Hyperthyroidism in Cats

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Quick Facts: Hyperthyroidism...

  • Is the most common glandular disorder in cats.
  • Can be found in cats of all breeds and sex.
  • Has an average onset at age 12-13 years (although in rare cases can occur in cats younger than 10).

Clinical Signs Include:

  • Weight loss despite an excellent appetite
  • Excess thirst
  • Restless and needy behavior
  • Intermittent vomiting or diarrhea
  • Urinating outside the litter box

Diagnosing the Disease

During your cat's physical exam, the thyroid glands will be palpated. In normal cats, the lobes of the thyroid gland cannot be felt with your fingers. In a hyperthyroid cat, at least one lobe is usually prominent and may be detected by your veterinarian. A full lab work panel, including a Free T4 by Equilibrium Dialysis, will determine your cat's thyroid function. Findings from the exam and lab work along with noted medical history and clinical signs will help determine a definitive diagnosis. 

Symptoms if Left Untreated

Hyperthyroidism is caused by a benign growth in the thyroid gland that is over-producing T4. Hyperthyroid cats often have a reduced quality of life through weight loss, muscle deterioration, chronic vomiting or diarrhea, heart disease and high blood pressure which can result in heart failure, sudden blindness or sudden death. Good news...all of the above can be prevented with treatment for thyroid disease!

Treatment Options

Radioactive Iodine (I 131)

The gold standard for treatment of hyperthyroidism in cats is a radioactive scan to confirm the disease location and size of the glands followed by a therapeutic dose of Iodine 131. This treatment involves an injection followed by 3-7 days of hospitalization. No anesthesia is required and the treatment is curative. Disadvantages to this option are that the owner is separated from their pet during the hospitalization period and children and pregnant women can have no contact with their cat for 1-2 weeks after therapy. Also during this time, a special flushable litter must be used. This treatment method is not appropriate for cats with kidney or heart failure. I 131 therapy is available locally by referral to the Feline Hyperthyroid Treatment Center of Charleston and is quoted at $975.

Medicating with Methimazole

The most common treatment for hyperthyroidism is a medication called methimazole which blocks the production of T3 and T4. After the treatment has been given for 2-4 weeks, the thyroid levels must be checked and regularly monitored to insure the correct dose is being administered. This option is often popular because the medication is relatively inexpensive and no hospitalization is required. Disadvantages to this method of treatment include the inconvenience and difficulty of medicating every 12 hours. While side effects are uncommon, those that do occur will typically become present within the first three months of treatment. Medicating with methimazole can sometimes unmask or worsen kidney disease. 

Prescription Diet y/d

For the occasional cat who is not a good candidate for routine medication and Radioactive Iodine is financially out of reach, there is a commercial diet available through your veterinarian. Hill's Prescription Diet y/d is reduced in iodine with the idea that excessive thyroid hormone levels can not be produced if there is not enough iodine in the diet to support their production. It is claimed that this diet can normalize a cat's thyroid in 8-12 weeks but must be fed exclusively, meaning no treats, access to other pets' food, or time allowed outside. Once a cat is transitioned to this diet, it is recommended that thyroid levels, kidney parameters, and urine concentration is checked at 4-8 weeks then monitored every 6 months thereafter.