Hyperthyroidism in cats: Causes, symptoms and treatment

Jul 13, 2022

What is hyperthyroidism in cats?

Hyperthyroidism in cats is a condition caused by the overproduction of thyroid hormone within the thyroid gland. This gland is a butterfly-shaped structure in the center of the neck. This excess thyroid hormone boosts your cat’s metabolism, which may explain a common symptom: weight loss, even though your cat is eating more.

Also called thyrotoxicosis, hyperthyroidism is common in older cats and is often caused by a non-cancerous tumor called an adenoma. It’s more rare, but hyperthyroid disease may also be caused by a malignant tumor called a thyroid adenocarcinoma.

The origins of this disease are unknown, although some current research points to thyroid-disrupting compounds and chemicals in food, carpet, furniture and elsewhere in the environment. Thyroid disease may cause numerous problems since thyroid hormones affect most of the body’s organs and systems.

The good news: hyperthyroidism is treatable in most cases.

What are the symptoms of hyperthyroidism in cats?

The most common symptoms associated with hyperthyroidism in cats include:

  • Weight loss
  • Increased appetite
  • Vomiting

Less common:

  • Diarrhea
  • Hyperactivity
  • Increased thirst
  • Increased urination
  • Changes in fur – pelt looks unkempt, matted, greasy

How is hyperthyroidism diagnosed in cats?

The signs of hyperthyroidism are usually subtle at the beginning. Because thyroid disease predisposes your cat to other more serious conditions, it’s wise to address the signs as early as possible to keep your feline healthy.

Bring your cat to a vet for an accurate hands-on exam. Your veterinarian will begin with palpating your cat’s neck to check for enlargement of the thyroid gland, along with a check of your cat’s heart rate and possibly a blood pressure check as well.

Depending upon what these basic tests suggest, your vet may order a blood panel to analyze:

  • Thyroid hormone levels
  • Kidney enzymes
  • Liver enzymes, and
  • Red and white blood cell count
  • Urinalysis and chest x-rays are also relevant since thyroid disease may affect the kidneys and the heart.

Blood tests for most cats with hyperthyroidism reveal elevated levels of the thyroid hormone, Total T4, but not all do. If your cat’s T4 levels are normal but your vet still suspects the possibility of thyroid disease, further tests can reveal more detailed information.

What is the treatment for hyperthyroidism in cats?

Treatment protocols for hyperthyroidism in cats may include:

  • Medication
  • Radioactive iodine therapy
  • Dietary Therapy
  • Surgery

Medication for hyperthyroidism in cats

Although anti-thyroid drugs do not cure the disease, they can offer short-term or long-term control of hyperthyroidism by reducing the production and release of the thyroid hormone from the thyroid gland.

Daily Treatments

For maximum effectiveness, a lifetime commitment to twice daily oral drug treatment may be required. As an alternative to pills, an anti-thyroid transdermal gel wiped on the cat’s ear twice daily may be an option. When using either medication, you’ll need to bring your cat to your vet for periodic blood tests to monitor for side effects, notably kidney function.

Side effects from medication

Some cats experience side effects from anti-thyroid medication, including vomiting, anorexia, fever, anemia, and lethargy. Cats rarely exhibit an allergic reaction to the medication. Symptoms of an allergic reaction include extreme facial itching. The medications cannot be used in feline patients who are overly sensitive to it.

Radioactive iodine therapy for cats with hyperthyroidism

This form of treatment for hyperthyroidism aims for a cure rather than just management of the disease. Note that all antithyroid medications need to be stopped for at least one week before initiating radioactive iodine therapy.

How radioactive iodine therapy works

The treatment works by injecting the cat with radioactive iodine, which is quickly absorbed into the bloodstream and taken up by the thyroid gland. The emitted radiation destroys abnormal thyroid tissue without harming surrounding glands or tissues.

Does radioactive iodine therapy require anesthesia?

