Hyperthyroidism in Cats: Signs, Diagnosis and Treatment

Feline hyperthyroidism is one of the most common endocrine diseases affecting older cats.

The condition develops when the thyroid gland produces excessive quantities of thyroid hormone. In most affected cats, this is associated with benign enlargement of thyroid tissue, such as adenomatous hyperplasia or a thyroid adenoma.

Excess thyroid hormone increases the body's metabolic rate and can affect several organ systems, including the heart, kidneys and gastrointestinal tract.

What Are the Signs of Hyperthyroidism in Cats?

The signs can initially be subtle and often develop gradually.

How Is Feline Hyperthyroidism Diagnosed?

Diagnosis is usually based on a combination of clinical signs, physical examination and blood testing.

Measurement of circulating thyroid hormone, particularly total T4, is commonly used when investigating suspected hyperthyroidism.

Some cats with early disease may initially have thyroid hormone concentrations within or close to the reference range. If hyperthyroidism remains strongly suspected, your veterinary surgeon may recommend repeat testing or additional investigations.

Blood pressure assessment, kidney parameters and general blood tests are also important because hyperthyroidism can affect other organ systems and may occur alongside other diseases common in older cats.

What Treatment Options Are Available?

There are several established approaches to managing feline hyperthyroidism. The most appropriate option depends on the individual cat, concurrent medical conditions, treatment availability and practical considerations.

• Medical treatment to reduce thyroid hormone production
• Radioactive iodine treatment
• Surgical removal of abnormal thyroid tissue
• Dietary management in selected cases

Each approach has advantages and limitations. Your veterinary surgeon can discuss which options are appropriate for your cat.

Medical Treatment for Hyperthyroidism

Anti-thyroid medicines reduce the production of thyroid hormones. Two active substances used in authorised veterinary medicines for cats in the UK are thiamazole, also commonly known as methimazole, and carbimazole.

These treatments control excessive thyroid hormone production but do not remove the abnormal thyroid tissue itself. Ongoing treatment and veterinary monitoring are therefore usually required while medical therapy is being used.

Why Is Monitoring Important?

Monitoring is a particularly important part of treating feline hyperthyroidism.

Reducing thyroid hormone levels can reveal kidney disease that was less apparent while the cat was hyperthyroid. This happens because hyperthyroidism can increase blood flow through the kidneys and temporarily alter some of the laboratory measurements used to assess kidney function.

Regular reassessment therefore helps the veterinary surgeon balance thyroid control with the cat's overall health.

Monitoring may include:

• Thyroid hormone concentrations
• Kidney parameters
• Liver-related blood parameters
• Red and white blood cell counts
• Platelet counts
• Body weight
• Blood pressure
• Assessment of appetite, activity and general wellbeing

Possible Adverse Effects of Anti-Thyroid Treatment

Most cats tolerate medical treatment well, but adverse effects can occur.

Possible effects may include:

• Reduced appetite
• Vomiting or gastrointestinal upset
• Lethargy
• Changes affecting blood cell production
• Liver-related abnormalities
• Skin irritation or itching in some cats

If a cat receiving treatment becomes unwell, develops marked lethargy, stops eating, develops facial itching or develops any other concerning signs, the prescribing veterinary surgeon should be contacted.

Hyperthyroidism and High Blood Pressure

Systemic hypertension can occur alongside feline hyperthyroidism and may also be associated with concurrent kidney disease.

Persistently high blood pressure can damage organs including the eyes, kidneys, brain and cardiovascular system, which is why blood pressure measurement is often included in the assessment of older hyperthyroid cats.

Radioactive Iodine Treatment

Radioactive iodine treatment targets abnormal thyroid tissue while generally leaving normal thyroid tissue and other organs relatively unaffected.

For suitable cats, it can provide definitive treatment without the need for ongoing anti-thyroid medication.

Availability is limited to specialist centres and cats usually need to remain within the treatment facility for a period after administration because of radiation safety requirements.

Surgical Treatment

Surgical thyroidectomy involves removal of abnormal thyroid tissue.

Surgery can be an effective treatment in selected cats, but the suitability of anaesthesia and surgery needs to be considered carefully, particularly because hyperthyroidism predominantly affects older cats.

Potential complications and the possibility of abnormal thyroid tissue being present in more than one location should also be considered when discussing this option.

Dietary Management

A restricted-iodine diet may help control thyroid hormone production in some cats when it is fed exclusively.

For this approach to work effectively, the cat must receive no significant alternative source of dietary iodine, which means other foods, treats and hunting may interfere with treatment.

Dietary management is therefore not appropriate for every household or every cat.

What Is the Outlook for Cats With Hyperthyroidism?

Many cats respond very well to treatment and can maintain a good quality of life for years after diagnosis.

The outlook depends not only on control of the thyroid disease but also on other conditions that may be present, particularly chronic kidney disease and cardiovascular disease.

Regular monitoring remains important even after thyroid hormone levels have stabilised.

Frequently Asked Questions