28 Jan 2021
Sarah Caney
Job Title
Hyperthyroidism – the clinical syndrome resulting from excessive circulating levels of thyroid hormones – is the most common endocrinopathy in cats and is especially common in older cats. The median age at diagnosis is usually around 12 to 13 years and diagnosis is typically straightforward.
A variety of management options are available, and the most appropriate one will depend on the patient and carer. Medical management with anti-thyroid medications is popular and success depends on a good team approach between clinician and carer, such that compliance is supported.
Hyperthyroidism affects about 10% of cats older than 10 years of age.
The clinical signs of hyperthyroidism vary in severity, and are generally most severe in cats that have been suffering with the illness for longer and in those that have concurrent illnesses. Chronic kidney disease (CKD) is one of the most common concurrent illnesses and this results in a worsening of many of the clinical signs.
Common clinical signs of hyperthyroidism that an owner may notice are listed in Table 1. Clinical signs often develop slowly, which can make it difficult for carers to spot signs of illness in their cat.
| Table 1. Clinical findings in cats with hyperthyroidism – listed in approximate order of decreasing frequency | |
|---|---|
| Clinical sign | Approximate frequency – percentage of cats (Baral and Peterson, 2011) |
| Weight loss | 88 |
| Polyphagia | 49 |
| Vomiting | 44 |
| Polyuria/polydipsia | 36 |
| Increased activity | 31 |
| Decreased appetite | 16 |
| Diarrhoea | 15 |
| Decreased activity | 12 |
| Weakness | 12 |
| Dyspnoea | 10 |
| Panting | 9 |
| Large faecal volume | 8 |
| Anorexia | 7 |
| Physical examination findings | |
| Large thyroid gland | 83 |
| Tin | 65 |
| Heart murmur | 54 |
| Tachycardia | 42 |
| Gallop rhythm | 15 |
| Hyperkinesis | 15 |
| Aggressiveness | 10 |
| Unkempt hair coat (Figure 1) | 9 |
| Increased nail growth | 6 |
| Alopecia | 3 |
| Congestive heart failure | 2 |
| Ventral neck flexion | 1 |
A veterinary clinical examination may find additional abnormalities in a hyperthyroid cat (Table 1). The patient may be difficult to examine through being more anxious or active in the consulting room. Weight loss, in spite of a normal or increased appetite, is the most common clinical finding.
The overwhelming majority of hyperthyroid cats have a palpable goitre (Figure 1).
As previously mentioned, hyperthyroidism may be suspected on the basis of historical and clinical findings.
In most cats, the diagnosis can be confirmed by measuring resting serum total thyroxine (T4) levels. Changes found on routine blood profiles include elevated levels of liver enzymes (alanine aminotransferase and alkaline phosphatase), leukocytosis, eosinopenia and erythrocytosis. Mild hypokalaemia and/or hyperphosphataemia are seen in a small number of patients.
Occasionally, a normal total T4 result is received in a cat suspected of having hyperthyroidism. This is especially common in cats that have concurrent disease resulting in suppression of total T4 levels; the so-called sick euthyroid phenomenon.
Repeat total T4 testing at a reference laboratory, free T4 and thyroid stimulating hormone (TSH) assessment can be helpful. Cats suffering from hyperthyroidism typically have elevated free and total T4 and low or undetectable levels of TSH (Peterson et al, 2015).
Diagnosis of hyperthyroidism is rarely an emergency as this is a slowly progressive illness. Therefore, if in doubt of the diagnosis, repeat assessment (a “watch and wait” approach) is sensible rather than treating the cat.
Two broad categories of treatment exist:
Curative options are favoured by the author, where possible – especially when hyperthyroidism is diagnosed in a relatively young and otherwise healthy cat.
Patients should be screened for the presence of systemic hypertension, which is estimated to be present in around 15% of cats suffering from hyperthyroidism (Figure 2).
All treatments for hyperthyroidism have the potential to worsen kidney function. This is because the hyperthyroid condition increases renal blood flow and glomerular filtration rate.
When the hyperthyroidism is treated, the increased blood flow to the kidneys decreases. For many hyperthyroid cats, this return to normality is not associated with kidney problems. However, in a proportion of patients, this reduction in blood flow has the potential to “unmask” kidney disease that was not previously known about and to worsen pre-existing kidney disease.
Unfortunately, no guaranteed way exists to predict which cats will suffer renal problems following treatment of their thyroid disease, although symmetric dimethylarginine (SDMA) assessment may be of some utility.
A study by Peterson et al (2018) showed that while non-azotaemic cats with elevated pre-treatment SDMA were more likely to develop renal complications following stabilisation of their thyroid disease, unfortunately, having a normal pre-treatment SDMA was no guarantee of protection from renal complications.
In other words, assessment of pre-treatment SDMA had a poor sensitivity, but high specificity, for predicting azotaemia following treatment of hyperthyroidism.
Hyperthyroidism is damaging to the kidneys, so optimal management of the hyperthyroidism is desirable, where at all possible. Typically, it is only cats with very serious CKD (for example, IRIS stage 4; creatinine greater than 440μmol/L) where optimal management of hyperthyroidism proves difficult/impossible without inducing a clinical and laboratory deterioration in renal function.
Regular check-ups are important – especially in those cats managed with reversible options.
The aim of check-ups is to ensure therapy is optimal without any significant side effects. Suitable protocols for check-ups are covered elsewhere (Daminet et al, 2014).