The procedure does not require anesthesia, and no serious side effects are common. Because handling radioisotopes is dangerous to humans, it is only performed in veterinary facilities designed with a special isolation area. It is often performed at veterinary teaching hospitals. Your cat will be hospitalized for 3 – 5 days following the procedure, until the radiation level in their body falls to an acceptable limit. At that time, your cat will be quarantined in a separate ward to minimize risks to other pets and hospital personnel. (No, your kitty will not glow in the dark!)

What happens after radioactive iodine treatment?

After your cat is discharged, isolation from the household is necessary. Contact with the treated cat should be limited and minimal for three weeks following the procedure. A cat that has undergone this treatment will briefly continue to emit low levels of radiation, and will excrete radioiodine in its saliva and urine. As an extra safety precaution, pregnant women and young children should avoid all contact with the cat, litter, food dishes and toys for a full three weeks.

Other adults in the family need to prevent prolonged contact for at least two weeks. This means no lap-sitting, and no sleeping with your cat. These are short term sacrifices, and your cat will forgive you. Resist the urge to pet your treated cat, since the saliva your cat uses to groom and bathe is radioactive for three weeks after the therapy. Cornell University details stringent safety guidelines for post-radiation care, including separate laundering of linens and the consistent use of disposable gloves to prevent contamination.

Because radioiodine is excreted in your cat’s urine, it is essential that your recently treated cat have its own private, exclusive litter box. Also, since radioiodine is also excreted in saliva, do not let your treated cat share food bowls or water bowls with other pets for three weeks.

Dietary therapy for hyperthyroidism in cats

Restricting iodine in the diet may be an option, although this treatment is somewhat controversial because it can be difficult for many pet owners to do. However, if other treatment options are not viable for your pet, discuss dietary therapy with your vet. Prescription foods developed specifically to decrease the production of thyroid hormone by reducing iodine (iodine is needed by the thyroid gland to produce it) are available. If your veterinarian chooses this method, keep in mind that it’s a strict, lifelong regimen: no other foods, treats, or hunted snacks. Even one bite of something that contains iodine can reduce the effectiveness of this therapy. Plan to bring your cat to the vet for periodic check ups following any treatment or therapy.

Surgery for hyperthyroidism in cats

Surgical removal of the thyroid glands, called surgical thyroidectomy, seems to be the most straightforward and permanent approach to addressing hyperthyroidism. However, this approach is rarely chosen by vets. The delicate nature of this surgery may also damage the nearby parathyroid glands, which are crucial to maintaining stable blood calcium levels. To further complicate things, some cats develop thyroid tissue in abnormal areas of the body, such as within the chest cavity. This tissue is impossible to find without highly specialized scans. If it is present, removing the glands in the neck alone will not solve the problem.

What’s the prognosis for cats with hyperthyroidism?

With appropriate therapy, the prognosis is generally good. The time frame needed to see results varies with the treatment, and with the cat. The best studied of these treatments is radioactive iodine therapy. Radioactive iodine therapy is curative within three months of therapy in approximately 95 percent of all thyroid cases, according to Cornell University. Other approaches, including medication, may produce favorable results in a matter of two weeks. Discuss reasonable expectations with your vet.

What other harmful conditions might result from hyperthyroidism in cats?

Address your cat’s hyperthyroidism quickly because the condition may produce other harmful conditions. For example, elevated thyroid hormones can:

  • Speed up the heart rate and produce stronger contractions of the heart muscle. This may cause thickening of the heart’s left ventricle, which, over time, may lead to heart failure.
  • Cause hypertension, or high blood pressure, which is another potential complication of hyperthyroidism. Hypertension can damage the eyes, kidneys, heart and brain. Often, when hyperthyroidism is successfully managed, secondary heart disease and hypertension may resolve without further treatment.

The thyroid gland may indeed seem like a butterfly– delicate and sometimes difficult to tame. However, your vet will recommend the best options for keeping this gland and the vital body functions it affects under control for greater health and longevity of your cat.

Does your cat have hyperthyroidism or do you think she’s exhibiting some of these symptoms? Contact Airvet and get connected with a vet in minutes to discuss your cat’s situation.