Hyperthyroidism in Cats

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Hyperthyroidism, also known as "hyperthyroid disease," occurs when the thyroid gland enlarges ​and starts producing excess amounts of thyroid hormone (thyrotoxicosis). In cats, thyrotoxicosis is usually caused by a benign tumor on one or both thyroid glands. Although thyroid tumors can be cancerous, the chances are less than 5 percent in cats. The majority of cats diagnosed with hyperthyroidism are 10 years of age or older.

Signs of Hyperthyroidism in Cats

Things to look out for include:

  • Increased appetite
  • Unexplained weight loss and loss of muscle mass
  • Irritability or nervousness
  • Frequent vomiting
  • Unkempt-looking coat
  • Diarrhea
  • Excessive thirst (polydipsia) and urination (polyuria)
  • Weakness
  • Lethargy

Not all of the following signs will be seen in every cat with hyperthyroidism, but the presence of one or more are indications for you to have your cat seen by your veterinarian

Calico cat drinking water in a dish outside.
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Diagnosis

  • Physical Exam: The veterinarian will palpate your cat's neck area, checking for enlargement of the gland(s). Heart rate and rhythm are also assessed as some cats with hyperthyroidism develop secondary cardiac problems. Your veterinarian will also ask about your cat's appetite and note if there's been weight loss and how rapidly it occurred.
  • Labwork: Hyperthyroidism is diagnosed by measuring elevated blood thyroid hormone levels, specifically thryoxine or T4. However, older cats may have T4 values within the normal range, but still have hyperthyroidism. In those cases, more comprehensive thyroid screening blood work is done. Hyperthyroidism can mimic the signs of other diseases, such as chronic kidney disease (CKD) or liver disease. A complete blood count, blood chemistry panel, and urinalysis can show abnormalities related to these disease processes and are part of the diagnostic work-up.
  • Other tests: As hyperthyroid disease can predispose to other conditions, such as hypertrophic cardiomyopathy (HCM) or hypertension, additional diagnostics such as chest x-rays, an echocardiogram, EKG, and blood pressure are recommended.
  • Results of each of these tests influence treatment recommendations (see below)

Treatment of Hyperthyroidism

There are three main treatment options for hyperthyroid cats: medical therapy, surgery, or radioactive iodine treatment. Each offer a strong possibility of returning thyroid hormone levels to normal values and the prognosis for cats with uncomplicated hyperthyroidism is good. Each treatment plan carries pros and cons and fortunately a caregiver does not usually need to make an immediate decision.

Anti-Thyroid Medication

The most commonly prescribed drug is oral methimazole, which controls the production of thyroid hormones from the affected gland(s).

Most cats tolerate methimazole well, but it will have to be given once or twice every day for life and requires routine blood tests to monitor thyroid hormone levels. The dosage may need to be adjusted over time. Initially, this is the least expensive treatment option, however cost adds up over time. If oral medications are not an option, this medication can be administered as a gel absorbed through the skin, usually along the inside of your cat's ear.

Advantages

  • Non-invasive
  • Relatively inexpensive, initially. Can be expensive in the long term.
  • Only treatment option for cats with kidney disease or hypertrophic cardiomyopathy

Disadvantages

  • Side effects occur in some cats, including vomiting, anorexia, fever, anemia, and lethargy. A rare side effect is an allergy to the drug, presenting as a rash, often on the ears and face. More serious side effects from long-term use include liver damage and bone marrow suppression, although these are rare.
  • Continual veterinary monitoring is needed.
  • The medication does not affect the tumor, which may keep growing.
  • Some cats (and/or owners) cannot handle twice-daily pilling, and the attendant stress can exacerbate other physical problems.

Surgery

Surgery (thyroidectomy) is an effective treatment, but not all veterinarians are experienced with this option and your cat may need to be seen by a board certified surgeon. A radionuclide scan is indicated prior to the surgery to determine the extent of the diseased thyroid tissue and to locate any extraneous thyroid tissue growing elsewhere in the neck (or chest) of the cat, which may contraindicate surgery if if cannot be entirely removed.

Because HCM is sometimes present in cats with hyperthyroidsim, a full cardiac workup is required prior to surgery to avoid complications related to anesthesia and recovery. Also, cats must be treated with anti-thyroid medication for a minimum of 15 days prior to surgery so kidney function can be accurately assessed alongside a normal thyroid hormone level. Hyperthyroidism can mask underlying kidney disease because elevated thyroid hormone increases blood flow to the kidneys. Untreated hyperthyroid cats with kidney disease can therefore have normal kidney values on blood work. When they are treated with methimazole, their thyroid hormone levels to fall to normal and the blood flow to the kidneys is reduced. Cats with kidney disease will show elevations in kidney values once their thyroid hormone level normalizes. A cat with HCM or kidney disease is not a candidate for surgery.

Advantages

  • Eliminates the need for long-term medication.
  • Favored where radioactive iodine therapy is not available.

Disadvantages

  • Surgery may damage adjacent parathyroid glands and affect calcium metabolism
  • If both glands are affected simultaneously, the cat may need two separate surgeries. Likewise, if one gland is initially affected and removed and the other gland becomes affected in the future, a second surgery is necessary.
  • There are normal risks of anesthesia.
  • Recurrence/regrowth of thyroid tissue is possible.
  • Possible development of hypothyroidism (can be treated with thyroid hormone supplementation.)
  • It's more expensive than medical treatment.

Radioactive Iodine Therapy

This is quickly becoming the treatment of choice in areas where it is available, and where caregivers can afford it. A single injection of radioactive iodine (I-131) is given subcutaneously. The substance "finds" and destroys all diseased tissue, including any ectopic (outside the normal area) thyroid cells without harming any normal tissue. The cat must remain in the veterinary hospital for five days to two weeks (depending on state laws) until its radioactive levels are deemed acceptable. Caregivers may be able to visit during that time, but will only be able to view their kitty through a special leaded window.

The cat is also given the anti-thyroid medication for at least 15 days prior to treatment with I-131. As with the surgical option, a cat with hypertrophic cardiomyopathy, kidney disease, diabetes, or any other serious condition is not a candidate for radioactive iodine therapy.

Advantages

  • It provides a permanent cure in 95 percent of cases.
  • It minimizes the stress to the cat.
  • There are no serious side effects and the procedure is very safe.

Disadvantages

  • It's expensive as it costs about the same as surgery.
  • The cat must be in otherwise good health prior to treatment.
  • The subsequent development of hypothyroidism is a possibility (can be treated with thyroid supplementation.)

Warning

Cat feces and urine are considered to be radioactive following radioactive iodine treatment. Different state and federal laws dictate the disposal of cat litter during that period so owners should ask their veterinarians about follow-up care.

If you suspect your pet is sick, call your vet immediately. For health-related questions, always consult your veterinarian, as they have examined your pet, know the pet's health history, and can make the best recommendations for your pet.