Hyperthyriodism in cats

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In this condition too much thyroid hormone (thyroxine) is produced by a usually benign (non- cancerous) tumour of the thyroid gland. There are two thyroid glands, one on each side of the neck just below the larynx (voice box). It is often possible to feel these swollen glands in this position when they are affected. Hyperthyroidism is relatively common but the reasons for this are still unclear.

Thyroid hormone controls metabolic rate and consequently the disease produces a speeding up of body functions. The problem occurs in older cats and there is weight loss despite a good or even ravenous appetite. Increasing restlessness with a tendency to irritability and fractious behaviour is usually noticed. Heart rate can be markedly increased and lead on to heart disease and breathing problems. There may also be increased thirst, occasional vomiting and diarrhoea. If the disease is not treated there is progressive deterioration and eventual death.

The disease is diagnosed by taking a blood test to asses thyroxine levels. A blood profile is also carried out to discover whether any other problems such as kidney disease exist so that treatment may be safely planned.

There are three possible methods of treatment:
1. Medical treatment with antithyroid drugs given daily as tablets.
2. Surgical removal of the affected thyroid gland/s (the most usual procedure).
3. Radioactive iodine treatment (available at University Veterinary Schools, Barton Veterinary Hospital and some other specialist centres).

Antithyroid Treatment:

In all cases this is used initially to control the problem. Tablets are needed three times daily for the first two weeks and then usually dropped to twice daily. This can then be continued lifelong, although it may be necessary to increase dosage as the glands slowly enlarge. This method of treatment depends largely on the success of giving the tablets consistently and freedom from side effects which include: vomiting, lack of appetite, blood cell changes, acute skin eruptions, and liver complications. These tablets are also a human medication and for safety reasons should only be handled with gloves.

Surgical Treatment:

This will involve the removal of one or both affected thyroid glands. The operation is complicated by the presence of the small parathyroid glands which are closely attached and sometimes engulfed by the diseased tissue. The parathyroid is essential to the control of blood calcium and must be maintained otherwise severe complications can result. If only one gland is removed there are normally no problems but if both are involved at least one parathyroid must be identified and preserved. If this is not possible the operation may have to be completed without removal of all affected tissue. When both glands are removed it is usual for the patient to be hospitalised for a time and to check blood calcium levels. Generally operations are successful even in older patients and a return to normality is the gratifying result. However if only one gland is involved initially problems can develop in remaining gland after a time needing further treatment it is not usually necessary to give thyroid supplements after surgery.

Radioactive iodine:

In many ways this is the safest and most effective treatment but only carried out at certain specific centres. There are facilities at London, Bristol, Glasgow and more recently Canterbury so availability has improved. Due to radiation regulations the treated patient is not allowed home for 4 - 6 weeks after treatment. This treatment can be of benefit for those cases where there are problems with surgical or medical treatment, for those where anaesthetic risks are considered too dangerous and where owners are prepared for the more extended hospital isolation period, travel and cost involved. Please enquire for an estimate of current costs.


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