Veterinary medicine professionals frequently say, “Cats are not small dogs,” meaning that while we see cats and dogs as equals as pets, they have different medical needs, metabolize drugs differently, and experience different diseases. This holds true for thyroid disease, which is common in both species, but affects each in opposite ways. WesVet Animal Hospital wants to clear up any confusion you may have about pet thyroid disease by highlighting the differences in dog versus cat disease types.

What thyroid diseases do pets experience?

Dogs: The most common thyroid disease in dogs is hypothyroidism (i.e., a thyroid hormone deficiency) that usually affects large or mid-sized dogs aged 4 to 10 years. Rarely, dogs may develop hyperthyroidism (i.e., a thyroid hormone excess). 

Cats: The most common thyroid disease in cats is hyperthyroidism, which affects middle-aged to older cats. Cats rarely develop hypothyroidism.

What causes pet thyroid disease?

Dogs: Hypothyroidism in most cases is caused by auto-immune thyroid tissue destruction or idiopathic (i.e., unknown cause) tissue atrophy. Rare hyperthyroidism is almost always because of a cancerous thyroid gland tumor.

Cats: Non-cancerous thyroid tumors or tissue hyperplasia account for most cat hyperthyroidism cases. Tumors are cancerous in only 1% to 2% of cases.

What are pet thyroid disease signs?

Thyroid hormones control your pet’s metabolism, so signs may affect nearly every body system, including heart rate, blood pressure, body temperature, nutrient use, weight, digestive transit time, and behavior.

Dogs: Hypothyroidism slows down a dog’s metabolism, resulting in the following signs:

  • Low heart rate or blood pressure
  • Lethargy
  • Dull attitude
  • Heat-seeking behavior and cold intolerance
  • Weight gain or difficulty losing weight
  • Hair loss or skin infections

Cats: Hyperthyroidism speeds up a cat’s metabolism. Elevated thyroid levels generally affect health more negatively, and can be dramatic compared with low thyroid’s more subtle changes. Signs in cats may include:

  • Weight loss
  • Ravenous appetite
  • Vomiting or diarrhea
  • Rapid heart rate or heart failure
  • Coughing or rapid breathing
  • High blood pressure
  • Sudden blindness
  • Anxiety 
  • Increased energy, “acting like a kitten again”

How is pet thyroid disease diagnosed?

Dogs: A basic blood panel that includes thyroid screening can point to a suspiciously low T4 hormone level. Sometimes, T4 levels appear normal in dogs with the autoimmune form, but a special thyroid disease panel with additional tests can help to make the diagnosis. If a thyroid tumor is causing a dog’s hyperthyroidism, additional imaging tests are used to visualize the tumor and determine whether it has spread to other body regions.

Cats: T4 is high on screening blood work in hyperthyroid cats. If levels are normal, a second test measuring freely circulating, unbound hormones (i.e., free T4) can confirm the diagnosis. 

How is pet thyroid disease treated?

Dogs: Hypothyroid dog treatment is relatively straightforward and involves a daily synthetic hormone pill. Blood tests are performed four to six weeks after dose adjustments to ensure T4 isn’t too high or too low. Most dogs respond well to treatment, and once your veterinarian finds the correct dose, blood tests are needed only once or twice per year. Hyperthyroid dogs with thyroid tumors usually need surgery with or without additional cancer treatments.

Cats: Hyperthyroidism treatment in cats can be tricky and must focus not only on lowering thyroid levels, but also on addressing secondary changes, such as high blood pressure or heart disease. Options to control thyroid hormone levels include:

  • Radioactive iodine — Injecting radioactive iodine selectively kills the overactive thyroid cells as they absorb the “poisoned” iodine to try and make more thyroid hormones. This approach is the gold standard, but requires a several-day hospital stay in a specialized facility.
  • Iodine-deficient diet — Without iodine, your cat’s thyroid cannot make the excess hormones, but this works best for mild cases, and only when cats eat solely the prescription diet. They must not chew on grass, have treats, or eat another pet’s food.
  • Daily thyroid suppressant medication — Medications given twice daily as pills or a transdermal cream suppress thyroid function. Medication is required life-long, so this option works well only for dedicated owners who can easily administer medications without causing their cat undue stress.
  • Surgery — Surgery can remove abnormal thyroid tissue, but cats may still need daily medication following surgery to supplement low thyroid hormones. Blood calcium balance can be affected if nearby parathyroid glands are also removed.

Kidney disease is extremely common in older cats, and often occurs hand in hand with hyperthyroidism. However, high thyroid hormones can make kidney blood values appear normal, and often kidney failure is only “unmasked” after thyroid treatment, so the pet may need additional treatments. Medications can address heart function or high blood pressure that do not normalize after thyroid treatment.

Regular wellness visits and annual blood work can screen your pet for thyroid disease. Call us to schedule a visit with the WesVet Animal Hospital team if your pet is due for their wellness examination or has thyroid disease signs, or if you have questions about your pet’s thyroid disease management.