Hyperthyroidism is the most common endocrine (hormonal) disorder of cats. It is more often seen in cats over eight years of age, and there is no sex or breed predisposition. It is due to an increase in production and secretion of the 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:
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 or diarrhoea.
Increased thirst and urination.
Matted, greasy and unkempt coat.
In hyperthyroidism a nodule is often 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, despite 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 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 the thyroid gland, but act by interfering with the production and secretion of the thyroid hormone. Their use does not result in cure, but rather controls the condition. Felimazole (Methimazole) is the drug of choice and initially given 1-2 times daily. When used as a long term treatment, twice daily dosage is usually sufficient. It can also be used prior to surgery to reduce the enlarged thyroid gland.
Mild (and often transient) side effects are seen quite commonly in cats on this medication (approx. 20%), and can include anorexia (inappetence), vomiting and lethargy. More serious side effects are seen less frequently (approx. 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. Intravenous fluids are generally provided throughout surgery and any associated cardiac disease will be assessed and may require medication.
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 results in muscle twitching, weakness and convulsive seizures. Blood Ca (calcium) levels are checked post surgically if there are any concerns. 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. The differentiation of normal from abnormal thyroid tissue is not always straightforward. Around 70% of hyperthyroid cases are bilateral (where both thyroid lobes are affected), 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), and is selectively concentrated within the thyroid gland.
I131 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.
In 1-2% of cases hyperthyroidism can persist after treatment. This can be corrected with a second dose of I131. Very occasionally permanent hypothyroidism (lack of thyroid hormone) has been seen after I13I treatment, but this can be easily managed with thyroid hormone replacement therapy.
The primary advantages of I131 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 within the thyroid gland is unimportant. Additionally, large doses of I131 are the only effective treatment of thyroid adenocarcinoma, which is responsible for around 1 to 2% of feline hypothyroid cases.
The problems of 131I treatment include:
Poor availability, due to safety regulations that cover the use of radioactive products. Currently four locations around the UK can perform this procedure: including the Royal Veterinary College, London, Bristol Veterinary School and Barton Hospital, Canterbury.
Hospitalisation for between 3 and 6 weeks following treatment, which is necessary to allow adequate decay of the I131.
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.
Cost: the average cost for this procedure is currently £1,000-£1,200 due to the extended hospitalization and regulations required.
Should you wish to discuss any of these issues in more detail, please consult your veterinary surgeon.