Feline hyperthyroidism is the most common hormonal imbalance in cats around the age of thirteen. The condition occurs when the cat’s thyroid gland becomes inflamed and produces excess thyroid hormone. Aside from age, the exact cause of hyperthyroidism had not been determined, although dietary iodine may play a role in influencing cats who are susceptible to the disease. Read on to find out more about hyperthyroidism in cats, its symptoms, and how to treat it.
What Is Hyperthyroidism in Cats
Cats have two thyroid glands in their neck that play a role in regulating their metabolism. Also called hyperthyroid disease, hyperthyroidism happens when the cat’s thyroid glands become enlarged and produce too much thyroid hormone. This can become a problem since the thyroid hormone affects the heart, as well as other organs.
The most common cause of hyperthyroidism in cats is a benign tumor on one or both thyroid glands. Although this growth can be cancerous or a sign of cancer, this is uncommon. Often the enlarged gland in the neck can be felt by a veterinarian and is usually found during the physical exam needed for diagnosis.
Symptoms of Hyperthyroidism in Cats
Initially, the symptoms of hyperthyroidism can be subtle but may become more severe over time. Not every cat will have all symptoms, but if you notice that your cat has one or more of these symptoms, make an appointment with your veterinarian and have your cat checked out.
Hyperthyroidism in cats symptoms include:
- Weight loss accompanied by increased appetite
- Nervous or irritability
- Restlessness or hyperactivity
- Depression
- Increased thirst
- Vomiting
- Increased and inappropriate urination
- Diarrhea
- Vocalizing
- Difficulty breathing
- Rapid heart rate
- Matted or greasy coat
Treatment of Hyperthyroidism
The good news is that, in most cases, this condition can be treated successfully, and most cats can live a long and healthy life. There are many treatment options available, include iodine-restricted diets, medication, radioactive iodine therapy, and, in some cases, surgery. Once your cat has been diagnosed, you and your veterinarian will choose a treatment plan based on factors such as how severe the disease is, your cat’s current health, and what will work best for your family’s lifestyle.
Management with Diet
For some cats, a diet low in iodine is all they need to control their symptoms. To produce the thyroid hormone thyroxine, the cat’s body needs iodine. Since a cat’s body doesn’t produce iodine, restricting iodine decreases thyroxine production. They may also take thyroid supplements for cats to support the normal function of the thyroid glands.
For an iodine-restricted diet to work, your cat must exclusively eat the special diet, with no treats or supplements that might have iodine in them. Since the main use of iodine in a cat’s body is the production of thyroxine, a diet the restricts iodine shouldn’t cause further health issues.
Anti-thyroid Medication
A human drug, Methimazole, is often prescribed for hyperthyroidism in cats. Another option is Felimazole, the only FDA-cleared thyroid medicine for cats. Both drugs work by controlling the production of thyroid hormones by the affected glands. Cats generally tolerate the drug well, but it has to be administered once or twice a day for the rest of the cat’s life. In addition, the cat must have routine blood tests to check on their hormone levels, and, if needed, the dosage will be adjusted. The drug is also available as a gel that can be applied inside a cat’s ear and absorbed through the skin.
Radioactive Iodine Therapy
This treatment option is growing in popularity in places where it is available, however, it is costly. Radioactive iodine is injected subcutaneously, and the substance then finds and destroys diseased tissue without doing harm to healthy tissue. The cat must stay at a veterinary hospital for 3-5 days until their radioactive levels are considered acceptable.
The cat is given anti-thyroid medication for a minimum of fifteen days before treatment. A cat is not a candidate for radioactive therapy if they have kidney disease, diabetes, hypertrophic cardiomyopathy, and other serious conditions.
Surgery
A thyroidectomy, or surgical removal of the thyroid, is an effective treatment. The procedure is usually performed by a board-certified surgeon. A radionuclide scan is done prior to surgery to see how much diseased tissue there is and locate any thyroid tissue growing in other areas in the neck or chest. This may contraindicate surgery if this tissue cannot be removed entirely.
Since hypertrophic cardiomyopathy can be present in cats with hyperthyroidism, a cardiac workup is a must before surgery to circumvent complications related to anesthesia and recovery. In addition, the cat must be given an anti-thyroid medication for at least fifteen days before surgery to normalize their hormone level so that their kidney function can be accurately evaluated.
Because elevated thyroid hormone levels increase blood flow to the kidneys, hypothyroidism can mask kidney issues. Their kidney values will be normal. When their thyroid hormone levels are normalized with medication, the blood flow to the kidneys is reduced, and their kidney values can be more accurately measured.
Diagnostic Tests for Hyperthyroidism in Cats
If you notice that your cat has any of the symptoms above, take them to your veterinarian. They will begin with a physical exam, run blood tests called a total T4. Depending on the results of these tests, other tests may be required.
- Physical Examination – your veterinarian will feel the area of their neck, checking for enlarged glands. The cat’s heart rate and rhythm will be assessed, as secondary cardiac problems can occur with hyperthyroidism. Your veterinarian will ask about your cat’s appetite, if they’ve lost weight, and how fast it happened.
- Blood Tests – measuring blood thyroid hormone levels, especially thyroxine or T4. However, senior cats may have normal T4 values but still have hyperthyroidism. In those instances, additional thyroid screening blood work will be required. Hyperthyroidism sometimes mimics signs of other issues, such as liver disease or chronic kidney disease (CKD). A complete blood chemistry panel, blood count, and urinalysis can find abnormalities related to these diseases and are a part of diagnostic testing.
- Other Diagnostic Tests – because hyperthyroidism can be accompanied by other conditions such as hypertension and hypertrophic cardiomyopathy (HCM), additional tests such as an echocardiogram, EKG, chest x-rays, and blood pressure are generally run.
The results of these tests will influence the treatment recommendations for your cat.
Hyperthyroidism and Kidney Disease
There is a clear link between kidney disease and hyperthyroidism in cats. As mentioned above, if your cat suffers from kidney disease, increased blood flows through the kidneys, helping them function better and mask kidney disease. When left untreated, hyperthyroidism can speed up the development of kidney disease. This means it is important to treat hyperthyroidism, so that kidney disease doesn’t become worse and untreatable.
What Breeds Are Prone to Hyperthyroidism
Any breed of cat can experience hyperthyroidism, especially as they get older. No specific breed is known to have a greater risk of hyperthyroidism, though some breeds such as Siamese, Persian, and Himalayan cats appear to experience the disease less compared with others.
Senior cats – thirteen years or older – have a higher risk of developing hyperthyroidism. Environmental factors may predispose some cats to hyperthyroidism, although the exact mechanism is unknown. No one breed has a greater risk of developing hyperthyroidism, but Burmese, Siamese, Persian, Himalayan, and Abyssinian breeds generally have a decreased incidence of hyperthyroidism than other breeds.
Currently, hyperthyroidism in cats cannot be prevented. Early diagnosis can decrease the incidence of secondary conditions and improve the cat’s prognosis. All cats middle age and older should have a physical examination by a veterinarian every six months to look for evidence of enlargement of the thyroid and symptoms of hyperthyroidism. Annual urine and blood test are crucial for all cats more than six years old to diagnose hyperthyroidism before irreversible damage occurs.
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