29 Aug 2016
Sarah Caney provides details on how to spot the clinical signs of this fairly common disorder, which mainly affects older cats, and how to appropriately manage it.
Sarah Caney
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Figure 3. Surgical thyroidectomy is often a straightforward procedure and offers the possibility of a cure from hyperthyroidism.
Hyperthyroidism is a common illness, especially in older cats. Typical clinical signs include weight loss in spite of a good or increased appetite. The majority of affected cats have a palpable goitre on physical examination. Most cases are straightforward to diagnose through measuring serum total thyroxine levels, but, occasionally, additional testing is required to confirm the diagnosis.
Four management options exist, including antithyroid medication, exclusive feeding of an iodine-restricted food, surgical thyroidectomy and radioiodine. All treatments have advantages and disadvantages and the treatment choice should be based on the individual cat and owner situation. In the long term, check-ups are especially important in those cats receiving ongoing management for their thyroid disease.
The prognosis for cats with hyperthyroidism is often very good with appropriate management. Where possible, curative treatments are recommended.
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Hyperthyroidism – the clinical syndrome resulting from excessive circulating levels of thyroid hormones – is a common disorder, primarily affecting older cats.
The median age at diagnosis is around 12 to 13 years. In most cases, the condition is caused by benign hyperplasia of thyroid tissue and is typically bilateral (involving both thyroid glands).
The clinical signs of hyperthyroidism vary in severity and are generally most severe in cats suffering with the illness for longer and those with concurrent illnesses. Chronic kidney disease (CKD) is one of the most common concurrent illnesses and results in a worsening of many of the clinical signs.
Table 1 lists the common clinical signs and physical examination findings in cats suffering from hyperthyroidism. Hyperthyroid cats can be difficult to examine through being more anxious as a result of their hyperthyroidism.
The overwhelming majority of hyperthyroid cats have a palpable goitre and are suffering from weight loss (Figure 1). The goitre is usually palpated in the neck, just below the larynx, on one or both sides of the trachea (Figure 2). The size of the goitre can vary enormously. In most cases, the goitre is not visible by eye and is the size of a garden pea.
In rare cases, the goitre can be as large as a golf ball. In a small number of hyperthyroid cats, the enlarged thyroid cannot be felt. This may be for several reasons (Table 2). In the very small number of cats with thyroid adenocarcinomas, the thyroid may feel adherent to underlying tissues and/or the skin.
Hyperthyroidism may be suspected on the basis of historical and clinical findings, as already outlined. The most common changes found on routine blood profiles include elevated levels of liver enzymes (alanine aminotransferase; ALT, and alkaline phosphatase; ALP), leukocytosis, eosinopenia and erythrocytosis.
The diagnosis of hyperthyroidism is usually straightforward to confirm through measuring serum total thyroxine levels (T4). In cats with mild/early disease, levels may be more equivocal due to fluctuating levels of thyroid hormones. Presence of concurrent illnesses can also make diagnosis more difficult since total T4 levels can be suppressed by other illnesses – what is referred to as the sick euthyroid syndrome.
If the total T4 result is in the lower half of the reference range, hyperthyroidism is unlikely. However, if the total T4 result is in the upper half of the reference range, hyperthyroidism remains a potential differential diagnosis. In these patients, a simple and often effective method of confirming the hyperthyroidism is to repeat the total T4 measurement after a few weeks.
Free T4 measured by equilibrium dialysis can be another useful diagnostic tool. This test is highly sensitive in diagnosing hyperthyroidism (Peterson et al, 2001), although a small number of false-positive results can occur, meaning the free T4 test should not be used as a screening test for diagnosis of hyperthyroidism.
An elevated free T4 (>40pmol/l), in addition to total T4 in the upper half of the reference range (>30nmol/l), is consistent with a diagnosis of hyperthyroidism, especially if the cat is known to be suffering from concurrent disease (Wakeling et al, 2008).
Untreated hyperthyroidism is associated with progressively worsening clinical signs. Delaying treatment increases the risk of complications, such as cardiovascular disease.
Treatment often produces a rapid improvement in quality of life with many patients having an excellent quality of life for years following diagnosis. There are four options for management of hyperthyroidism:
Table 3 summarises the advantages and disadvantages of treatment options.
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 presence of systemic hypertension, which is estimated to be present in around 15% of cats suffering from hyperthyroidism.
