Hyperthyroidism is the most common endocrine (hormonal) disorder of cats. It is rarely seen in cats under eight years of age, and there is no sex or breed predisposition. It is due to an increase in production and secretion of thyroid hormone by the thyroid gland in the neck.
Cats may present with a combination of the following clinical signs which tend to develop gradually:
- Weight loss
- Increased appetite
- Hyperactivity and restlessness
- Increased heart rate, with a variety of cardiac rhythm irregularities and murmurs
- Increased frequency of defecation, with abundant, bulky stools
- Increased thirst and urination
- Occasional vomiting
- Matted, greasy and unkempt coat
In hyperthyroidism a nodule is usually palpable in one or both of the thyroid lobes. 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 either not palpable or small and symmetrical.
Once hyperthyroidism is suspected on the basis of clinical signs, the diagnosis is confirmed by 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).
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.
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. Carbimazole (Neo Mercazole) is commonly used, initially given three times daily, reducing to a maintenance dose once the thyroid hormone levels have returned to normal. When used as a long term treatment, twice daily dosage is usually required, so keeping the cat medicated can be difficult with this treatment.
Mild (and often transient) side effects are seen quite commonly in cats on this medication (~15% of patients), and can include poor appetite, 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, 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.
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 concurrent diseases e.g. chronic renal failure. 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 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 in the veterinary practice are 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.
This uses radioactive iodine (I131) which is administered subcutaneously (injected under the skin) or given by mouth, and is selectively concentrated within the follicles of the thyroid gland.
131I selectively destroys the affected thyroid tissue, including any areas of thyroid tissue which may be inaccessible to surgery, and spares adjacent normal tissue, including the parathyroid glands.
An initial tracer-dose of 131I may be given in order to precisely calculate the correct treatment-dose of 131I for any individual. However, recent experience suggests that a standard dose (150-250MBq/cat) is likely to be curative in around 90% of cases. Where hyperthyroidism persists after treatment, a second dose can be given. Very occasionally permanent hypothyroidism (lack of thyroid hormone) has been seen after 131I treatment, but this can be easily managed with thyroid hormone replacement therapy.
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 or sedation, 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, due to safety regulations that cover the use of radioactive products.
- Hospitalisation for between 1 and 2 weeks following treatment, which is necessary to allow adequate decay of the 131I.
- 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.
- The radiation risk to personnel treating the cats.