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What is Hyperthyroidism?
Hyperthyroidism is caused by the over-production of thyroid hormone by the thyroid glands. The thyroid glands sit on either side of your cat's airway, about halfway down the neck. Thyroid hormone is involved in regulating the body's metabolic rate, and so excess thyroid hormone 'speeds up' metabolism, causing clinical signs such as weight loss.
Most cases of hyperthyroidism are caused by benign tumours of the thyroid glands. These tumours do not spread beyond the thyroid but they lose the ability to switch off thyroid hormone production, even when excess levels are circulating in the blood. A minority of cases are caused by malignant adenocarcinomas (a form of cancer).
Causes of Hyperthyroidism
Although not all of the factor leading to the development of hyperthyroidism in cats are fully understood, there are some dietary and environmental factors which have been identified. For more details follow this link: causes of hyperthyroidism
Signs of hyperthyroidism
Hyperthyroidism affects many bodily functions. The most common clinical signs are:
- Increased thirst
- Weight loss in spite of a good or increased appetite
- Panting or open-mouth breathing
In a smaller proportion of cats, hypertension (high blood pressure) can occur. Complications of hypertension such as stroke or sudden-onset blindness may be the first symptom of this problem. This increase in blood pressure can also cause damage to the kidneys, which can complicate treatment.
If untreated, hyperthyroidism will also lead to heart problems, including thickening of the walls of the heart, arrythmias, and potentially congestive heart failure. This type of heart disease is often reversible with treatment.
Your veterinarian will perform a full physical examination, the results of which will often point towards hyperthyroidism. The majority of hyperthyroid cats have goitre- enlargement of the thyroid glands which can be felt. Heart murmurs and signs of high blood pressure are also common examination findings. Further laboratory investigations will be needed.
Full blood profile and haematology: should show signs suggestive of the disease. Also very important to check kidney function before considering treatment.
Urinalysis: to rule out other causes of increased thirst and to help assess kidney function.
Thyroid hormone measurement. There are various methods for measuring thyroid hormone and more than one may be required to confirm the diagnosis. As with many hormone disorders, the levels of the hormone being measured can be affected by many factors, and so blood results often do not give a simple positive or negative result.
It is also quite likely that your veterinary surgeon will wish to check the condition of your cat's heart. Hyperthyroidism can cause the muscle of the heart wall to thicken or hypertrophy which may lead on to heart failure. Chest xrays, ultrasound, and electrocardiography may be recommended.
Treatment of Hyperthyroidism
There are three possible treatment protocols: surgery, medication, or radioactive iodine. All have their pros and cons. Radioactive iodine therapy is extremely effective and safe, however it is not readily available in many parts of the world and is likely to be very expensive. Medical treatment requires daily administration of an antithyroid drug which will be required for the rest of the cat's life. Antithyroid drugs such as methimazole and carbimazole can cause significant side effects such as jaundice and blood cell disorders. Surgery to remove thyroid tumours is generally very successful and removes the need for ongoing treatment but does carry the risk of a general anaesthetic and potential post operative problems such as hypocalcaemia.
Whatever decision is taken regarding treatment, there will be a need for some ongoing monitoring, particularly to monitor kidney function. The changes in blood flow which occur during treatment can reveal underlying kidney disease, which must be recognised at an early stage. Without this complication, the prognosis with treatment is generally very good for cats with benign thyroid tumours. Malignant tumours carry a much worse prognosis.
Note: I have recently returned from a conference at which a fourth treatment for thyroid disease in cats was unveiled: an ultra-low iodine diet. While this seems an exciting development, many experts are very skeptical as low iodine diets in people tend to produce a different sort of thyroid overgrowth. One of the speakers at the conference asserted with confidence that cats fed on these diets would probably go on to develop nodular thyroid disease within two years.