What is Hyperthyroidism?
Hyperthyroidism is one of the most common endocrine disorders seen in older cats. It occurs when the thyroid glands, located in the neck, become overactive and produce excessive amounts of thyroid hormone (thyroxine or T4). This hormone plays a crucial role in regulating metabolism, so an excess leads to an increased metabolic rate, affecting various organs and body systems.
The excessive production of thyroid hormone is most commonly caused by the development of a benign (non-cancerous) tumour in the thyroid gland. Only in a very small number of cases is the enlargement of the thyroid gland due to a malignant or cancerous growth. It is thought that certain dietary imbalances or exposure to thyroid-disrupting chemicals may contribute to the development of disease.
Understanding the signs, diagnosis, and treatment options for hyperthyroidism is essential to keeping your cat healthy.
So, here’s what you need to know to help your fluffy family member…
What Are The Clinical Signs of Hyperthyroidism?
Cats with hyperthyroidism often show subtle signs that typically become more pronounced over time. The most common signs include:
- Ravenous appetite
- Weight loss (despite increased appetite)
- Poor coat condition/unkempt fur (dry, matted, greasy)
- Hyperactivity, restlessness, aggression
- Increased thirst (polydipsia) and increased urination (polyuria)
- Vomiting and diarrhoea
- Increased heart rate (which can sometimes lead to heart murmurs or high blood pressure)
Since many of these symptoms can also be caused by other conditions, such as diabetes or kidney disease, we’d strongly recommend seeking veterinary advice for an accurate diagnosis.
How is Hyperthyroidism Diagnosed?
A thorough vet exam is necessary to confirm hyperthyroidism. The diagnostic process our vets use typically involves:
- Physical Examination – The vet may feel an enlarged thyroid gland in your cat’s neck and check for weight loss and any irregularities with their heart.
- Comprehensive Blood Testing – A high level of thyroid hormone (thyroxine or T4) in the blood is a key indicator of hyperthyroidism. Additional blood testing will be performed alongside the T4 test to assess kidney function and overall health. Sometimes, a cat will have hyperthyroidism, but bloodwork will show a T4 within the normal range. If hyperthyroidism is still suspected, additional diagnostics may be recommended.
- Additional Tests – If required, the vet may recommend imaging studies (such as ultrasound or scintigraphy) to visualise the thyroid glands or further tests to rule out concurrent diseases.
As hyperthyroidism is such a common disease in older cats, we’d always recommend an annual health screening on your cat, including routine bloods and a thorough physical examination. This allows for early detection of disease and the commencement of appropriate treatment promptly to improve your pet’s quality of life.
How is Hyperthyroidism Treated?
There are numerous options for managing and treating the condition, which can significantly improve your cat’s quality of life. The options are as follows, ordered from most common to least common.
- Medication (Antithyroid Drugs) – Daily oral medication can reduce the production and release of T4 into the body. An alternative way of medicating in pets that will not take oral medications, is a topical gel that can be rubbed onto the skin of the ear. This is a non-invasive option but requires lifelong treatment and regular monitoring with health checks and bloodwork.
- Radioactive Iodine Therapy – This is considered the treatment of choice where appropriate and, in many cases, will cure the condition permanently. A capsule containing radioactive iodine is administered orally and selectively destroys overactive thyroid tissue while sparing normal thyroid cells. Due to the handling of a radioactive substance, only a select few facilities/clinics will perform the treatment. Your vet can help you determine if this is a viable option for your pet. If you don’t know where to start, our team are happy to help. Get in touch.
- Surgical Thyroidectomy – Removing the affected thyroid gland can be a solution for some cats, but it carries risks, particularly in older cats or cats with comorbidities. The surgery is relatively straightforward for a healthy patient and has strong success rates for permanent cure. The main risk associated with removing the thyroid glands is accidental damage to the parathyroid glands, which lie very close by and are important in regulating calcium levels.
- Prescription Diet – A specially formulated low-iodine diet can help manage the condition by limiting the thyroid gland’s ability to produce hormones. This approach requires strict adherence to the diet with no other food sources. Research into this option is ongoing and whilst it may be viable for patients that are not good candidates for other treatments, it also may come with adverse effects on overall health due to the restriction of iodine in the diet.
What Regular Testing is Required for Ongoing Monitoring?
Once hyperthyroidism is diagnosed, medication is commenced, and blood and urine testing are repeated at 3 and 6 weeks after starting the medication. This allows vets like ours to assess how your cat responds to the medication and whether they are receiving the correct dose.
Regular blood and urine testing in the initial phase of treatment is also vital for assessing kidney function.
Hyperthyroidism can hide or mask kidney disease.
Higher blood pressure caused by increased T4 levels can increase the blood flow to the kidneys and make it seem like they are working more effectively than they are. Once the thyroid levels are controlled, we may notice kidney changes in bloods that were not previously there.
Once we are satisfied that the dose is correct, the kidneys have been assessed, and the hyperthyroidism is under control, blood and urine testing can be stretched to 3-6 monthly intervals ongoing. At this time, a discussion may be had about whether to continue management with medication or whether to consider radioactive iodine therapy.
With appropriate treatment, most cats with hyperthyroidism live happy, healthy lives. Regular veterinary check-ups, bloodwork, and monitoring of T4 levels and kidney function are essential.
Early diagnosis and treatment can make a significant difference if your cat is showing signs of hyperthyroidism. Contact your veterinarian to discuss the best management plan for your feline companion. If you need a vet, or have some questions, our team are happy to help.
Frequently Asked Questions
It is one of the most common endocrine disorders seen in older cats. It occurs when the thyroid glands, located in the neck, become overactive and produce excessive amounts of thyroid hormone (thyroxine or T4). This hormone plays a crucial role in regulating metabolism, so an excess leads to an increased metabolic rate, affecting various organs and body systems.
The most common signs include:
- Ravenous appetite
- Weight loss (despite increased appetite)
- Poor coat condition/unkempt fur (dry, matted, greasy)
- Hyperactivity, restlessness, aggression
- Increased thirst (polydipsia) and increased urination (polyuria)
- Vomiting and diarrhoea
- Increased heart rate (which can sometimes lead to heart murmurs or high blood pressure)
By a physical examination, comprehensive blood testing, and/or additional tests - If required, the vet may recommend imaging studies (such as ultrasound).
The most common treatments are:
- Medication (Antithyroid Drugs)
- Radioactive Iodine Therapy
- Surgical Thyroidectomy
- Prescription Diet
Blood and urine testing are repeated at 3 and 6 weeks after starting the medication. Once the dose is correct, the kidneys have been assessed, and the hyperthyroidism is under control, blood and urine testing can be stretched to 3-6 monthly intervals ongoing.
