What Is Hyperthyroidism?
The thyroid gland is a gland located in a cat’s neck below the “Adam’s Apple” on either side of the windpipe (trachea) that, among other functions, is responsible for producing thyroid hormone that regulates metabolism. Hyperthyroidism is a disorder in which too much thyroid hormone is produced by one or both of the thyroid glands.
The most common cause is a benign tumor of the gland. The condition is most commonly seen in cats over 8 years old, with the average age at diagnosis 13 years. Less than 3-5% of hyperthyroid cats have a cancerous thyroid growth.
What Are The Signs / Symptoms Of Hyperthyroidism?
The most common signs / symptoms of hyperthyroidism are:
- Weight loss
- Increased appetite
- Rapid breathing
- Increased drinking
- Increased urination
- Unkempt fur
Key Point: The classic sign of a hyperthyroid cat is weight loss despite an excellent appetite.
How Is Hyperthyroidism Diagnosed?
Apart from the signs/symptoms above, your veterinarian may well detect a rapid heart rate (and potentially a heart murmur) and be able to feel that the thyroid gland is enlarged. Cats with hyperthyroidism usually have high blood pressure, but measuring this at the clinic can be an unreliable way to rule in hyperthyroidism, as most cats’ blood pressure will be high when stressed.
Standard blood tests will often show raised liver enzyme levels and a raised thyroid hormone level (T4). A significantly elevated T4 forms the basis of the diagnosis. In cases where the T4 levels are borderline increased, it may be necessary to perform a T3 suppression test which is a series of blood tests that helps to understand how the thyroid gland responds to administration of T3 (the active form of thyroid hormone). Other blood tests exist and scans of the thyroid using radioactive Iodine can also be used to help diagnose this condition.
How Is Hyperthyroidism Treated?
There are a number of options for treatment:
The most common medication prescribed to treat feline hyperthyroidism is methimazole (In the UK and Australia, carbimazole is used, but this is converted to methimazole in the body). This drug acts to block the production of T4 and T3 (the thyroid hormones).
Oral medication is the least expensive form of treatment and no hospitalization is required. However, medication is for life and giving tablets each day to cats can be difficult. Side effects of the drug include vomiting, lethargy and reduced appetite. If these are observed, medication is stopped until the symptoms stop and a reduced dose is then restarted before increasing to the required dosage. Rarely, facial itching can be caused by this drug. Your veterinarian will arrange regular checkups including blood tests to ensure treatment is going to plan.
Surgery can be performed to remove the abnormal thyroid tissue. In most cases, patients will need to be stabilized first via medication to bring thyroid hormone levels down to the normal range and to allow your veterinarian to unmask any potential kidney problems that would make an anesthetic too much of a risk.
The benefit of surgery is that usually it is a permanent solution, but disadvantages include the risk of anesthesia, cost and risk of hypothyroidism and calcium crisis. Calcium crisis is a serious risk for cats that have both thyroid glands removed. Calcium levels are regulated by two small glands attached to the thyroid gland (the parathyroid glands). Blood calcium is usually monitored for a few days after surgery to ensure there isn't a problem. Calcium supplementation (if required) should only be done under strict supervision of your veterinarian and only supplements recommended by your veterinarian should be used.
Treatment by radiotherapy is generally considered the safest and most effective method of treatment for hyperthyroidism. In this method, radioactive iodine is used to destroy the abnormal thyroid tissue. The treatment only needs to be performed once in most cases, but it is necessary to quarantine the patient during the treatment process. Radiotherapy may not be a good idea for a cat with kidney disease.
No matter which path is chosen, it is critical to get treatment underway as early as possible as hyperthyroidism has a major effect on the body. An untreated cat is at risk of heart disease, sudden blindness and sudden death.
Content reviewed by a veterinarian.