Thyroid Conditions Part 2: Hyperthyroidism

Gray cat drinks clean water from water fountain

While dogs are more likely to develop hypothyroidism, caused by an underproduction of thyroid hormones, cats are more likely to develop hyperthyroidism, caused by an overproduction of thyroid hormones.

Hyperthyroidism is the opposite of hypothyroidism in many ways and has vastly different symptoms. Dr. Kathleen Aicher, a clinical assistant professor at the Texas A&M School of Veterinary Medicine & Biomedical Sciences, discusses these symptoms and more in the second part of this two-part series on thyroid conditions.

“Thyroid disorders occur commonly in middle-aged to older dogs and cats and, therefore, should be on the radar of pet parents as well as their primary care veterinarians,” Aicher said. “Both disorders can have a significant impact on the health and well-being of patients, particularly untreated hyperthyroidism.”

Hyperthyroidism often appears in cats when excess thyroid hormone is released from either benign enlargement of the thyroid glands or, far less commonly, a cancerous growth. This is rare in dogs, however, which typically only develop hyperthyroidism if their hypothyroidism is being inadvertently over managed, if they develop a tumor that produces thyroid hormones, or if they eat meat containing neck tissue with thyroid glands.

“Patients with hyperthyroidism have weight loss despite eating well or even eating more than usual,” Aicher said. “These patients can also have increases in thirst and urination, be agitated or vocal, and may have gastrointestinal symptoms such as vomiting or diarrhea. Internally, patients with hyperthyroidism can have increases in their heart rate, structural changes within their heart, and hypertension.”

Cats with hyperthyroidism may also appear to have an unkempt haircoat and changes with their body condition, which may be mistaken by their families as changes expected with advanced age. For this reason, annual blood tests for hyperthyroidism are beneficial for all cats once they have reached middle age, or about 7 years old.

“There are four major ways that cats can be treated for hyperthyroidism: daily medication to reduce thyroid hormone synthesis, radioactive iodine that destroys thyroid tissue, surgery to remove the overactive thyroid tissue (thyroidectomy), or exclusively feeding an iodine-restricted diet,” Aicher said.

“It is really important to get hyperthyroid cats to a euthyroid (normal) state to stop the systemic effects that thyroid hormone excess has on their bodies,” she said. “Therefore, definitive treatments, such as radioactive iodine, are preferred, as they have a very high likelihood of curing the cat without the need for a long-term medication or diet change. The downsides to radioactive iodine treatment include a higher initial cost and that treated cats must remain in the hospital for a few days following treatment.”

Because canine hyperthyroidism is usually caused by the dog’s diet or hypothyroidism over management, treatment is usually as simple as adjusting or stopping the dog’s medications or foods.

“In the rare case that a dog has developed hyperthyroidism due to a thyroid hormone-producing tumor, they should be evaluated by a veterinary oncologist to talk more about treatment options for the tumor,” Aicher said.

“Most of the time, thyroid disorder treatments can substantially improve the quality of life for affected dogs and cats,” she said.

If your pet is diagnosed with hyperthyroidism, do not fret; once a treatment plan is established and put into place, most pets will recover from hyperthyroidism and go on to live a long, wonderful life.

Pet Talk is a service of the School of Veterinary Medicine & Biomedical Sciences, Texas A&M University. Stories can be viewed on the web at Suggestions for future topics may be directed to

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