Thyroid disease is a frequently diagnosed problem in dogs and cats as they age and results in a myriad of clinical signs that affect the entire body. The thyroid is an endocrine gland that produces hormones that affect metabolism and regulate body functions. Normal body temperature, heart rate, weight, behavior, immune function, and organ health are largely dependent on normal thyroid hormone levels, and pets with levels that are too high or too low can have changes in these functions. Thyroid problems affect people, dogs, and cats differently, which can be confusing for pet owners, so the Town and Country Animal Hospital team is answering your frequently asked thyroid disease questions. 

Question: What’s the difference between canine and feline thyroid disease?

Answer: Dogs and cats can develop thyroid disease in middle or older age, but each species has different disease profiles. Dogs typically develop a thyroid hormone deficiency (i.e., hypothyroidism) caused by autoimmune or idiopathic (i.e., unknown reason) gland destruction, while cats develop a thyroid hormone excess (i.e., hyperthyroidism) caused by a benign tumor or tissue overgrowth. Rarely, tumors can cause hyperthyroidism in dogs or hypothyroidism in cats.

Q: What are thyroid disease signs in cats?

A: Cats experience a thyroid hormone excess, which speeds up their metabolism. With every body system working faster and harder, cats can show the following signs:

  • Weight loss
  • Ravenous appetite
  • Increased heart rate or heart failure
  • High blood pressure
  • Sudden blindness from retinal damage
  • Vomiting or diarrhea
  • Anxiety or other behavioral changes
  • Increased energy and activity levels

Q: What are thyroid disease signs in dogs?

A: In contrast to cats, dogs suffer from thyroid hormone deficiency, which slows down their metabolism and body functions. Signs in dogs can mimic the normal aging process, but in fact are the result of an underlying disease. Hypothyroid dogs may:

  • Gain weight or have trouble losing weight
  • Have a normal or decreased appetite
  • Feel cold and seek warm places in your home
  • Have changes in their hair, skin, or nails
  • Have chronic infections
  • Have decreased energy and activity levels
  • Develop constipation
  • Have an abnormally low heart rate

Q: How is thyroid disease diagnosed and monitored?

A:  A blood test measuring the main thyroid hormone can detect high or low levels in your pet. This hormone, called T4, is often bound to other substances in the blood, which can mask abnormal levels. If your veterinarian suspects thyroid disease but total T4 is normal, they may order a “free” T4 level to confirm. Your veterinarian also can check the thyroid stimulating hormone (TSH) level in dogs, which signals to the thyroid to produce more hormone. Hypothyroid dogs produce excess TSH as their bodies try to coax the thyroid gland into producing more hormones.

Q: How is thyroid disease treated in cats?

A: Owners of hyperthyroid cats have several treatment options, including:

  • Radioactive iodine treatment — Injected radioactive iodine selectively kills abnormal thyroid cells and normalizes hormone levels four to six weeks after treatment. This is the ideal treatment for most cats, requiring only a single injection followed by a three- to four-day hospital isolation stay.
  • Iodine-deficient diet — The thyroid gland needs dietary iodine to manufacture hormones, so depriving the body of iodine can reduce production. This method is best for mild to moderately affected cats and requires strict compliance without any outside food or treats.
  • Daily medication — Thyroid-suppressing medication can reduce hormone production, but only does so temporarily, so medication must be given by mouth or transdermally twice per day, indefinitely.
  • Surgical removal — Complete thyroid gland removal can resolve severe disease not responsive to other therapies, but cats often require daily thyroid hormone supplementation after surgery. If the parathyroid glands also are removed, treated cats may need medications to control calcium balance as well.

Q: How is thyroid disease treated in dogs?

A: Treatment for dogs is simpler, with a twice-daily pill to replace their missing thyroid hormone. Finding the right dose can take several months, but most pets respond well and have complete symptom resolution with this treatment. Medication must be given each day for the remainder of the dog’s life to remain effective. 

Q: Is my pet’s thyroid medication the same as mine?

A: Never share your medication with your pet. While treatment for thyroid disease in people is similar to pets, drug doses are vastly different. Thyroid replacement doses for dogs are much higher than for people, and cats cannot properly metabolize some of the thyroid-suppressing drugs used in people. 

Q: Does my fat dog or thin cat have thyroid disease?

A: Possibly, but losing or gaining weight is a sign of many other diseases in pets. Obesity is common in dogs and cats, and usually a result of improper nutrition, inadequate exercise, or genetic factors that predispose them to weight gain. Weight loss occurs frequently in older cats suffering from kidney failure, heart disease, cancer, diabetes, or intestinal disease. Your veterinarian can determine the best diagnostic tests if you have concerns about your pet’s weight.

Thyroid disease signs can be vague or subtle in the early stages, but your vigilance as a pet owner can help us diagnose the problem sooner. Contact the Town and Country Animal Hospital team if you notice changes in your pet’s weight, appetite, or energy level so we can determine if thyroid disease is to blame. Ask your veterinarian whether annual or semi-annual blood tests and blood pressure screenings to detect early thyroid disease may be beneficial for your aging pet.