Hyperthyroidism in Cats: Signs, Tests and Treatment Options

What hyperthyroidism is

Cats have two small thyroid glands in the neck that make hormones controlling how fast the body burns energy. In hyperthyroidism one or both glands enlarge and overproduce hormone, so the cat's metabolism runs too fast. Cornell Feline Health Center explains that the enlargement is usually a benign growth; according to VCA Animal Hospitals only about 1 to 2 percent of cases are cancerous.

It is overwhelmingly a disease of middle-aged and senior cats. VCA puts the average age at diagnosis around 12 years, with relatively few cats affected before age 10. The exact cause is still not known. Researchers have looked at diet, iodine levels and household chemicals such as flame retardants as possible contributors, but none has been proven, and there is currently no known way to prevent it.

Signs of hyperthyroidism in cats

The most common signs are weight loss despite a good or even ravenous appetite, increased thirst and more urine in the litter box. Many cats become restless, hyperactive or irritable, and some start yowling loudly at night. Vomiting and diarrhea are common, and the coat often looks unkempt, greasy or matted because the cat grooms less.

The changes usually creep in over months, so owners often put them down to old age, or are pleased that an elderly cat suddenly seems lively and hungry. Some cats eventually lose their appetite and become weak instead. A fast heartbeat is common, and your vet may feel an enlarged gland in the neck. None of these signs on its own proves the thyroid is the problem, because diabetes and kidney disease can look similar.

When to call your vet today

Book an appointment within a few days if an older cat is losing weight, eating noticeably more, drinking more, peeing more, vomiting regularly or acting unusually restless. Weighing your cat at home on a bathroom scale, by weighing yourself holding the cat and subtracting your own weight, can help you notice a trend before it is obvious.

Call the same day if your cat stops eating, is vomiting repeatedly, seems weak, or suddenly seems not to see well, bumping into furniture or with dilated pupils. VCA notes that about 1 in 5 hyperthyroid cats has high blood pressure, which can damage the eyes, and sudden blindness from a detached retina is a reason to be seen promptly.

When it's an emergency

Go to your vet or the nearest emergency vet immediately if your cat is breathing fast or with effort, breathing with its mouth open, or collapses. Untreated hyperthyroidism can thicken the heart muscle and, as Cornell explains, may lead to heart failure.

Sudden blindness, a seizure, or a cat that becomes very weak and stops eating and drinking also need urgent care. These are not things to wait out overnight. Phone ahead if you can, so the emergency team is ready, and bring any recent test results or a list of medicines your cat already takes.

What you can safely do at home

Do not try to treat suspected thyroid disease at home or with supplements, and never give a cat human thyroid or heart medicine. The right treatment depends on test results, and some medicines that are safe for people are dangerous for cats. Keep fresh water available at all times, since these cats are genuinely thirsty, and scoop the litter box more often to keep up with the extra urine.

Note your cat's appetite, weight, thirst, vomiting and any night yowling for a week or two before the appointment; that record is useful to your vet. If your cat is already being treated, give everything exactly as prescribed and do not stop or change it without talking to your vet, because the signs return when treatment stops.

What the vet will likely check

Your vet will weigh your cat, listen to the heart, feel the neck for an enlarged gland and measure blood pressure. The key blood test is total T4, the main thyroid hormone, which is raised in most hyperthyroid cats. Borderline cases may need a repeat test a few weeks later or additional thyroid tests. General blood work and a urine test check the kidneys, liver and blood sugar, because other conditions often show up in older cats at the same time.

There are four main treatment routes, and your vet will help you choose. Daily medication controls the disease for as long as it is given and needs regular blood checks. Radioactive iodine, where available, cures most cats with a single treatment but needs a hospital stay. Surgery to remove the gland is possible but less often chosen. An iodine-restricted prescription diet can control it in some cats, but it must be the only food they eat. Kidney values are rechecked once thyroid levels come down, because treatment can reveal kidney disease the high hormone level was hiding.

Prevention and long-term outlook

Because the cause is not known, hyperthyroidism cannot be prevented, but it can be caught early. VCA recommends yearly blood and urine tests for cats over 7, and a physical exam every six months for middle-aged and senior cats. Catching it while the cat is still in good body condition, before the heart, eyes or kidneys are affected, gives the best results.

With treatment the outlook is generally good, and most cats regain weight and return to their normal selves. Complications in the heart or kidneys can make things more complicated, which is another reason not to wait. Keep up with the follow-up blood tests your vet recommends, since the dose or plan often needs adjusting over time.

This page is general information, not veterinary advice, and it cannot replace an examination by the vet who knows your pet.