Iatrogenic hypothyroidism is an important adverse effect to monitor in all cats receiving treatment for their hyperthyroidism, since it is associated with a worse prognosis.
Carer education and discussion is important to determine a management plan based on shared decision-making.
The “best” treatment depends on each individual cat and owner situation. All treatments have benefits and disadvantages (Table 2), which is why it is important carers discuss management of their cat’s hyperthyroidism in detail with the veterinary clinic.
| Table 2. Advantages and disadvantages of each management option | ||
|---|---|---|
| Advantages | Disadvantages | |
| Medical management: thioureylenes | • Readily available. • Initially less expensive than curative treatment options. • Fairly rapid onset of action – most patients euthyroid within a few weeks. • Most cats suffer no side effects of treatment, even with very long-term use. • Can be titrated “to effect”; especially helpful in cats that have concurrent chronic kidney disease. • Reversible – especially an advantage in cats with concurrent kidney disease where any treatment for hyperthyroidism can cause a worsening in their kidney function. • Very helpful in stabilising a patient in preparation for surgical treatment or while awaiting radioiodine. • No hospitalisation, sedation or anaesthesia required. • Hypothyroidism is rare with medical treatment and can be easily corrected by reducing the dose of medication. |
• Side effects can be serious in some patients and may require withdrawal of medication. • Regular monitoring, including blood tests, is recommended so that any side effects can be identified and treated quickly. • The medication does not cure the condition, so treatment is required for the rest of the cat’s life. • Treatment monitoring is required to ensure the correct dose of medication is being given – over time, the required dose of medication may change. • Some studies have suggested compliance with long-term medical treatment can be a problem, making medical management less effective in the long term compared to curative treatment options such as radioiodine. • Hyperthyroid cats can be difficult to medicate, so long-term treatment can be hard work. • Very occasionally, some cats are resistant to the thioureylene drugs, meaning they may need very high doses or an alternative treatment to control their illness. • The cost of medication and check-ups can be more expensive than curative options in the long term. |
| Nutritional management | • Readily available. • Initially less expensive than curative treatment options. • No side effects reported (other than reduced renal function, which can occur with any treatment for hyperthyroidism). • No need for anti-thyroid medication in those cats that accept the food. • Reversible – especially an advantage in cats with concurrent kidney disease where any treatment for hyperthyroidism can cause a worsening in their kidney function. • Very helpful in stabilising a patient in preparation for surgical treatment or while awaiting radioiodine. • No hospitalisation, sedation or anaesthesia required. • Easier to administer than medication. |
• Successful management depends on 100 per cent compliance to the food. • Limited information available on long-term use. • It can take up to 12 weeks to achieve euthyroidism (slower than anti-thyroid medications). • The food does not cure the condition, so treatment is required for the rest of the cat’s life. • Not ideal for multi-cat households (supplementation of healthy cats recommended if it is not possible to feed their normal food). • Many treats and some nutritional supplements contain iodine and are therefore “banned”; cats that are keen hunters may have poor control if they eat their prey. • Some water sources may contain iodine, which could affect efficacy. • Long-term compliance may be an issue. • Some cats will not accept the food. • Some cats do not fully respond to the food. • Not an ideal food for cats in International Renal Interest Society stage 3 or 4 kidney disease. • The cost of food and check-ups can be more expensive than curative options in the long term. |
| Surgical thyroidectomy | • Available in most practices. • Potentially curative with no further requirement for anti-thyroid medication. • Straightforward surgery in many patients. • Rapidly effective. • Short hospitalisation period (typically a few days). |
• Requires general anaesthesia. • Technically more difficult than other treatments. • Only possible if the thyroid nodule is accessible to surgical removal (most cases). • Can be expensive. • Complications, especially to the parathyroid glands, are possible and can be life-threatening (hypoparathyroidism). • Recurrence possible if not all of the abnormal tissue is removed. • Occasional permanent hypothyroidism requiring supplemental thyroid hormones. |
| Radioiodine | • High cure rate (95%) with no further requirement for anti-thyroid medication. • No anaesthesia required. • All abnormal thyroid tissue is treated, regardless of its location in the body (in other words, ectopic thyroid tissue treated, too). • Safe to adjacent structures, such as the parathyroid glands. • Recurrence of disease is very rare. • Side effects are rare. • The only effective treatment for thyroid carcinomas (higher dose required). |
• Special facilities required – not routinely available. • Human health and safety considerations require separate hospitalisation of patients for a period following treatment (typically one to two weeks). • While hospitalised, other treatments may not be possible, so radioiodine is usually only suitable for reasonably well hyperthyroid cats. • Can be very expensive. • It may take several months for euthyroidism to be achieved. • Occasionally (around five per cent of cases), a second treatment with radioiodine is required to achieve euthyroidism. • Occasional permanent hypothyroidism requiring supplemental thyroid hormones. |
An agreed management plan – concordance – supports carer compliance in the future.
Carers appreciate support for whatever management is chosen – whether transition to a new diet, oral medication or otherwise.
In general, the prognosis for management of hyperthyroidism is very good, depending on the severity of the disease and presence of other concurrent illnesses, such as CKD.
For the long-term care of hyperthyroid cats, check-ups are really important – and for those cats receiving reversible options, regular blood tests are necessary to ensure the hyperthyroidism is under control and the cat is not suffering from any side effects.
Most cats do very well when being treated for hyperthyroidism and the sooner it is diagnosed, the better the outcome generally.
