Also known as overactive thyroid, goitre or thyrotoxicosis, feline hyperthroridism is a common condition in older cats where excessive levels of thyroid hormones are produced from the thyroid gland which has become enlarged.
Why Are Thyroid Hormones Important?
Thyroid hormones act on nearly every cell in the body and are responsible for increasing the basal metabolic rate i.e. the amount of energy per unit time that a cat needs to keep the body functioning at rest.
Elevated thyroid hormone levels increase the metabolic rate above normal and so the cat requires more energy to maintain normal bodily functions.
Is Feline Hyperthyroidism Common?
Recent studies have shown a 9% of cats over the age of ten will develop the condition.
What Causes Hyperthyroidism In Cats?
In most cases, enlargement of thyroid glands stems from a non-cancerous tumour called an adenoma.
Rarely it is caused by malignant tumours known as thyroid adenocarcinomas.
The factors that cause these tumours leading to hyperthyroidism are unknown as yet.
Why Should I Be Concerned About It?
Thyroid hormones affect nearly all of the organs in the body; therefore, thyroid disease often causes secondary problems with blood pressure, maintenance of bodyweight, heart disease and kidneys to name some common side effects.
What Are The Signs Of Feline Hyperthyroidism?
A history is often suggestive and some common clinical signs noted by owners might include:
- Increased appetite
- Increased urination and thirst
- Poor coat quality or over-grooming
- Weight loss
How Is It Diagnosed?
History and clinical exam by a veterinarian will raise the suspicion for the condition.
- A palpable goitre (swelling the neck related to an enlarged thyroid gland) can sometimes be felt.
- A heart murmur and high heart rate are sometimes detected as well an elevated blood pressure.
- A blood test is required to measure the thyroid hormone T4 in the blood – an elevated value is always significant.
- Kidney function will need to be checked on bloods at this time too
If a heart murmur is present a heart scan or echocardiogram is advised to diagnose the nature any heart disease that may be present.
What Are The Treatment Options?
Thankfully we have a range of treatment options from medical and surgical treatment to radioiodine therapy, which will suit most cats and owners.
Medical management is always started and drugs called thiamazole or methimazole are used to inhibit the production of thyroid hormone in the thyroid gland and acts to normalise levels within the body.
Regular blood tests are needed every 3 weeks initially to ensure that the correct dose is being administered and the thyroid hormone levels are normalising. Medical therapy can be continued for the lifetime of the cat if needed.
Once stabilisation has been achieved surgery or radioiodine therapy can be considered and a cure of the condition is possible.
What Are The Risks Of Treating The Condition?
There are no specific risks but about a third of cats have been shown to have an underlying renal (kidney) disease that only becomes apparent once the thyroid function has been brought under control. Regular blood tests will show this once treatment has begun.
Medical therapy is usually well tolerated but sometimes vomiting and inappetence can be seen. These effects are usually transient.
Surgery to remove the thyroid gland will result in the need to supplement thyroid hormone with oral medications. It also carries the risk of damage to the parathyroid gland which regulates calcium levels in the body.
Radioiodine therapy requires radiation therapy and so cats will be radioactive for a period of time following treatment. There are relatively few side effects but they usually remain in hospital for a number of weeks. Only a small percentage of cases fail to achieve a return to normal thyroid function.