Hyperthyroid is common in older cats and it occurs when the thyroid glands become overactive by producing too much thyroid hormones

Thyroid hormones control many processes within the body and when too much is produced it can cause cats to become very ill. However, many cats that develop hyperthyroidism are treated correctly and make a full recovery.

The Causes

Hyperthyroidism is caused mainly non-cancerous, benign changes. Cats have two thyroid glands and in 7 out of 10 cats both glands are involved as they become enlarged. Although uncommon, in 1-2% of cases a cancerous tumour can be the reason for the change.

The Signs

Although it is more common in older cats, occasionally it is seen in cats younger than 7 years old and it affects male and female cats in the same way.

There are many signs and initially they are subtle but over time they develop and become more severe as the disease becomes worse.

The main signs are:

• Decrease in weight

• Increased appetite

• More active, irritable and restless

• Heart rate increase

• Messy coat

• Vomiting, diarrhoea and look for a cool place to sit

Despite an increase in appetite in most cases, some cats do become weaker and lethargic while also losing their appetite.

Possible Complications

If hyperthyroidism is not controlled it can cause problems with the heart. Heart rate can increase as well as the muscular wall of the heart which can lead to heart failure.

High blood pressure can also become a complication although this is uncommon. However, it can cause problems to the organs such as the kidneys, heart, brain and eyes.

Diagnosing Hyperthyroidism

Typically, the thyroid glands will enlarge although this is not visible so it will require a touch examination. The illness is diagnosed using a blood test that measures the amount of thyroid hormones in the blood. Blood pressure is often checked and an electrocardiogram is carried out to check the activity of the heart. This could also be followed up by an x-ray or an ultrasound if heart disease is present.

Technetium Scan

Some specialist centres are able to offer a technetium scan. This can confirm that hyperthyroidism is present but it will also be able to locate the abnormal tissue which can aid surgery. The scan involves the use of technetium which is a radioactive chemical.

Treating Hyperthyroidism

Cats that are successfully treated will often experience a complete reversal of the signs of the disease. Further tests will be carried out to ensure that the kidneys are functioning correctly.

There are four forms of treatment available:

Drug Therapy

Drugs from a group known as thioamides are used and this includes Felimazole (methimazole) and Vidalta (carbimazole). They come in tablet form and decrease the amount of thyroid hormones that are produced and released. This is not a cure but it is a form of control over the disease. Medication is administered on a daily basis.

This treatment comes with minimal side effects although there can be a loss of appetite, vomiting and lethargy.

Surgery

Surgery can lead to a permanent cure as the affected tissues are removed. This is often successful and can offer a long-term cure in most cats but it can develop again in the future. To minimise the chance of complications surrounding surgery, drug therapy is used to stabilise the disease before surgery takes place. This should also involve the management of heart disease if it is present. There is a risk surrounding damage to the parathyroid glands which are located close to the thyroid glands. These glands control the blood calcium levels and so the cat should be monitored for a few days following surgery to ensure that blood calcium concentrations are correct.

Radioactive Iodine Therapy

This is a safe and effective treatment for the disease. In most cases it offers a cure and requires no on-going treatment. This is given as a single injection and the iodine is then consumed by the abnormal tissue. The radiation destroys the abnormal tissue without damaging the surrounding tissue.

There are no significant side-effects following this treatment although cats are radioactive temporarily. This means that they have to remain hospitalised for a short period. This often cures around 95% of cases but blood tests are taken following treatment to ensure that thyroid hormone levels are correct.

Treatment through diet

A new treatment is managing the issue through a special diet that contains controlled levels of iodine. Iodine is used by the thyroid gland to create thyroid hormones. This means that if there is only enough iodine in the diet to produce normal levels this will help to bring the disease under control.

This is a treatment that is relatively successful although it does require the cat to be fed a therapeutic which will be provided by your vet.

Thyroid Adenocarcinoma

Thyroid adenocarcinoma is rare but this is harder to treat although it can be managed using increased doses of radioactive iodine.