Feline Hyperthyroidism

Feline Hyperthyroidism
By Dr. Ed Mapes
Stonebridge Animal Hospital
McKinney, Tx.

Thyroid hormone, thyroxin, controls metabolic rates in cats and other species. When too much thyroxin is produced, the speed of body processes increases and results in the disorder known as hyperthyroidism. This is rather commonly seen in cats over 8 years of age, but rarely in dogs.
A cat’s thyroid (Figure 1) is actually two glands located on either side of the trachea in the neck just below the larynx. The normal glands are not visible and cannot be felt with the fingers, but can enlarge and become evident in hyperthyroid cases.

Figure 1 location of the feline thyroid gland

Figure 1 location of the feline thyroid gland

This condition is now diagnosed much more often than in years past, and differing theories exist as to why. We do know that industrial pollutants containing polybrominated diphenyl ethers (PBDEs). PBDEs are flame retardants commonly used building materials, electronics, home furnishings, automobiles, plastics, foams and textiles. The chemical tends to accumulate in the environment and becomes concentrated in fish and therefore fish-flavored cat foods.
Elevated thyroxin levels and increased metabolic rates affect all organ systems. The kidneys, heart, liver, muscles, nervous, and digestive systems are all over-stimulated, leading in time to a set of characteristic symptoms in cats, including:

  • Increased appetite
  • Severe weight loss
  • Increased thirst and urination
  • Weakness
  • Irritability and restlessness
  • Vomiting and diarrhea
  • Enlarged thyroid gland
  • Rapid heart rate often exceeding 200 beats per minute
  • Dull, dry hair coat
  • Dehydration


The most serious consequences of hyperthyroidism usually involve the kidneys and heart. Elevated blood pressure and levels of metabolic by-products can damage the kidneys, and hyperthyroid cats often present with a urinary bladder infection. In other situations, controlling blood pressure and metabolic rate with treatment of hyperthyroidism can “unmask” kidney disease, and the patient can develop renal failure after thyroid treatment is begun. Veterinarians must monitor kidney function before and after treatment for hyperthyroidism and adjust therapy accordingly.

The over-stimulated heart contracts faster and more vigorously in hyperthyroid cats, eventually causing the heart muscle to increase in size. This results in a condition known as hypertrophic cardiomyopathy, which must also be considered when dealing with these cats. It isn’t unusual to hear a heart murmur when examining these patients.

Veterinarians usually do a urinalysis, radiographs, blood pressure test, and blood testing to assess the body’s functions and detect elevated levels of the T4 hormone (thyroxin) to confirm a diagnosis in suspected cases.

The mode of treatment aims to control the elevated thyroxin levels and ultimately decrease metabolic rates. Since many of these cats have other disorders caused by the elevated metabolism, we also treat those to restore the patients’ overall health.

Drug therapy is usually very effective in decreasing thyroxin levels within 10-14 days. We dose the medication (Methimazole) according to findings of the heart and kidney function and other factors. Surgical removal of abnormal thyroid tissue is another option we sometimes choose, and treatment with radioactive iodine that destroys thyroid tissue is another mode of therapy.

We monitor these patients carefully after treatment is begun, since so many issues are involved. Within a couple of months, though, the difference in these cats can be remarkable. They begin to gain weight, eat and drink less, and even have a dramatic change in demeanor.

A recent patient, Fuzzy Meyer, now sits on her owner’s lap and lets Mrs. Meyer pet her – this is remarkable considering Fuzzy would NEVER allow that before. Mrs. Meyer feels as though she has regained her cat.

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