Hyperthyroidism treatment and long-term management options

Once a diagnosis of hyperthyroidism has been made, decision making turns to management. Research by the author has shown owners appreciate being involved in the treatment decision-making process (Caney, 2013).

There are four options for management of hyperthyroidism.

• Reversible options (lifelong treatment needed):
  • antithyroid medication
  • exclusive feeding of an iodine-restricted diet (Hill’s y/d)
• Potentially curative options:
  • surgical thyroidectomy
  • radioiodine

The author favours curative options, where possible, especially when hyperthyroidism is diagnosed in a relatively young and otherwise healthy cat.

Antithyroid medications

Veterinary licensed antithyroid medications include methimazole (Felimazole, Dechra Veterinary Products; Thiafeline, Animalcare) and carbimazole (Vidalta, MSD). 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, which is of particular benefit when stabilising patients with concurrent chronic kidney disease (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The dose of medication can be fine-tuned to suit the individual patient and withdrawn completely if necessary.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 thyroxine (T4) levels to the lower half of the reference range is aimed for. Total T4 levels should be checked two to three weeks after starting treatment or changing the dose (Daminet et al, 2014).

Transdermal methimazole gel is also available in the UK via Summit Pharmaceuticals. This is not a veterinary licensed preparation, but can be used under cascade regulations where appropriate. Dosing is as for oral preparations, with the same range of potential adverse effects.

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 bloodstream. Transdermal antithyroid medications can take longer to be effective than the oral forms.

Side effects have been reported with oral and transdermal administration of thioureylenes (Peterson et al, 1988; Mooney et al, 1992). 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 some cats, the side effects are more severe and may necessitate stopping treatment or having a treatment “holiday”. Starting treatment at a low dose, before gradually increasing this as needed, helps minimise the occurrence and severity of side effects.

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 most commonly develop in the first few months of therapy and include:

• persistent and more severe gastrointestinal signs such as nausea, vomiting and anorexia
• pruritic dermatitis, often affecting the head and neck area
• serious haematological problems including thrombocytopaenia, leucopaenia, immune-mediated haemolytic anaemia
• hepatopathy
• myasthenia gravis

Advantages of thioureylene medications include:

• readily available
• initially less expensive than the 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 CKD
• reversible: especially an advantage in cats with concurrent CKD 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 is required
• hypothyroidism is uncommon with medical treatment and can be easily corrected by reducing the dose of medication

Disadvantages of thioureylenes include:

• side effects can be serious in some patients and may require withdrawal of medication
• regular monitoring, including blood tests, is recommended so 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 give pills to, so long-term medication 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 hyperthyroidism
• the cost of medication and check-ups can be more expensive than curative options in the long-term
• long-term treatment with thioureylenes increases the likelihood of multiple and larger thyroid nodules developing and may make the hyperthyroidism more difficult to treat with thioureylenes or radioiodine (Peterson et al, 2015).

Feeding an iodine-restricted diet

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 (Hill’s Prescription diet y/d) 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 International Renal Interest Society [IRIS] stages three and four CKD).

Advantages of y/d include:

• 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 antithyroid medication in those cats that accept the food
• reversible – especially an advantage in cats with concurrent CKD 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 is required
• may be easier to administer than a pill

Disadvantages of y/d include:

• successful management depends on 100% 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)
• not curative, so 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 hunt frequently and eat their prey
• some water sources may contain iodine, which could affect efficacy
• long-term compliance may be an issue in some cats
• some cats will not accept the food
• some cats do not fully respond to the food
• not an ideal food for cats in IRIS stage three or four CKD
• cost of food and check-ups can be more expensive than curative options in the long term

Surgical thyroidectomy

Thyroidectomy is a potentially curative treatment, but it has the 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).

Presurgical 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.

Advantages of surgical thyroidectomy include:

• available in most practices
• potentially curative with no further requirement for antithyroid medication
• straightforward surgery in many patients
• rapidly effective
• short hospitalisation period

Potential disadvantages of surgical thyroidectomy include:

• 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

Radioiodine 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 shown the best long-term prognosis for treatment of hyperthyroidism is achieved with radioiodine (Milner et al, 2006).

Advantages of radioiodine include:

• high cure rate with no further requirement for antithyroid medication
• no anaesthesia required
• all abnormal thyroid tissue is treated, regardless of its location in the body (ectopic thyroid tissue treated too)
• safe to adjacent structures such as the parathyroid glands
• recurrence of disease is very rare
• side effects are rare
• radioiodine is an effective treatment for thyroid carcinomas, although a higher dose is required

Potential disadvantages of radioiodine include:

• special facilities required – not routinely available
• human health and safety considerations require separate hospitalisation of patients for a period following treatment (typically one to four weeks)
• while hospitalised, other treatments may not be possible so radioiodine is usually only suitable for reasonably “healthy” hyperthyroid cats
• can be very expensive
• it may take several months for euthyroidism to be achieved
• occasionally (around 5% of cases), a second treatment with radioiodine is required to achieve euthyroidism
• occasional permanent hypothyroidism, which would require supplemental thyroid hormones

Deterioration in renal function

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 (GFR). 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.

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 at all possible. Typically, it is only cats with very serious CKD (for example, IRIS stage four, creatinine greater than 440µmol/L) where optimal management of hyperthyroidism proves difficult/impossible without inducing a clinical and laboratory deterioration in renal function.

Long-term monitoring

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 (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 an article by the author (Caney, 2015).

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.