hypertension together. However, the development of systemic hypertension in hyperthyroidism is also the consequence of the biological effects of excess thyroxine or T4, and triiodothyronine or T3. Increased thyroid hormone levels can increase cardiac output by increasing both heart rate and contractility, and this tends to increase blood pressure.

At the same time, thyroid hormones tend to decrease vascular resistance. This would have an opposing action and decrease blood pressure, and perhaps it does limit the severity of hypertension that is seen in cats with hyperthyroidism. But nevertheless, the pathophysiology of hyperthyroidism is recognised as a key factor in why some cats with this condition develop hypertension.

So, is it worth performing a fundic examination in cats with hyperthyroidism and suspected hypertension? Well of course I'm going to say yes to this question. If we see evidence of hypertensive ocular target organ damage, then that helps us to confirm our diagnosis and we'll want to start treatment.

However, it is worth being aware that overall, relatively fewer cats with hypertension associated with hyperthyroidism seem to develop ocular target organ damage, compared to cats, for example, with chronic kidney disease and hypertension. The reason for this isn't completely clear. Perhaps they have slightly less severe hypertension, or perhaps there are differences in the risk of developing vascular pathology in the eye between these two conditions.

Another hypothesis that is put forward is that cats with hyperthyroidism can be quite hyperactive and also quite anxious and have behavioural changes that can make it much more challenging for us to measure blood pressure without the influence of situational or what was previously called white coat hypertension. This is definitely something that we need to consider and account for when we're measuring blood pressure in cats with hyperthyroidism. So what do you think is likely to happen when we measure blood pressure when we treat cats with hyperthyroidism?

Well, if hypertension is secondary only to the hyperthyroid state, then we might expect the hypertension to resolve when we treat these cats with hyperthyroidism and they become euthyroid. But actually, many studies have shown that this isn't what happens, and that in fact there can be a relatively little change in blood pressure pre to post treatment. So when we diagnose a cat with hyperthyroidism and systemic hypertension together, we do recommend starting management of both.

If you have got a concern for situational hypertension, then you might decide to start your anti-thyroid medications first and to continue to monitor blood pressure. But if you do this, the continuing to monitor blood pressure part is really important because if we document persistently high blood pressure values, then we really do want to treat hypertension and hyperthyroidism together. Whenever we diagnose hypertension with another disease condition, it's key to try and aligner the monitoring so that we don't need to have too many re-examination visits.

For cats that are diagnosed with hypertension, we want to monitor their response to treatment, and our targets are going to be a systolic blood pressure less than 160 millimetres of mercury, or ideally less than 140 millimetres of mercury, whilst at the same time avoiding hypotension. So if you have started amlodipine at 0.625 milligrammes per cat peros once daily and not achieve the desired blood pressure control, then a dose escalation to 1.25 milligrammes per cat once daily is indicated.

Again with further monitoring after approximately 3 to 10 days to check response. In the longer term, continued monitoring of blood pressure can usually be coordinated with those re-examination appointments for monitoring the thyroid disease, but should be at a minimum of every 4 to 6 months. A question for you, if a cat is norm attentive at the diagnosis of hyperthyroidism, does this mean that all is good and there's no need to measure blood pressure again?

Well, it would certainly be nice if that were the case, but the answer is no. Data from studies shows us that approximately 20% of cats that are diagnosed with hyperthyroidism are initially norm attentive but will become hypertensive after we treat them. Now for some cats, this might be because as we control their thyroid disease, we unmask underlying chronic kidney disease.

So assessing renal function is important here too. But that's not the case for all of these cats. Some of these cats develop hypertension despite not having any biochemical evidence of CKD.

Either way, treating hypertension in these cats is going to be important, and amlodipine can be used as a monotherapy alongside the anti-thyroid medications. If you diagnose and treat systemic hypertension in a cat after they have become e-thyroid, then monitoring targets are going to be the same as we've previously discussed. And once again, in the longer term, it's really great if we can just align the monitoring of both the hypertension and the hyperthyroidism.

So if we think about the take home messages for managing a cat with hypertension and hyperthyroidism. These are definitely conditions that we see together, so in any cat that is hyperthyroid, we should be measuring blood pressure. Where we diagnose hyperthyroidism and hypertension together, we can start treatment for both simultaneously.

But some cats will only develop hypertension after treatment for hyperthyroidism. So we should measure blood pressure not only at diagnosis, but also after we start treatment for hyperthyroidism. Longer term, if we're managing both hyperthyroidism and hypertension, we need to make sure that we monitor to optimise control of both of these conditions together.