• Pet Gazette

Hyperthyroidism – Skinny Hungry Cats


They are seen quite frequently – middle aged cats, once quite plump, that have started dropping weight rapidly even though they eat like a pig. Sometimes, they stop eating because they feel awful. When I see cats like this, I always suspect hyperthyroidism.


Unlike humans and dogs who get HYPOthyroidism, cats get HYPERthyroidism. Thyroid hormones normally function to determine the metabolic rate of the cat’s body. Most often, thyroid tumors are benign, with only 3% to 5% being malignant. Left untreated, hyperthyroidism will shorten your cat’s life and significantly decrease his quality of life. Untreated hyperthyroidism leads to hypertension (increased blood pressure), kidney failure, and heart failure.


Hyperthyroidism is typically a disease of older cats with the average age of diagnosis at 13 years. While many causes have been speculated, including chemicals in the lining of cans used for cat food, nothing has been definitively identified. More than likely this disease is caused by many factors, including environment (endocrine disruptors), diet, and genetics. Additionally, domestic cats live longer today than they did 30 years ago and we test more and catch the development of this disease more frequently.


The clinical signs of hyperthyroidism include weight loss despite a great appetite, a decrease in muscle tone and mass, restless behavior, chronic vomiting and diarrhea, fast heart rate even when resting, and a heart murmur. These signs can mean other diseases, too, so it is important to do a full diagnostic work-up, including blood work, urinalysis and maybe x-rays and an ultrasound. In fact, thyroid levels can be artificially suppressed by other illnesses. If your cat is showing signs consistent with hyperthyroidism but his thyroid levels are normal, your vet will need to rule out other diseases


An overactive thyroid can mask kidney disease, which is also common in older kitties. Once a cat is successfully treated for hyperthyroidism, kidney disease may become apparent. This is why your vet will recommend multiple blood tests — to monitor response to treatment. Cats on therapeutic drugs will need to have regular blood tests to monitor their thyroid levels and make sure that the doses are correct. Hyperthyroidism is typically treated in one of two ways: 1) a twice daily medication that blocks the production of thyroid hormones, or 2) a one-time treatment called I131.


A drug called Methimazole will reduce the production of thyroid hormone. It is given twice daily to cats and is typically well tolerated. Methimazole will not cure hyperthyroidism – it basically is just a band-aid, but it can be used safely for years. Once the drug is stopped, the disease symptoms come back so it is a life-long drug. It is important to routinely check bloodwork to monitor thyroid levels and liver function. Methimazole treatment is the least expensive treatment up front, but over time you may spend a considerable amount of money on lab work and drugs.


I131 Radioactive Iodine therapy is the safest and most effective way to treat hyperthyroidism in an otherwise healthy cat – it can cure the disease with a single treatment. Not only do you avoid those multiple trips to the vet for blood work, but you can you avoid giving a pill to your cat twice daily for the rest of her life. Radioactive Iodine I131 is effective 96% of the time. We refer you to an approved facility to receive an injection of radioactive iodine. During this procedure, your cat will be housed for about one week in a specially-designed facility that protects against exposure to radioactive elements. Side effects are rare, but cats can become hypothyroid and need to have thyroid supplementation if too much of the thyroid gland is destroyed. This only happens about 4% of the time.

The cost of I131 Radioactive Iodine Therapy typically runs about $1500. But when you consider years of vet visits, medications and blood tests, it can be well worth it. Your vet can discuss whether this treatment is advised for your cat. Very old cats and cats with serious disease are not good candidates.

In rare cases, surgery to remove a malignant cancerous thyroid gland may become necessary. But the significant majority of cases can be treated safely and effectively with medication or radiotherapy.

I recommend all cats over the age of 10 years get a senior blood panel which includes testing the thyroid. Catching it early will help protect your cat’s health for years to come.


Dr. Carnohan retired from her career in finance at the age of 50 to become a veterinarian and then graduated from the Kansas State College of Veterinary Medicine. She bought the Cat Care Clinic of Asheville in August 201 and has been practicing there ever since. Dr. Carnohan also helped found Asheville Cat Weirdos.

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