Feline hyperthyroidism

Advice on the medical management of feline hyperthyroidism when faced with concurrent disease

19 September 2018, at 12:37pm

Feline hyperthyroidism is a complex metabolic disease which can be further complicated by concurrent or secondary illnesses. In many instances, curative treatment may not be feasible, in which case it is necessary to manage the disease with medical treatment. As hyperthyroidism is more prevalent in cats over nine years old, it is common for patients to be affected by some form of concurrent disease. This article aims to summarise the management of hyperthyroidism when faced with the more common concurrent diseases.

Concurrent diabetes mellitus

In most cases of concurrent disease, one disease has developed before the other and should therefore be stabilised first. The development of hyperthyroidism in a stable diabetic cat will likely lead to deterioration of blood sugar control and a higher dose of insulin potentially being required for stabilisation. The available antithyroid medications are, according to their Summary of Product Characteristics (SPC), contraindicated for use in cats with diabetes mellitus.

In Sarah Caney’s Veterinary Bulletin on FAQs about hyperthyroidism in cats, she notes that “Although many antithyroid medications contain sugar, the author does not consider this to be a significant contra-indication to their use in cats suffering from concurrent diabetes mellitus.”

When managing feline hyperthyroidism, your protocol does not need to be changed and medications can be used at the standard dose of methimazole, thiamazole or carbimazole. As per the product SPC, doses should be titrated as necessary until the cat is euthyroid. However, following the control of hyperthyroidism in a diabetic cat, the insulin dose may simultaneously need to be decreased and care should be taken to monitor patients for hypoglycaemia during this initial stabilisation period. Furthermore, due to accelerated protein turnover, fructosamine levels will often be lower than expected in uncontrolled hyperthyroid cats with concurrent diabetes and this is therefore considered a less reliable indicator of diabetes control in these patients.

If a stable hyperthyroid cat develops diabetes, the management of the hyperthyroidism does not need to be changed, and the clinician can therefore concentrate on treatment and stabilisation of the diabetes in these patients (Caney, Vet Professionals).

Concurrent chronic kidney disease

Treatment of hyperthyroidism has the potential to worsen kidney function and, as hyperthyroidism is prevalent in cats over nine years of age, it is common for hyperthyroid patients to be affected by some degree of renal disease. In some cases, because hyperthyroidism can cause a secondary increased glomerular filtration rate (GFR), clinical evidence of kidney disease may not be apparent prior to starting treatment. Controlling hyperthyroidism can reduce GFR by up to 50 percent, which may act to worsen any pre-existing kidney disease or unmask renal disease that was not previously detected. In spite of this, optimal management of concurrent hyperthyroidism is still desirable for most patients since hyperthyroidism can damage the kidneys further.

Typically, it is only cats with severe chronic kidney disease (eg IRIS stage 4, creatinine greater than 440μmol) that are likely to suffer a clinical deterioration in renal function following the start of antithyroid treatment. It is for this reason that medical treatment is often trialled initially as it is reversible should renal problems be encountered.

It may be prudent to start treatment at a lower dose of methimazole/thiamazole and if this level is tolerated but is insufficient to control the hyperthyroidism, the dose can be increased. If possible, hyperthyroidism should be treated optimally (ie reducing total T4 to lower half of the reference range) (Caney, Vet Professionals). However, if a clinical deterioration in renal function is experienced then the dose of antithyroid medication should be reduced and the clinician may have to accept a limited hyperthyroid control in order to maintain kidney perfusion. Often an iodine restricted diet is not recommended for managing hyperthyroid cats with significant renal disease as a renal diet is indicated for these patients (Caney, Vet Professionals).

Concurrent hypertrophic cardiomyopathy

Ettinger et al. (2000) reported that as many as 20 percent of hyperthyroid cats suffered from congestive heart failure and that the prevalence of congestive heart failure had significantly declined due to improved awareness and earlier diagnosis and treatment of hyperthyroidism, thus demonstrating the importance of early testing and diagnosis.

Hyperthyroidism in cats is associated with the development of hypertrophic cardiomyopathy but the exact mechanism is unknown. It has been suggested that the left ventricle of the heart becomes enlarged as a secondary response to the direct and indirect effects of elevated amounts of thyroid hormones, which include systemic hypertension, increased heart rate, increased myocardial contractility, sympathetic nervous system activation and an increase in the cellular oxygen demand (Ware, 2009).

Treatment of hyperthyroidism is likely to lessen some of the contributing factors and improve the patient’s overall condition; however, in clinically significant cases, an ACE inhibitor and beta-blocker treatment may be required. Cardiac disease can make patients extremely susceptible to stress and hyperthyroid patients should therefore be treated with the utmost care during handling and restraint (Rooney, 2011).

Concurrent liver disease

Feline hyperthyroidism will often result in elevated levels of liver-derived enzymes such as alanine aminotransferase (ALT) and alkaline phosphatase (ALKP). The mechanism for the increase in these liver enzymes is unknown. Usually the enzymes are only mildly elevated, and it has been suggested that an elevation up to 500U/L for ALT and ALKP can be due to hyperthyroidism, whereas elevation above 500U/L may be suggestive of primary hepatobiliary disease (Berent et al., 2007). Therefore, further investigation should be considered at enzyme elevations above 500U/L, including bile acids, ultrasound and liver biopsies. It has been suggested that bone-derived ALKP may be contributing to the overall increase in ALKP in hyperthyroid cats.

Mild to moderate elevations in liver enzymes should not be a barrier for treatment of hyperthyroid cats. However, as antithyroid medications are noted to occasionally cause hepatopathies, close monitoring is recommended. Should the liver enzymes increase beyond an acceptable level, antithyroid treatment should be stopped and further investigation carried out.


Hyperthyroidism is mostly prevalent in elderly cats, which inevitably increases the possibility of other diseases occurring concurrently with the hyperthyroidism. As a result, management of hyperthyroid cats can be challenging. Typically, one disease will develop before the other and should therefore be addressed first, but, as always, clinical judgement should be applied.

Antithyroid medication is contraindicated for use in cats suffering from systemic disease and when used in these cases, it constitutes off-licence use. However, in most cases a good outcome can be achieved when using antithyroid medication to treat hyperthyroid cats with concurrent disease, providing good monitoring practice is applied.

Patients that receive ongoing medical management of hyperthyroidism should undergo three-monthly T4, haematology and biochemistry analysis to ensure that medication is titrated to maintain euthyroidism and to monitor or diagnose any adverse event or concurrent/secondary illness. In the stabilisation phase and when treating patients with concurrent disease, more frequent monitoring is needed.

The above information is for general guidance and each individual case should only be treated after careful risk benefit consideration by the attending veterinary surgeon.

All Summary of Product Characteristics can be accessed at: