Hyperthyroidism is a condition in which changes within the thyroid gland result in excessive concentrations of thyroid hormone within the circulation. The glands are located in the neck to the side of the windpipe.
The changes within the gland are mostly benign and is referred to as a thyroid adenoma. This is a form of tumour, but is benign in its own right, but problematic in its effects upon the body. Occasionally cases may result from a thyroid carcinoma. This is a far more malignant tumour which means that it may spread elsewhere in the body as is the case for other forms of malignant cancer. It must be stressed that only 2-5% cases are carcinomas.
Whilst many cases involve only one thyroid gland, some 70% of cases are bilateral, but do not necessarily present at the same time.
The hormone itself is responsible for controlling the bodyʼs metabolism, i.e. how quickly the body works. Excessive amounts of the hormone result in the metabolism speeding up, and many different effects may be seen.
Some of these cats are described as being Morris Minors that turn into Porsches! They are usually mature cats whose level of activity may change and from being quite peaceful and inactive, they become restless. They may eat far more than usual and yet lose weight and body condition. Their coat appearance may become quite poor and they may start to look quite gaunt.
In terms of changes within the body, one major problem is that the heart muscle becomes thicker than normal. The volume available for pumping blood becomes less and consequently the speed of pumping increases to accommodate the change. This may ultimately lead to heart failure as the heart cannot adequately pump the required volume of blood and fluid may accumulate around or in the lungs lead to problems with breathing. Another follow on effect may be a loss of rhythm which compounds these problems and makes treatment very difficult.
We may also record elevated blood pressure. This may be detrimental to the general well being and mood of the cat as well as potentially accelerating kidney damage, and potentially leading the bleeding from the retina which may blind the cat.
Kidney function often suffers due to a combination of the described effects. Occasionally, whilst hyperthyroidism may be detrimental to the kidneys of the majority of cats, just occasionally the elevated blood pressure and increased blood flow actually maintains kidney function when it would otherwise deteriorate. It must be emphasised that this only occurs in the minority of cats.
Diagnosis is based upon the clinical signs when we examine patients, especially identification of a ʻgoitreʼ an increase in the size of the affected thyroid gland. Blood samples are used to identify elevations in the thyroid hormone, and to assess the function of the kidneys and other organs.
Treatment is usually medical in order to stabilise these patients and improve their heart function. Once this improves, surgery may be considered.
Anti-thyroid drug therapy
There are three drugs now available. Two of these are the same but one is a slow release version of the other. The medicine is not a cure but once control is achieved, it may be used as long-term control. Side effects are not common but occasionally do occur, generally with reduced appetite and vomiting. The drugs may also occasionally produce problems with white blood cell counts, platelet counts, and liver disorders.
Long-term treatment usually requires 1 or 2 tablets per day, whilst initially treatment usually requires three times daily dosing, although the new slow release version has improved the dosing frequency which can be a huge issue for getting tablets into cats. Regular monitoring of thyroid levels may be very beneficial as well as monitoring of kidney function.
Surgery may provide a cure for hyperthyroidism. Generally we do need to control the signs of hyperthyroidism mainly with respect to heart function which may make anaesthesia challenging. The greater the control of the condition, the better the patient may deal with the procedure.
As stated, the condition is ultimately bilateral in many cases but only one gland may be enlarged at surgery. Removal of both glands simultaneously increases the chances of inadvertent damage to the parathyroids, 2 very small glands which are part of the thyroid. If these glands are permanently damaged, the calcium levels within the bloodstream may fall dangerously low. In some cases these glands recover their blood supply, whilst others require long term calcium supplementation.
Nevertheless, removal of both glands is successfully performed relatively frequently. Alternatively, the glands may be removed in staged procedures. We can discuss the advantages and disadvantages with you prior to your decision. It would however be fair to say that surgically managed patients not only avoid the need for tablets but do generally make far better clinical progress.
This is a very successful treatment that many humans may receive but is not widely accessible. Its disadvantage is that it requires your cat to be isolated at a treatment facility for around 3 weeks. The number
of facilities available for this treatment is also very limited.
For more information, please contact the surgery. Other sources of information include web pages at www.fabcats.org