THYROID DISEASE IN CATS

Thyroid disease is a very common problem in older cats. The two thyroid glands in the neck become enlarged and produce excessive amounts of thyroid hormones. Some cats may have misplaced thyroid tissue in the chest cavity that can be similarly involved. Typically signs develop gradually over a period of months or years in cats of 12 to 13 years of age. Hyperthyroid cats are generally bright and alert but are easily stressed and  may show signs of weight loss (despite a very good appetite), excessive thirst and intermittent vomiting or diarrhoea. Later in the course of the disease severe emaciation and over-loading of the heart occurs which may result in the build up of fluid in the chest and respiratory signs.  However, many of the signs shown may be mistaken for the signs of old age. Other diseases, such as high blood pressure or kidney disease, will be present in up to a third of patients with thyroid problems.

TREATMENT

All cats confirmed with over-active thyroids need treatment for the disease. In many cases, effective treatment will give the cat a normal life-expectancy. There are several options available:-

MEDICAL TREATMENT – The only licensed treatment for thyroid disease in cats is the drug felimazole. This tablet is initially given twice a day for a period of three weeks. After this time a blood test should be taken to determine the thyroid hormone levels and the dose adjusted up or down as required. Most cats require 1 or 2 tablets a day to keep their disease under control although a very few cats may require up to 4 tablets daily. Felimazole is effective in treating all types of thyroid disease and is generally very well tolerated by cats. The tablets are quite small, and most cat owners have few problems administering them to their cats after they have had a few weeks practice.

A small proportion of cats may get side-effects with this drug, usually in the first few months of treatment. In particular, some cats may get stomach irritation and vomiting, liver disorders can occur and, in some rare cases, severe skin sensitivity may occur. For this reason the manufacturer of the drug recommends that treated cats are given routine blood tests 3, 6, 10 and weeks after starting the treatment and every three months thereafter. Blood tests should be taken 6 hours after the morning pill.

One “side-effect” of therapy is that some cats may develop signs of poor kidney function after stabilisation of the thyroids. This is because cats with thyroid disease have high blood pressure, high blood volumes and high renal blood flow that may, over the short term, improve renal function in cats with pre-existing kidney disease. This unmasking of kidney disease may be seen with any form of therapy. If medical treatment is chosen, the felimazole dose rate can be adjusted to leave the cat slightly hyperthyroid to optimise kidney function.

The drug must not be given to pregnant cats and women that are pregnant must not be exposed to the drug. Thus you must not eat or drink while handling the product, you must not break or crush the tablets and you must wash your hands after use. Since the drug is excreted in the urine of treated cats, you must wash your hands after emptying litter trays and women of child-bearing age should wear gloves if they have to handle the litter.

SURGICAL TREATMENT – On the face of it, surgical treatment to remove the over-active thyroid glands would seem to be an ideal option. Unfortunately, cats with thyroid disease have severe volume and pressure over-loading of the heart that puts them at great risk under anaesthesia. This requires that they be stabilised with felimazole tablets for 3 weeks prior to surgery. However, the greatest risk associated with removal of the glands is damage to the parathyroid glands resulting in an inability to maintain blood calcium levels after surgery. This often results in fatalities, usually 2 days post-operatively. After this time, calcium levels usually normalise. The risk of problems with the parathyroids would suggest that it might be best only to remove the largest thyroid gland. However, in over 70% of cases both glands are affected and the removal of only the worst affected gland will often result in the recurrence of clinical signs within months of surgery.

After surgery cats usually do not require any other treatment. However, some cats with thyroid tissue in the chest cavity may show no improvement after surgery. This tissue cannot be easily removed and if a cat remains hyperthyroid after surgery another form of therapy is required.

IODINE 131 THERAPY

A single injection of this short-acting radioactive compound is the treatment of choice for over-active thyroids in human medicine. It is the simplest, safest and most effective form of treatment for feline thyroid disease as well.  The radioactive iodine is taken up by the over-active areas of the thyroid glands and emits beta particles resulting in the over-active areas undergoing regression. Beta particles are able to penetrate through less than 2mm of tissue.

However, the regulations regarding the use of this treatment in domestic pets make it the least used option. The drug can only be used at facilities with the appropriate licences. The pet’s urine and faeces must be treated as radioactive waste after injection with Iodine 131 and the pet must stay in isolation at the facility until radioactivity in the wastes have declined to negligible values. This means that the cat may have to stay for up to 4 weeks and, on occasion, up to 6 weeks after injection. This means that therapy typically costs a four figure sum. We can arrange referral for this therapy if you wish.

THE INTERNET AND THYROID DISEASE -. In many countries,  pets treated with Iodine 131 are allowed home straight away (<country-regionw:ston>Australia/<country-regionw:ston><placew:ston>New Zealand) or after a weeks stay in isolation(many American States). Thus Iodine 131 is commonly used in these countries. Spot-on preparations of felimazole are also commonly used elsewhere. Be warned that much of the information on the Internet concerning therapy of this disease is not appropriate to the <country-regionw:ston><placew:ston>UK.