Hyperthyroidism: Thyroid Disease in Cats

Laurie Anne Walden, DVM

The thyroid gland produces hormones that regulate the body’s rate of metabolism. The most common thyroid disease in cats is hyperthyroidism, or abnormally high thyroid hormone production. Hyperthyroidism has serious health effects for cats. This disease is rare in dogs.
 
Causes
Hyperthyroidism in cats is usually caused by a benign thyroid gland tumor that produces excess thyroid hormone. Malignant thyroid gland tumors are much less common but do occur in some cats.
 
Hyperthyroidism has become more common in cats over the last few decades. The disease now affects up to 10% of cats over 10 years old.[1] The reasons for the increase in hyperthyroidism are unclear but might include genetics, substances in the environment, or dietary factors.
 
Signs
Thyroid hormones affect many body systems, so the signs can vary from cat to cat. However, some signs are fairly common in cats with hyperthyroidism. Weight loss in a cat with a good appetite is a classic sign of hyperthyroidism.
 
Cats with hyperthyroidism often have other disorders at the same time. Some of the most common are heart disease, high blood pressure, and kidney disease. The symptoms of these other conditions can overlap the symptoms of thyroid disease.
 
Signs of hyperthyroidism include the following:

• Weight loss
• Normal or increased appetite
• Increased thirst
• Increased urine volume
• Vomiting
• Diarrhea
• Rough or unkempt fur
• Agitation or restlessness
• Vocalizing more than usual
• Rapid heart rate

 
Diagnosis
Hyperthyroidism is diagnosed with a blood test for thyroid hormone level. Because thyroid disease is common in cats, many feline blood panels include a thyroid test. Your veterinarian might recommend routinely screening middle-aged and senior cats for hyperthyroidism, kidney disease, and other common diseases. Blood tests can detect some of these disorders before a cat has any symptoms. Your veterinarian is also likely to suggest thyroid testing if your cat is losing weight or has any other signs of hyperthyroidism.
 
In most cats, a test for a single type of thyroid hormone (thyroxine, or T4) is enough to diagnose hyperthyroidism. Sometimes additional thyroid hormone tests are needed for diagnosis.
 
Cats with suspected hyperthyroidism should have other blood tests and urinalysis to screen for conditions like kidney disease. Cats with thyroid disease also benefit from regular blood pressure monitoring. Your veterinarian might recommend imaging studies like radiographs (x-rays), ultrasound, or echocardiography (ultrasound of the heart) to further assess the internal organs.
 
Treatment
The goals of treatment are to return the cat’s thyroid function to normal, keep adverse effects of treatment to a minimum, and avoid lowering the thyroid hormone levels below the normal range (a condition called hypothyroidism). Untreated hyperthyroidism causes significant illness and can be life threatening.
 
Four treatment methods are currently available:

• Radioactive iodine: administration of a radioactive isotope of iodine that destroys overactive thyroid tissue but does not affect normal thyroid tissue
• Medication: daily administration of medication (usually by mouth) that blocks thyroid hormone production
• Surgery: surgical removal of all or part of the thyroid gland
• Diet therapy: feeding a prescription iodine-restricted diet as the cat’s only food

 
Radioactive iodine and surgery can potentially cure the disease. Medication and diet therapy must be continued for the rest of the cat’s life. Each of these treatment methods has benefits, adverse effects, and costs that must be taken into account for each individual cat. The cat’s overall health and other medical conditions also affect the choice of treatment. Discuss the options with your veterinarian so you can make an informed decision that works for your cat and your family.
 
Cats being treated for hyperthyroidism need regular monitoring to assess thyroid hormone levels and check for evidence of other diseases. The type and frequency of monitoring depend on the treatment method and the cat’s condition. Kidney disease is fairly common in cats with hyperthyroidism and sometimes doesn’t show up on laboratory tests until the thyroid function is under control, so kidney function is routinely checked in cats being treated for thyroid disease.
 
For More Information
Feline Hyperthyroidism (American Association of Feline Practitioners): https://catfriendly.com/feline-diseases/hyperthyroidism/
 
Hyperthyroidism in Cats (Cornell Feline Health Center): https://www.vet.cornell.edu/departments-centers-and-institutes/cornell-feline-health-center/health-information/feline-health-topics/hyperthyroidism-cats
 
Reference
1. Carney HC, Ward CR, Bailey SJ, et al. 2016 AAFP guidelines for the management of feline hyperthyroidism. J Feline Med Surg. 2016;18(5):400-416.
 
Photo by Dave Francis

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