Cat - Hyperthyroidism

What is hyperthyroidism?

Hyperthyroidism is the most common endocrine (hormonal) disorder of cats. It is most common in older cats and is rarely seen in cats under eight years of age. There is no sex or breed predisposition. Hyperthyroidism is due to an increase in production and secretion of thyroid hormone by the thyroid gland in the neck.

What are the clinical signs of hyperthyroidism?

Cats may present with a combination of the following clinical signs which tend to develop gradually:

• Weight loss
• Increased appetite
• Hyperactivity and restlessness
• Moderate elevation of body temperature
• Increased heart rate, with a variety of cardiac rhythm irregularities and murmurs
• Increased frequency of defaecation, with abundant, bulky stools
• Increased thirst and urination
• Occasional vomiting
• Panting
• Matted, greasy and unkempt coat

How is hyperthyroidism diagnosed?

In hyperthyroidism a nodule is usually palpable (can be felt) in one or both of the thyroid lobes in the neck (goiter). As the enlarged lobe may be freely movable and can slide along and behind the trachea, it may be difficult to detect, and require careful palpation. In the normal cat, the thyroid lobes are not usually palpable. Once hyperthyroidism is suspected on the basis of clinical signs, the diagnosis is confirmed by a blood test detecting elevated serum thyroid hormone levels. Other laboratory tests may also be abnormal, such as elevation of the liver enzymes, or changes on an electrocardiograph (ECG). In some cats, the overactive thyroid tissue is present in the chest rather than in the neck, and this can only be identified by performing a special type of scan (scintigraphy), only usually performed at specialist centres.

How can hyperthyroidism be treated?

There are three therapeutic options for the treatment of hyperthyroidism.

Which treatment option is most suitable for your cat depends on a number of factors and your vet will discuss this with you.

1.  Anti-thyroid drug therapy

Anti-thyroid drugs are readily available and economical. They do not destroy thyroid gland, but act by interfering with production and secretion of thyroid hormone. Their use does not result in a cure, but rather controls the condition. The drugs are in the form of tablets and are usually given once or twice a day. The anti-thyroid drugs most commonly used are carbimazole (Vidalta®) and methimazole (Felimazole®).  The thyroid hormone levels are monitored periodically, particularly in the early stages of treatment, to check on the effectiveness of treatment and to allow any adjustments to the dosage to be made. Indefinite treatment will be required to prevent signs from returning.

Mild (and often transient) side effects are seen quite commonly in cats on this medication (~15% of patients), and can include anorexia (inappetance), vomiting and lethargy. More serious side effects are seen less frequently (~5% of patients) and can include a fall in the number of white blood cells, skin problems, blood clotting problems, or liver disorder. Blood should therefore be tested routinely to monitor for potential side effects, and in some patients the occurrence of severe adverse reactions may necessitate withdrawal of the drug.

2.  Surgical thyroidectomy

Surgical thyroidectomy (removal of the thyroid glands) has the immediate advantage over drug therapy in that it provides a cure. This treatment is readily available, although surgical skill and experience are necessary to minimise potential side effects.

Anaesthesia can be problematic in hyperthyroid patients both as a direct result of the condition being treated, and also because a number of patients have other diseases e.g. chronic renal failure, heart disease. To reduce hyperthyroid-related surgical risks, patients should be pre-treated with anti-thyroid drugs for 3 to 4 weeks prior to surgery to reduce their thyroid hormone levels back to normal. Any associated cardiac (heart) disease should be carefully controlled.

Side effects of the surgical procedure may include nerve damage, or hypoparathyroidism (lack of the hormone that controls the level of calcium in the blood). The parathyroid glands are located very close to the thyroid glands, and so can be easily damaged when the thyroid glands are being removed. The resultant hypocalcaemia (low blood calcium level) can result in muscle twitching, weakness and convulsive seizures. Patients should be observed closely for the first 2-3 days after surgery.

There is generally a low rate of recurrence of hyperthyroidism following surgery, although some cases do recur. This can happen when a case of bilateral hyperthyroidism (i.e. where both thyroid lobes are affected) is mistakenly treated as a unilateral case (where only one side is affected) - the differentiation of normal from abnormal thyroid tissue is not always straightforward. Around 70% of hyperthyroid cases are bilateral, and in unilateral disease the gland on the opposite side of the neck is normally reduced in size. Occasionally, adenocarcinoma (malignant tumours) are present, and although they do not usually spread through the body, local invasion may prevent satisfactory surgical excision.

  3. ¹³¹I  (radioactive iodine) therapy

This uses radioactive iodine (¹³¹I) which is administered subcutaneously (injected under the skin), and is selectively concentrated within the thyroid gland.

¹³¹I  selectively destroys only the affected thyroid tissue, including any areas of thyroid tissue which may be inaccessible to surgery, and spares normal tissue, including the parathyroid glands.

The primary advantages of ¹³¹I treatment are that it is curative, has no serious side-effects (no toxicity, no hypoparathyroidism), does not require an anaesthetic, is associated with a low recurrence of hyperthyroidism and the location of the tumour is unimportant. The cost of treatment is comparable to surgical treatment, but depends in part on the length of hospitalisation. Additionally, large doses of ¹³¹I  are the only effective treatment for thyroid adenocarcinoma, which is responsible for around 1 to 2% of feline hyperthyroid cases.

The problems of ¹³¹I treatment include:-

• Poor availability. The treatment is only available in a few specialist centres, due to safety regulations that cover the use of radioactive products.
• Hospitalisation for between 3 and 6 weeks following treatment, which is necessary to allow adequate decay of the ¹³¹I . (All of the radioactive Iodine needs to have left the cat before it's allowed to go home)
• It is not suitable for use with patients requiring intensive care as, particularly in the early days following treatment, excessive handling of the cat must be avoided.