Hyperthyroidism in Your Cat FAQs

In cats with hyperthyroidism, the thyroid gland develops nodules of abnormal tissue, either as a single nodule or multiple ones. These nodules, known as hyperplastic or adenomatous nodules, are areas where the thyroid cells grow abnormally. Over time, regular thyroid tissue changes and becomes hyperplastic (overgrown), eventually forming a thyroid adenoma.

A thyroid adenoma is a benign type of tumor, meaning it does not spread to other parts of the body, so it isn’t as harmful as other forms of cancer. However, as this abnormal tissue continues to grow, especially over a long period, there is a chance it could develop into a more serious form called thyroid carcinoma, which is a malignant cancer. This transformation is more commonly seen in cats who are treated with medications like methimazole for an extended period.

The important thing to remember is that most cases of hyperthyroidism are treatable, and with regular monitoring and the right approach, your cat can live a happy and healthy life!

Common signs of hyperthyroidism include weight loss, even though your cat may have an increased appetite and seem to be eating more. If caught early, the weight loss can be mild but more severe over time. This condition is a frequent cause of weight loss in older cats.

Other symptoms can include vomiting, hyperactivity, increased thirst and urination, diarrhea, and larger stools. Some cats may also lose interest in grooming, and their nails may grow long and become brittle.

High blood pressure (hypertension) is often linked to hyperthyroidism, with about 10-25% of untreated cats affected, though some studies suggest the number may be higher.

Hypertension can seriously impact cats, damaging organs such as the eyes, heart, brain, and kidneys. Eye changes, including bleeding, retinal detachment, and retinal degeneration, can lead to vision problems and even blindness. Monitoring blood pressure in senior cats, especially those with kidney disease or hyperthyroidism, is essential to catch any issues early and prevent complications.

Learn more on hypertension in cats.

Thyroid hormones directly affect the heart, often causing a thickening of the heart muscle, known as hypertrophic cardiomyopathy. This leads to a much faster heart rate; about 20-30% of cats may develop a heart murmur or irregular heart rhythm.

Early diagnosis and treatment can help minimise these effects. However, severe heart disease can develop if hyperthyroidism is left untreated for a long time, or a cat may have pre-existing heart disease alongside hyperthyroidism.

If heart failure occurs, symptoms may include rapid, shallow breathing, open-mouth breathing, loss of appetite, and in some cases, hind leg paralysis due to blood clots.

Hyperthyroidism is diagnosed by measuring thyroid hormone levels in the blood. The most common test is the measurement of Total T4 concentration. An elevated T4 level, combined with typical symptoms, is a strong indicator of hyperthyroidism.

Cats may sometimes show signs of hyperthyroidism, but their thyroid blood test results fall within the normal range. When this happens, additional testing or repeating the Total T4 test later may be necessary to confirm the diagnosis.

The best way to evaluate a cat’s heart is through a heart ultrasound (echocardiogram), which gives detailed information about the heart’s structure and function. Sometimes, an ECG (electrocardiogram), which assesses the heart’s electrical activity, or chest X-rays may also be needed.

At CSS, we are fortunate to have daily access to the largest and most experienced team of veterinary cardiology specialists, ensuring that your cat receives the highest level of care.

Cats with hyperthyroidism are usually older and also at risk for developing chronic kidney disease. Diagnosing kidney disease in these cats can be challenging because the metabolic effects of hyperthyroidism can make kidney function appear better than it really is.

Kidney function is evaluated using blood tests, such as urea, creatinine, SDMA, and urine tests to check urine concentration. However, hyperthyroidism can affect all of these test results.

Thyroid levels need to be normal for at least three months to allow an accurate assessment of kidney function in a hyperthyroid cat.

Hyperthyroidism is a progressive disease that can be life-threatening if left untreated. There are four main treatment options:

1. Medical management: Using medications like methimazole or carbimazole in tablet form or as a transdermal ointment.
2. Radioactive iodine treatment: This targets and destroys overactive thyroid tissue.
3. Dietary management: A low-iodine diet, such as Hills y/d, helps manage thyroid hormone levels.
4. Surgical removal of the thyroid gland (thyroidectomy): This involves removing the affected thyroid tissue.

Only radioactive iodine treatment and surgical thyroid removal offer a potential cure for hyperthyroidism.

Each treatment option has its advantages and disadvantages. The best treatment depends on factors like availability, cost, how easy your cat is to medicate, and any underlying conditions like chronic kidney disease. It’s essential to have a detailed discussion with your vet to determine the best option for your cat.

Anti-thyroid medications work by blocking the production of thyroid hormones, but they don’t fix the root cause of hyperthyroidism, which is abnormal thyroid tissue.

These medications are only effective when given consistently; your cat must take them for life. If you stop the medication, the hyperthyroidism will return quickly.

As the abnormal thyroid tissue grows, it produces more hormones, meaning your cat may need higher doses over time to keep hormone levels stable. Regular monitoring is essential to ensure your cat’s dose is working.

Long-term use of these medications can also increase the risk of the benign thyroid condition turning into a more severe form called thyroid carcinoma, which is harder to treat.

It’s also expected that the thyroid tumor will gradually grow in cats receiving anti-thyroid medications.

Anti-thyroid medications can cause some side effects, but most are mild and manageable. About 25% of cats may experience vomiting or diarrhea, although using transdermal ointment (applied to the skin) can reduce this risk. These issues usually go away on their own without needing to stop treatment.

In some cases, cats may develop itchy skin or crusty patches on their face. While this can be uncomfortable, it’s not too common.

More serious side effects, though rare, include liver problems and bone marrow issues, which can lead to low white blood cell counts or platelet disorders. These are very uncommon but can be detected early with regular blood tests. If these issues arise, your vet will stop the medication.

In rare cases, anti-thyroid drugs can affect the bone marrow and cause anemia, which you might notice as pale gums in your cat. If your cat reacts poorly to one medication (like methimazole), they will likely react the same way to another (like carbimazole), so alternative treatments may need to be explored.

Medication can successfully manage hyperthyroidism for most cats, with noticeable improvements in thyroid hormone levels within 4-6 weeks. However, lifelong treatment is needed with regular monitoring and dosage adjustments.

It’s important to note that long-term use of these medications (more than 2-3 years) increases the risk of the thyroid condition progressing to a more severe form called thyroid carcinoma, which is harder to treat.

Surgical removal of one or both thyroid glands can be a permanent cure for hyperthyroidism in cats. However, this procedure involves general anesthesia, which can pose risks, especially for older cats with conditions like heart disease or other health issues that may be related to their hyperthyroid state.

In some cases, cats may have thyroid tissue in other areas, such as the base of the tongue or within the chest. Unfortunately, this ectopic tissue cannot be removed surgically, limiting the procedure’s effectiveness for those cats.

Surgery is typically considered for cats who cannot undergo radioactive iodine treatment or those who don’t tolerate medication. To minimise complications, removing one thyroid gland at a time is usually recommended in two separate surgeries. However, even with both thyroid glands removed, there is still a risk of the condition recurring. A recent study found a 22% recurrence rate of hyperthyroidism in cats with both glands removed (Covey et al, JVIM 2018).

While surgery can be a good option for some cats, the risks and benefits should be thoroughly discussed with your vet to find the best solution for your cat’s needs.

If you are concerned about your cat, please reach out to our friendly team. Call us on 1300 228 377 or fill out the contact form below. In an emergency, please contact your local vet.

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