The thyroid glands are small organs found just under the skin, either side of the windpipe in the neck. These glands produce thyroid hormone which helps to regulate your cat's metabolism, or rate of bodily activity. Production of thyroid hormone is normally closely regulated in the body as the effects of too much (or too little) can be very serious. Hyperthyroidism is a disease caused by overactivity of these glands. When the thyroid gland produces too much hormone, your cat's 'internal motor' effectively goes into overdrive. Untreated this would eventually be fatal, but the condition can be successfully treated.

Hyperthyroidism: whole cat ©Mike Martin

Hyperthyroidism appears to be caused by a form of benign cancer in the thyroid gland. However, it is still not clear what causes the cancer to develop. In affected cats the thyroid gland increases in size and starts to produce thyroid hormone all the time without taking heed of the body's normal regulatory messages. In seven out of ten cats both glands are affected. The disease is rare in young cats (less than seven years old) but becomes more common in later life. It is now the most common hormonal disease in middle-aged and older cats.

If untreated, the effects of excess thyroid hormone are seen in many organs. Heart rate is increased and changes occur in the muscle of the heart making it thickened and stiff. This can lead to signs of heart failure, irregular heart rhythms and blood clots may form in the circulation which can damage other organs. High blood pressure is another side effect of the high thyroid hormone levels. Cats with high blood pressure may suffer damage to many other organs particularly the eyes (causing blindness), kidneys and brain. Cats with hyperthyroidism also often have kidney disease - it may be that the two conditions are both just common in the older cats. However, the high blood pressure in cats with hyperthyroidism may actually help to support kidney function and when treatment for hyperthyroidism is instituted the drop in blood pressure may cause kidney problems to get worse.

The first indication that anything is wrong is usually a marked increase in your cat's appetite. Even though your cat is eating more it may lose weight, and its coat may become rough and unkempt. Other changes include restlessness and aggression, body tremors, increased drinking and urinating, vomiting and diarrhea. In about one case in ten the signs are not typical and can include depression, loss of appetite and physical weakness.

Apart from recognizing the disease from your description, there are a number of other steps your veterinarian can take to make a diagnosis. When your veterinarian examines your cat's throat the thyroid gland may feel lumpy or enlarged. Your veterinarian will also want to check your cat's heart - an abnormally fast or irregular heartbeat is often a feature of the disease and may indicate changes in the heart muscle caused by the high hormone levels. If your veterinarian is concerned about your cat's heart they may take an x-ray, perform an ultrasound examination or an ECG. Blood pressure can be measured in the clinic or at home, and a high blood pressure may be an indicator of hyperthyroidism. Blood tests are usually taken to rule out other diseases of the liver or kidneys. Thyroid hormone levels can be measured in the blood but in a few cats the thyroxine levels may be normal although your cat has the disease. Early diagnosis and treatment is important to prevent, and even reverse, damage to the heart and kidneys.

Medical

Advantages

• Simple and does not require an anesthetic.
• Multiple routes of administration (tablet, syrup or transdermal gel).
• Side effects are not common (particularly after the first few weeks of treatment).
• Suitable for cats with severe kidney disease which might be made worse by the other types of treatment and is generally used before other types of irreversible treatment to monitor the effect of reducing hormonal levels on kidney function.

Disadvantages

• Does not tackle the underlying problem and so treatment must continue throughout your cat's life.
• The temperament of some cats is not suitable for daily medication.
• You must remember to give the pills, syrup or gel every day. Gloves should be worn when administering any of these medications.
• In some cats there are side effects of the drug ranging from fatigue and itchy skin to anemia.
• Gloves must be worn if transdermal gel is administered, and you need to avoid touching the area until it is absorbed so this may not be suitable for homes with children.

• In the early stages your cat must be carefully monitored to make sure that the dose of drug is right.

Surgical

Advantages

• This avoids the need for regular medication, because once the gland is removed no more thyroid hormone can be produced.

