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Hyperthyroidism is typically a disease of the cat caused by the excessive production of thyroid hormones by the thyroid gland. Hyperthyroidism most commonly occurs in older cats (average age 12 years). The excessive thyroid hormones influence most organs of the body and can also increase metabolic rate, therefore clinical signs are variable. The classic presentation of a hyperthyroid cat consists of weight loss despite a ravenous appetite and hyperactivity (restlessness).

Other common signs of feline hyperthyroidism include:

  Senior Cat with Feline Hyperthyroidism
  • Increased drinking and urination
  • Poor hair coat
  • Aggressive/cranky behavior
  • Occasional vomiting and diarrhea
  • Weakness/depression
  • Trouble breathing

Other conditions found with feline hyperthyroidism include hypertension, heart disease, and an underlying renal dysfunction. Chronic hyperthyroidism that is left untreated can be a life threatening condition. A "thyroid storm" is a term that describes a multi systemic disorder due to excessive levels of thyroid hormones (thyrotoxicosis), which often manifests as cardiac failure, fever, delirium, and death, most often seen in untreated cats.

A diagnosis of hyperthyroidism is done via a blood test, which would indicate high levels of the thyroid hormone. The blood panel is also indicated to detect any concurrent abnormalities in a cat's kidneys and liver.

There are four types of treatment for feline hyperthyroidism:

  • Lifelong oral anti-thyroid medication
  • Radioactive iodine
  • Surgical removal of the affected thyroid glands
  • Specialized diet

The oral anti-thyroid medication of choice is called Tapazole. This drug is given twice daily and is continued life-long. Thyroid levels should be measured ever 3-6 months when on this drug to determine control. For pet owners that find it difficult to pill your cat twice daily, there is a transdermal version of this drug, and other options for treatment may be more beneficial as well.

Radioactive Iodine typically cures feline hyperthyroidism with one dose (>95% cases). This treatment is considered gold standard. Ideal candidates are cats with overactive thyroid glands and no underlying disease (heart/kidney). Typically the only disadvantage to this treatment is cost ($1500-3000) and time away from owner (2-14 days in isolation).

Hill's y/d is an iodine-restricted diet formulated to treat feline hyperthyroidism with benefits in as little as 3 weeks. This diet should be the sole source of nutrition for your cat (no treats).

Prognosis is good. Treatment is usually successful and properly treated cats lead normal healthy lives.

Dr. Alia Habhab ("Dr. Alia") has been working at Animal House of Chicago since July 2013. A recent graduate of Ross University Veterinary School, she finished her clinical year at Purdue University College of Veterinary Medicine in May 2013. Dr. Alia has a special interest in exotic animal medicine.