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Feline Hyperthyroidism or “overactive thyroid” is a common condition of middle aged to older cats.

It is occasionally seen in much younger cats.

Feline Hyperthyroidism is usually caused by a benign tumour affecting one or both thyroid glands located within the cat’s neck. A very small proportion of patients will have a more malignant cancerous thyroid tumour. The abnormal thyroid glands produce an excessive amount of thyroid hormone (Thyroxine). Thyroid hormone levels control the body’s metabolic rate with excessive levels resulting in the variety of changes that you or your vet may notice.

Things to look out for:

A wide variety of symptoms are associated with hyperthyroidism. Most affected cats will show some but not all; some cats early in the course of the disease will appear completely normal to their owners but show subtle signs that your vet can detect.

Appetite: the vast majority of hyperthyroid cats have an inappropriately good or excessive appetite. Most “normal” older cats do not need to eat as much as they did in their youth, so while owners are often pleased to see their old cat eating so well, this may indicate a problem.

Weight Loss: most hyperthyroid cats will be losing weight despite eating very well. Early in the disease process, the increase in appetite may be sufficient to make the rate of weight loss quite slow. However, with time weight loss can become extreme and life threatening.

Demeanour/Behaviour: most “normal” older cats are expected to be slowing down sleeping more. Hyperthyroid cats however are often inappropriately bright and active, even hyperactive and restless. They often do not look after themselves very well so may have matted coats and long nails.

Sickness & Diarrhoea: tummy upsets are common and can be severe either from a direct effect of the hormone excess or from over eating.

Excessive Thirst: it is important to note that many of the other diseases commonly seen in older cats will also cause increased thirst and urination.

Heart Problems: heart problems arise from prolonged exposure of heart muscle to high thyroid hormone levels. They are more difficult to detect at home unless very advanced. Initially a very rapid heart rate or pronounced heartbeat may be noticed. Eventually rapid, laboured breathing can develop requiring urgent treatment.

It is important to understand that none of these findings is specific for hyperthyroidism and they all can be seen with other diagnoses.

How do vets diagnose hyperthyroidism?

Regular checks of your older cats will enable your vet to look for clues that they may have developed hyperthyroidism. Sometimes it is possible to feel the thyroid tumour as a small lump (goitre) in the cat’s neck.

The diagnosis is usually confirmed with a blood test looking for the high thyroid hormone levels. Borderline cases may need repeat sampling. The same blood samples can be used to screen for other coincidental problems.

Treatment options

A number of treatment options exist and the choice made will depend on individual circumstances.

Oral Medication: a number of oral medications (both tablets and liquid) are available. They work by reducing the amount of hormone being produce by the abnormal thyroid gland. Careful monitoring (including regular blood testing) and dose adjustment is required to establish the most appropriate dose for each individual patient. Treatment must be life long and it is likely that the required dose will increase over time. Most cats tolerate the medication very well but side effects can occur.

Transdermal Gel: the same drug that is found in the oral medicines is also available as a transdermal gel (Cream to apply to the skin that allows the drug to be absorbed into the blood stream). This can be a useful alternative for fractious cats!

Surgery: it is possible to surgically remove abnormal thyroid glands. This can be curative and may be a good option for fit younger cats but may be less appropriate for older sicker cats. Removing one abnormal thyroid gland may only provide temporary cure because the second gland can become diseased at a later date. The removal of both thyroid glands has a greater risk of complications. The risks and benefits of surgery must be discussed in detail on a case-by-case basis.

Radioactive iodine: thyroid tissue is the only place in the body that accumulates a significant amount of Iodine. Therefore, the administration of a controlled dose of radioactive iodine will selectively destroy the abnormal thyroid tissue. This form of treatment can be arranged at a local referral hospital but requires a period of hospitalisation in very controlled conditions with minimal human contact so is only possible for otherwise healthy patients.

Diet: because Iodine is essential to the production of thyroid hormone, a prescription diet containing an extremely low level of iodine can be used to restrict the amount of thyroid hormone that an abnormal thyroid gland can produce. It is essential that cats on the special diet get nothing else to eat or the iodine restriction will be insufficient; this can be a challenge with cats who gain access to food away from home and in households with multiple cats.