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    With all the news lately about overweight pets, having a cat or dog that's on the slim side may not seem like a cause for alarm. However, if your older cat is losing weight despite having a healthy appetite, make an appointment with the vet, especially if the cat is over 10. The cause may be hyperthyroidism, or an overactive thyroid. In fact, this condition is one of the most common health issues in elderly cats. (If you’re worried about a pet that’s too overweight, follow the advice in our story, Is Your Pet Too Pudgy?) 

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    Hyperthyroidism in Dogs vs. Cats

    While hyperthyroidism (not to be confused with hypothyroidism, an underactive thyroid) is one of the most common causes of weight loss in cats, it is almost never found in dogs, according to Dr. Sandy Willis, DVM, based in Seattle. "We really don't know why cats get it and why dogs don't," Willis says. "And we don't know what causes it. We're seeing it more often these days in cats--probably because owners are taking better care of their pets and bringing them to their veterinarian for wellness exams and at the first sign of problems."

    For cat owners, being aware of the symptoms of thyroid problems is key to keeping your aging pet healthy. "Cats can be second-class citizens among pet owners because of their independent nature," says Willis. "Pets, particularly cats, hide disease. They can be really subtle."

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    What Exactly Is Hyperthyroidism? 

    Hyperthyroidism is a high metabolic state caused by an increased concentration of thyroid hormones. It is usually caused by an increase in thyroid cells that can result in an enlarged thyroid. It may also be triggered by an adenoma (a small noncancerous mass) on the thyroid. (While thyroid issues in pets should be taken seriously, here are 9 Pet Health Myths you can ignore.) 

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    The Dangers of Hyperthyroidism

    In addition to significant weight loss, if left untreated, an overactive thyroid can lead to other serious health problems, including high blood pressure and blindness. Many cats with untreated hyperthyroidism die of heart disease, Willis says.

    The good news is, once identified, hyperthyroidism is treatable and possibly even curable, with an excellent prognosis for an improved quality of life for your pet. 

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    Symptoms To Watch For

    The symptoms of hyperthyroidism can be subtle and can mimic other conditions like diabetes and cancer, as well as many common signs of aging, Willis says. Talk to your vet if your cat exhibits any of the following: (PLUS: 3 Pet Symptoms You Should NEVER Ignore) 

    - Subtle or significant weight loss
    - Increase in appetite
    - Increase in thirst
    - Hyperactivity
    - More frequent caterwauling or howling
    - Pacing and anxiety
    - Vomiting
    - Diarrhea
    - Frequent urination

    On the other hand, early in the disease, hyperthyroidism can also be asymptomatic, Willis says. It's important to bring your pets in for regular checkups, so a vet can assess their health. 

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    Next: What to Do When Your Pet's in Pain

    Diagnosis And Treatment

    Hyperthyroidism can be detected during the routine blood work and urinalysis that is part of your cat's regular vet examination. An enlarged thyroid gland can also be felt during a physical exam. "Your older pet should have a full exam at least twice a year," Willis says. "It's hugely important for senior wellness." (Here’s how you can Help Your Pet Live Longer.) 

    If a high level of thyroid hormone is detected, there are several ways to treat it. The most common, and often the easiest, is a radioactive iodine injection. Your cat will stay in the clinic for a couple of days to recover from the shot, but it is a one-time injection that usually cures the condition.

    Another option is to have the thyroid completely removed. This is less expensive than the iodine injection but not as highly recommended because surgery can be risky for older cats.

    In some cases, it may be better to control an overactive thyroid rather than cure it. "A lot of older cats also have kidney disease, and a hyperbolic state actually helps kidneys," says Willis. Your vet may recommend a drug called methimazole, which decreases thyroid production without harming the thyroid. Owners administer a pill or an ear paste one to three times a day for the rest of the cat's life.

    Once your cat is treated for hyperthyroidism, her health should improve significantly in 1 to 2 weeks. Her appetite will decrease, and so will any hyperactivity. Any weight lost should also come back.

    The most important thing you can do to keep your pet healthy is to stay closely attuned to her normal behavior and appearance--including regularly checking for lumps and masses, just as you would for yourself.

    --Tracy Miller