Hyperthyroidism, otherwise known as an overactive thyroid, is a common condition in older cats. Hormones produced by the thyroid glands play an important role in controlling the body's metabolism; however too much thyroid hormone causes the body to go into "over-drive".
Typically the cause of hyperthyroidism is a benign (non-cancerous) change in one or both thyroid glands, however in rare cases this change can be malignant (thyroid adenocarcinoma). Hyperthyroidism is much more common in older cats, with cases rarely seen before 8-9 years of age. The sexes appear to be affected equally and there also seems to be no particular breed predisposition. Many studies have been performed to try to determine if there are environmental factors that may trigger hyperthyroidism. So far, no definitive conclusions as to the cause have been reached.
Many owners mistake the signs of hyperthyroidism for the cat getting older. Typically there is weight loss despite a good appetite and increased food intake. The cat may have personality changes such as increased restlessness, aggressiveness or attention seeking. Some cats can have and increased thirst and urination, however this should always be differentiated from other common conditions such as kidney disease or diabetes. Some cats develop diarrhoea and may occasionally vomit.
The vet may detect an increase in heart rate and other evidence of heart disease such as murmurs or heart rhythm problems. An overactive thyroid may lead to significant changes within the heart, to the point that severely affected cats may develop heart failure. In these cats there is a build-up of fluid in the lungs or chest cavity. Luckily many of the heart changes can be reversed with appropriate treatment if detected in time.
The vet may also check your cat for high blood pressure. If the cat does have hypertension, this may lead to damage to the eyes, brain and kidneys if left untreated. Often, the blood pressure will return to normal once the hyperthyroidism is treated. If not, drugs that specifically lower the blood pressure may be prescribed.
Any cat that is suspected of having hyperthyroidism should undergo a complete physical examination. Ideally blood and urine should be collected for a general profile to check the functioning of vital organs such as the liver and kidneys and to asses for other underlying problems prior to treatment.
Hyperthyroidism is usually straightforward to diagnose. Often the enlarged thyroid gland can be felt as a small lump either side of the wind-pipe in the neck, by your vet. Blood tests that measure the thyroid hormones will usually be elevated. Sometimes if the disease is not very advanced, or the cat is ill for other reasons the thyroid levels will still be within the normal range. The vet may choose to repeat the thyroid test at a later date or detect the problem through the use of other, less commonly used tests.
There are three main treatments for hyperthyroidism. The treatment that is chosen will generally depend on your budget and whether there are any other concurrent problems, for example kidney disease.
A drug called carbimazole (Neomercazole) is used to reduce the production of thyroid hormone, however this is not a permanent solution and the problem will quickly return if the drug is stopped.
Carbimazole is given as a tablet twice daily for the rest of the cat's life. If your cat is difficult to medicate, alternative formulations such as oral paste or liquid or even a gel rubbed into the inside of the ear are available, however these formulations are more expensive and do not necessarily deliver the same level of drugs.
The fact that the treatment is reversible is useful if the reduction of the metabolism causes a worsening of pre-existing kidney disease. In these cases, a smaller dose may be tried.
Oral medication is more available and affordable in the shorter-term than the other treatments for hyperthyroidism. Once the cost of medication over the cat's lifetime plus the cost of ongoing blood testing is taken into account, however, the expense is generally equal to the other treatments.
Oral medications may also cause side effects such as nausea, vomiting and loss of appetite. These usually improve once the dose is reduced, but occasionally are severe enough to mean that the treatment must be stopped. Other problems such as skin irritation, blood and liver disorders are rarely seen, but theoretically may occur.
As previously mentioned, testing of thyroid levels and other blood parameters should be performed on a regular basis.
The overactive thyroid(s) may be removed surgically. This may produce a permanent cure, however one major problem is damage to adjacent glands, called the parathyroids. These glands regulate calcium levels in the body and if function is lost, serious drops in blood calcium levels occur, which may be life-threatening if not treated appropriately. Generally the calcium levels can be stabilised with medication and many cats can be weaned off these drugs over time.
Because cats with hyperthyroidism are often elderly, and may have secondary heart disease, anaesthesia for the procedure can be quite risky. Prior to surgery cats may be given carbimazole to reduce the effects of hyperthyroidism or other drugs may be prescribed to slow the heart rate.
This is the treatment of choice in cats with no obvious kidney disease. It is generally very safe and in 95-99% of cases will produce a permanent cure. If your cat has underlying kidney problems the sudden drop in blood flow through the kidneys once the metabolism returns to normal, may cause a worsening of disease signs. Therefore, it is recommended that carbimazole be prescribed first before considering an irreversible option such as radioiodine.
The small dose of radioactive iodine is given either via an oral capsule or an injection. The iodine is then absorbed by cells within the overactive portion of the thyroid gland(s). The radiation kills off these abnormal cells, leaving the normal cells of the thyroid intact. Thus, following treatment the cats thyroid function usually returns to normal. Some cats will need to be treated a second time, however this is generally less than 5 % of cases. Conversely a few cats are left with under-active thyroids. This is not usually clinically detectable, however if the cat does show signs of hypothyroidism supplementation with replacement hormone is recommended.
Although radioiodine therapy is very safe for the cat, government regulations stipulate that treated cats need to be housed in special facilities until most of the radiation has left the body. This generally means a week in hospital. After that your cat can return to life as normal, although some facilities advise that you not allow close contact with your cat (for example, sleeping in the same bed) for approximately two weeks after treatment.
There are radioiodine facilities in most states of Australia and you should contact your regular veterinarian to determine where the closest facility is if you would like to pursue this option.
Whichever treatment you choose, the most important factor is to have a close relationship with your local veterinarian. He or she can advise you on the best treatment for your cat and answer any questions you may have.