Disadvantages

• Not suitable for all cats, such as those with severe kidney disease or the very elderly because of the risk of anesthetic.
• Needs a general anesthetic which always carries a slight risk but more so in sick animals.
• Your cat may need drug treatment for a few weeks before surgery to stabilize their condition before anesthesia. The drug treatment will also give an idea of how your cat will be after surgery.
• Possibility of damaging the parathyroid glands, which lie close to the thyroid and control the use of calcium in the body.
• In a few cases a bit of the abnormal thyroid tissue may be growing separate to the main gland and may be missed at surgery.
• After surgery cats should be carefully monitored, including having blood tests, for a couple of weeks to make sure there are no changes in blood calcium caused by parathyroid gland damage.

Radiation

Advantages

• No anesthetic is required as the dose is given by injection under the skin.
• Very few unwanted side-effects.
• A single treatment will permanently cure the disease in nine out of ten cases and most of the rest will be cured by a second treatment.
• Radiation will also work in much rarer cases in which the tumor is malignant or where a portion of thyroid tissue has broken away from the main gland and therefore would not be removed during standard surgery.

Disadvantages

• Availability - treatment cannot be offered at all veterinary practices because of tight regulations covering the use of radioactive substances and there may to be a waiting list at specialist centers.
• Your cat will have to stay in complete isolation until the radiation level has died down, usually around four weeks and so will have to be boarded at the facility.
• Your cat cannot be handled during this time and so this method is unsuitable for cats needing regular treatment for other serious conditions.
• The cost of treatment and prolonged boarding can be high.
• Blood tests are needed at intervals to ensure that thyroid hormone levels remain controlled in the long term.

Dietary treatment

Advantages

• Dietary food is available from veterinarians and is easy to administer at home.
• Effective in most cases with no side effects.

Disadvantages

• Cats must be fed the special diet exclusively - if they get access to other food, eg by hunting or visiting other homes then they will get extra iodine and the food will be ineffective.
• Requires continued, regular monitoring of thyroid hormone levels to ensure disease is controlled.
• Prescription diets can be expensive long-term.

Whatever treatment has been used, it is important that regular monitoring is continued. If your cat is on medication to control their disease they may also need regular monitoring to ensure the medication is having the optimal effect. There may be many changes caused by the high hormonal levels such as heart changes which will reverse over time after treatment. Sometimes permanent damage will have been done before treatment, eg if your cat is blind that will not resolve after treatment, and so there may be long term management for other conditions. Kidney function should be measured regularly after treatment to make sure that function does not deteriorate as hormonal levels drop.

After treatment, signs of hyperthyroidism usually resolve in almost all cats. The decision on which method to choose should be made after careful discussion with your veterinarian. Each method has advantages and disadvantages and not all may be suitable for your cat. There are a number of things to consider before selecting a treatment: your cat's age, the severity of the condition, the presence or absence of other diseases and the risk of complications, etc. Cost may also be a factor as both surgery and radiation treatment can involve a significant expense. However, medication may also be costly in a cat diagnosed with the disease relatively early in its life and treated continuously for several years. Your cat’s temperament should be considered when making a decision about their treatment. If your cat does not allow you to administer oral medication easily, the transdermal gel, surgery, radiation or diet may be easier. Whereas a cat who doesn’t eat well away from home may not be suitable for radiation treatment.

Most cats respond well to treatment, with improvement in symptoms usually seen within a few weeks. Your cat should start gaining weight, become calmer, and show better grooming habits. Regular check-ups and blood tests will be necessary to monitor your cat's response to treatment and adjust medications if needed.

If your cat is taking daily medication, establishing a consistent routine is important. Some cats take pills easily with food or treats, while others might need special techniques. Your veterinarian can show you how to give medication effectively. Keep fresh water available at all times, as treated cats may still drink more. A water fountain or a couple of water bowls maybe helpful. Watch your cat's appetite and weight, and  discuss any changes with your veterinarian.

Contact your veterinarian immediately if your cat stops eating, develops any tremors, seems lethargic, has difficulty breathing, or shows any sudden change in behavior. During treatment, regular check-ups are essential, typically starting more frequently and spacing out as your cat stabilizes. Any recurrence of previous symptoms, such as weight loss despite good appetite or increased restlessness, should prompt a call to your veterinarian.