Feline Hyperthyroidism

What is it?

Hyperthyroidism is most often caused by a benign growth of the thyroid gland that overproduces the the thyroid hormones. There is no known cause of this disease. It has been speculated that common household chemicals may play a role, that indoor cats are at greater risk, and that certain breeds may have a decreased incidence implying some genetic factors may be involved. Less than 3-5% of aected cats have a malignant thyroid tumor. Typical physical signs include weight loss in the face of a strong appetite, increased thirst and urination, behavior changes, high blood pressure, and heart and organ disease. Blood for elevated thyroid hormone are the most common diagnostic tools. A thorough physical exam can reveal enlarged thyroid glands as well. A confirming diagnosis can be made with a nuclear medicine scan with necessary equipment available in specialized facilities.

How is it treated?

Treatment consists of oral medication, radioiodine therapy, surgery, or a combination of these. The most common oral medication prescribed for hyperthyroidism in the U.S. is called methimazole (tradename Tapazole ® or Felimazole® ). It blocks the production of thyroid hormones, with a 2-4 week induction time before levels will start to lower into the normal range. A more permanent treatment for the disease is radioiodine therapy. Radioactive iodine is administered by injection and travels via the bloodstream directly to the thyroid gland where it destroys the abnormal thyroid tissue. Surgery to remove the abnormal tissue is also an option, but is performed only on cats healthy enough to handle general anesthesia.

What is the prognosis for hyperthyroidism (in cats)?

Whatever the course of treatment you pursue, the outcome is generally favorable if the thyroid levels can be normalized. Complications arise when thyroid levels become too low or when the thyroid gland is completely removed. If concurrent kidney disease is present, treatment of the hyperthyroidism can actually unmask kidney failure. Kidney blood values must be monitored closely before and after treatment. Without treatment, the cat will continue to lose muscle mass and weight, develop high blood pressure, lose kidney and liver function, and more. A personalized treatment plan is important to slow the progression of hyperthyroidism. Talk to your veterinarian regarding the best treatment protocol for your cat.

What symptoms can present as the disease progresses?

Early stages:

  • weight loss
  • increased appetite
  • behavior changes
  • increased energy
  • changes in litter box usage

Late stages:

  • persistent early stages
  • extreme weight loss
  • gaunt appearance
  • increased thirst
  • increased urination
  • constipation
  • heart, liver, and kidney disease
  • severe lethargy
  • sunken eyes
  • rough hair coat/matting
  • loss of appetite
  • sensitive to touch

Crisis – Immediate veterinary assistance needed regardless of the disease

  • Difficulty breathing
  • Prolonged seizures
  • Uncontrollable vomiting/diarrhea
  • Sudden collapse
  • Profuse bleeding – internal or external
  • Crying/whining from pain*

*It should be noted that most animals will instinctually hide their pain. Vocalization of any sort that is out of the ordinary for your pet may indicate that their pain and anxiety has become too much for them to bear. If your pet vocalizes due to pain or anxiety, please consult with your tending veterinarian immediately.

Common Signs of Pain

  • Panting
  • Lameness
  • Difficulty sleeping
  • Pacing
  • Abnormal posture
  • Body tensing
  • Poor grooming habits
  • Tucked tail
  • Dilated Pupils
  • Licking sore spot
  • Muscle atrophy
  • Decreased appetite
  • Vocalizing/yowling
  • Reclusive Behavior
  • Aggressive Behavior
  • Avoiding stairs/jumping
  • Depressed
  • Unable to stand

Download Feline Hyperthyroidism brochure