Veterinary licensed antithyroid medications include tablet forms of thiamazole and carbimazole, and, more recently, an oral liquid methimazole.
Thioureylenes block production of the thyroid hormones and, therefore, symptomatically manage the hyperthyroidism. Lifelong treatment is required unless a curative treatment, such as surgery or radioiodine, is subsequently pursued. In the long term, difficulties with owner and patient compliance may reduce the overall success of this treatment modality.
Nevertheless, medical treatment is popular – not least since it is a reversible treatment of particular benefit when stabilising patients with concurrent CKD. The dose of medication can be “fine tuned” to suit the individual patient and withdrawn completely, if necessary. Ideally, a dose resulting in reduction of total T4 levels to the lower half of the reference range is aimed for and total T4 levels should be checked two to three weeks after treatment starts or the dose changes (Daminet et al, 2014).
Transdermal thiamazole gel is not a veterinary licensed preparation (however, it can be used under cascade regulations, where appropriate). Dosing is as for oral preparations with the same range of potential adverse effects. Cats receiving this therapy may be at a lower risk of gastrointestinal side effects compared to those treated with oral preparations.
The gel is usually applied to the inside of the pinna (a hairless area); carers should wear gloves and avoid direct contact with the gel. The medication is absorbed through the skin and into the blood stream. Transdermal antithyroid medications can take longer to be effective than oral forms.
Side effects have been reported with oral and transdermal administration of thioureylenes. Around 10% to 20% of patients may suffer from temporary and manageable side effects, including lethargy, inappetence, diarrhoea, nausea and vomiting.
In most cats with these side effects, the clinical signs are mild and only last a few days. In others, they are more severe and may necessitate stopping treatment or a treatment “holiday”. Starting treatment at a low dose before gradually increasing it, as needed, helps minimise their occurrence and severity.
Severe side effects may be seen in up to 5% of treated cats and necessitate withdrawal of therapy before an alternative treatment is started.
Side effects that most commonly develop in the first few months of therapy include:
Production of thyroid hormone requires iodine molecules; therefore, limiting the amount of iodine fed reduces the amount of thyroid hormone produced and released by the thyroid gland.
As with medical management, lifelong treatment (with 100% compliance) is required unless a curative treatment is subsequently pursued. Patient and owner compliance is essential to the success of this approach – even small deviations from the prescribed feeding can allow “escape” of thyroid control.
Unlike medical treatment there are no “drug-related” side effects to worry about, but compliance to the food may be an issue, especially if using this treatment long term. The food is phosphate restricted and moderate in protein, making it an acceptable nutrition for cats with mild to moderate CKD, but not recommended for cats in IRIS stages three and four CKD.
This potentially curative treatment has disadvantages of requiring general anaesthesia (which may be contraindicated in some patients) and is only suitable for those cases with easily accessible hyperfunctional thyroid tissue.
Up to 20% of patients may have ectopic hyperfunctional thyroid tissue and this is commonly located in the anterior thorax, not an area suited to straightforward thyroidectomy (Harvey et al, 2009; Figure 3).
Pre-surgical stabilisation with antithyroid medication or an iodine-restricted food is recommended. In routine cases, side effects of thyroidectomy, such as damage to the parathyroid glands resulting in hypocalcaemia, are possible.
This treatment is usually administered by subcutaneous injection. The radioactive iodine targets the abnormal thyroid tissue resulting typically in a 95% success rate. Published studies have so far shown the best long-term prognosis for treatment of hyperthyroidism is achieved with radioiodine.
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 previously unknown and worsen pre-existing kidney disease.
Unfortunately, there is no way to predict which cats will suffer renal problems following treatment of their thyroid disease. For this reason, medical treatment of hyperthyroidism is often recommended initially since this is a reversible treatment that can be reduced or stopped if problems are seen.
Hyperthyroidism is damaging to the kidneys, so optimal management of the hyperthyroidism is desirable, where possible. Typically, it is only cats with very serious CKD (such as IRIS stage four, creatinine >440mol/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 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 (IH) is an important adverse effect to monitor for in all cats receiving treatment for their hyperthyroidism, since it is associated with a worse prognosis. Diagnosis, prevention and management of IH is discussed in more detail in another article by the author (Caney, 2014).
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.
