Feline Hyperthyroid Disease

Hyperthyroidism

Hyperthyroidism is generally a disease of older cats and the main clinical sign is weight loss despite an excellent appetite. Hyperthyroid cats are also extra thirsty and commonly restless and especially demanding of attention and/or food. Many cats have chronic intermittent vomiting or diarrhoea and may be urinating in inappropriate locations. In a normal cat, the lobes of the thyroid gland cannot be felt with your fingers. In the hyperthyroid cat, at least one lobe is usually prominent and may be detected during a physical exam.

Blood tests need to be run to diagnose hyperthyroidism and screen the blood for other changes. An elevated thyroid hormone level forms the basis for diagnosis. Also a mild increase in liver enzymes is most commonly observed indicating mild (usually clinically insignificant) damage to the liver.

What causes Hyperthyroidism?

Hyperthyroidism is caused by a benign growth in the thyroid gland that is over-producing thyroid hormone (T4). It is important to realise that these tumours are almost always benign and represent a form of goiter rather than a form of cancer. Less than 5 % of hyperthyroid cats have a cancerous thyroid growth.

Hyperthyroid cats frequently experience reduced quality of life through one or more of the following changes; weight loss, muscle deterioration, chronic vomiting or chronic diarrhoea. Hyperthyroidism also puts cats at risk of developing heart disease and high blood pressure. These problems can result in heart failure, sudden blindness, or sudden death and all can be prevented with timely treatment for thyroid disease.

Treatment of Hyperthyroidism

Carbimazole and methimazole are two drugs used to treat hyperthyroidism. The most common medication prescribed is carbimazole, which is converted to methimazole in the body.  Medication must be given at least daily (usually twice daily). Some cats simply will not take oral tablets at this frequency therefore the option of methimazole as a transdermal gel is possible. This is administered on the hairless inner surface of the cat's ear.

Alternatively radioactive iodine therapy or surgery to remove thyroid glands is available. These specific treatments are performed by specialist vets or veterinary hospital with specialised facilities.

The usual side effects are: lethargy, loss of appetite, and vomiting. If one of these side effects occurs, medication is discontinued until the symptoms resolve. Medication is then restarted at a lower dose and gradually increased to the former dose. These side effects do not generally recur if medication is increased gradually in this way.

Facial itching is a more serious side effect. This side effect also resolves with anti-itch medication and discontinuing methimazole. Cats who have this side effect can be expected to have it again if medication is restarted, so another form of treatment should be used. Facial itching occurs in less than 4% of cats on methimazole.

Most side effects occur during the first 3 months of medical treatment.

Kidney Disease & Hyperthyroidism

Pre-existing kidney disease can be masked by hyperthyroidism, thus monitoring kidney function and thyroid levels are particularly important during therapy. Kidney problems can be minimized by starting with a lower dose of medication and working up over weeks or months so as not to cause as abrupt a change in kidney blood flow. If kidney problems become significantly worse on therapy for hyperthyroidism the medication can be discontinued.

Regular blood testing to examine the thyroid hormone level, white blood cell patterns, kidney function, and liver enzymes must be performed. After beginning the initial therapy, monthly blood tests need to be carried out until a stable dose of medication has been achieved and kidney function is stable. After this time 6 monthly blood test will need to be performed for ongoing monitoring. Hyperthyroidism needs long term medication and unfortunately it cannot be cured.