Hyperthyroidism in Cats

Early in my career, the veterinary journals had articles about a disease that was initially recognized in cats in 1979. I had not learned about it in veterinary school. That is hard to believe now, as that disease, hyperthyroidism, is routinely screened for in middle age to older cats these days. It is now considered the most common endocrine disease in cats. According to Veterinary Pet Insurance, hyperthyroidism is the fifth most common condition treated in cats.

The thyroid gland is responsible for a multitude of bodily functions by producing thyroid hormone. It regulates the body’s metabolism; if it produces too much or too little hormone, a cascading effect on health occurs. Cats that are hyperthyroid present to the veterinarian with a myriad of symptoms that may include weight loss, diarrhea, vomiting, increased appetite, increased thirst, poor hair coat, and restlessness. On the other hand, a few cats will have decreased appetite and lethargy. The average age of diagnosis is 13, but it can be much younger. It seems Himalayans and Siamese have a decreased risk of the disease.

The severity of the disease varies greatly from cat to cat. Many cats these days have milder symptoms at the time of diagnosis due to routine screening on wellness visits. Physical examination of affected cats may reveal a low body condition score, increased heart rate, heart murmur, muscle wasting, and an enlarged thyroid gland. The earlier the disease is diagnosed and properly treated, the better the outcome. If problems resulting from hyperthyroidism, such as heart disease, high blood pressure, or weight loss, are advanced, the damage done may be irreversible.

A cat is diagnosed with hyperthyroidism based on signs and blood tests. An elevated serum thyroxine level ( T4) is indicative of the disease and is a screening test that is often run in combination with routine senior blood work recommended for all cats. When a cat is sick, the T4 may be appear normal due to the illness; in these cases more advanced thyroid screening may be required.

A thyroid hormone secreting tumor of the thyroid gland is the cause of hyperthyroidism. In most cases, these tumors are benign adenomas. Thyroid cancer is rare in cats. The thyroid gland has two lobes. In the majority of cases, both lobes are affected. While it is not known why the disease develops, there have been a number of theories including the iodine in canned cat food and the fire retardants used on household fabrics. In my personal experience, neither of these explanations are probable for my own cat that developed hyperthyroidism. Barney, the barn cat, developed thyroid disease at the ripe old age of 15 or 16. He lived in the barn, coming in the house about once a year for a 24 hour nap- which leads to minimum exposure to fire retardant chemicals. He also ate a diet of rodents, supplemented by dry cat food. Oh, well.

The good news is that there is treatment for this disease. The most recent advancement is Hill’s Prescription diet Y/D. This diet controls the disease by severely limiting the amount of iodine that feeds the tumor. The thyroid gland requires iodine to function. By restricting the amount of iodine the gland gets, the amount of hormone it produces will be limited. When choosing this option, it is imperative that the cat only gets this food. Any cheating and it is not going to be an effective treatment. I have found this to be a good option for an only pet household. Follow up for these patients include regular blood monitoring to ensure control of the disease.

The gold standard for treatment is radioactive iodine treatment. I-131 is administered to the cat by injection. The chemical destroys functional thyroid tumor tissue without harming other tissue.. Most cats normalize after one treatment. This treatment is performed at specialty centers. State law varies how long the cat must be hospitalized after treatment. The cat’s urine is slightly radioactive for up to 14 days afterwards, so must be handled with care. Most cats have normal thyroid function after one treatment and no further treatment is needed.

One of the most common ways to treat hyperthyroidism is with oral medication. While medication is not a cure, it can manage the disorder well for an extended period of time. Methimazole is the drug most commonly used for this. Once hyperthyroidism is diagnosed, the medication may be started at a low dose to avoid the occasional gastrointestinal upset. The dose may be increased over time depending on T4 blood results and signs. While this is effective management, the tumor may increase in size and the dose of medication needed may need to be increased as well; regular blood testing is needed.

Surgical intervention is not commonly recommended for treatment. When removing the thyroid glands, it may cause the cat to become hypothyroid and require medication for that. More seriously, surgery also risks the parathyroid glands which regulates calcium metabolism.

In conclusion, hyperthyroidism should be on the radar for every older cat during a wellness examination. Annual blood testing is a good way to catch this problem as well as others early, before the advancement of the disease causes irreversible damage.