Feline Hyperthyroidism

Feline Hyperthyroidism, a Treatable Condition

Recent prevalence studies show that around one out of 10 cats aged 9 or older have a common disease called feline hyperthyroidism. While this condition is treatable, it is frequently mistaken for aging, and if left untreated, feline hyperthyroidism will be fatal.

Hyperthyroidism is a progressive disease with a slow, subtle onset, and early signs, such as increased appetite, high levels of activity, and a gradual deterioration in coat and body condition, can be wrongly attributed to aging.

In fact, market research in Europe found that 70 percent of veterinarians believe feline hyperthyroidism is underdiagnosed because pet owners don’t recognize the signs associated with this disease.

Through an interactive risk-assessment quiz, cat owners can identify if their cat may be at risk for this disease and if they should consult their veterinarian to test their cat for feline hyperthyroidism.

The thyroid gland, located on either side of the windpipe in a cat’s neck, produces the hormone thyroxine, which is essential for growth in young animals. In adult animals, this hormone is involved in the control of metabolic rate and red blood cell production, helps control the breakdown of fatty tissues and affects heart rate.

Hyperthyroidism occurs when the thyroid makes and secretes very high levels of the hormone thyroxine. These abnormalities are usually benign and manageable with medication or other treatments.

Common signs of excess levels of thyroxine include:

  • Increased appetite and thirst
  • Weight loss
  • Weakness and poor body condition
  • Hyperactivity or nervousness
  • Breathing difficulties
  • Rapid heart rate
  • Lump in the neck

Feline hyperthyroidism is diagnosed through a thorough physical examination and laboratory tests, including blood samples to measure the levels of thyroxine circulating in the bloodstream. Since thyroxine levels can fluctuate even in hyperthyroid cats, specialized diagnostic imaging may be necessary.

Once the condition is diagnosed, the aim of treatment is to reduce excessive thyroxine levels. The most common treatment options are:

  • Drugs that block the manufacture of thyroxine in the thyroid gland (mercaptoimidazoles)
  • Radioactive iodine treatment for malignant thyroid tumors
  • The removal of one or both lobes of the thyroid gland (surgical thyroidectomy)
  • Iodine restricted diet as sole food source

While medical treatment controls the disease, discontinuing the medication will result in a rapid return of signs of disease. Many anti-thyroid medications come in tablet form and a variety of dosing options, including sustained release, once daily dosing, or two to three times daily.

Side effects of anti-thyroid medication are generally mild (lethargy, inappetence, vomiting) and resolve quickly. Rare, more serious side effects such as liver problems, skin reactions, blood disorders and bleeding tendencies, can also occur.

To learn more about symptoms of and treatment options for feline hyperthyroidism, visit www.hyperthyroidism-cat.com.


Boretti FS, Siber-Ruckstuhl NS, Gerber B, et al. (2009) Thyroid enlargement and its relationship to clinicopathological parameters and T(4) status in suspected hyperthyroid cats. Journal of Feline Medicine and Surgery. 11:286-292. 
Wakeling J, Elliott J, Syme H. (2011) Evaluation of predictors for the diagnosis of hyperthyroidism in cats. Journal of Veterinary Internal Medicine. 25:1057-1065. 
Gallagher B, Mooney CT. (2013) Prevalence and risk factors for hyperthyroidism in Irish cats from the greater Dublin area. Journal of Veterinary Internal Medicine. 27:689. 
Sassnau R. (2005) Epidemiologische Untersuchungen zur Pravelenz der feline Hyperthyreose. Deutsche Veterinarmedizinische Gesellschaft Service GmbH.

Data on file at Merck. (2014). (Market research conducted in UK, France, Spain, Denmark and Italy)