Is Your Cat Always Hungry? It Might Be Hyperthyroidism

By Jake Stokes, DVM
angell.org/generalmedicine
generalmedicine@angell.org
617-522-7282

April 2025

X

X

X

The thyroid gland is located in the neck of our pet dogs and cats and produces two hormones that play a vital role in regulating their metabolism, growth, and development. These hormones impact our pet’s heart rate, blood pressure, body temperature, nervous system, bone health, and muscle function. It is also the organ responsible for the most common endocrine disorder in cats: hyperthyroidism. Hyperthyroidism affects approximately 1 in every 300 cats. Hyperthyroidism is characterized by an overactive thyroid gland, thus resulting in an increased metabolic rate. This results from a growth on the thyroid gland that produces too much thyroid hormone. Fortunately, most of these growths are benign instead of malignant. However, it is still essential to recognize the clinical signs of hyperthyroidism so a diagnosis can be made and treatment can be initiated.

Hyperthyroidism is a disease of middle-aged to older cats. There is no sex or breed predisposition. Therefore, any cat can risk developing hyperthyroidism as they age. The clinical signs can be vague in the early stages of the disease, though they can progress as time elapses. The most common clinical sign owners notice is rapid weight loss despite a ravenous appetite. The increased thyroid hormone production causes cats to have a significant appetite, but despite the calorie consumption, they will lose weight. While other diseases in cats can cause this, hyperthyroidism should be your first consideration when this is noticed. Other clinical signs include restlessness, increased vocalization, panting, increased thirst and urination, diarrhea, vomiting, or an unkempt hair coat. If you notice any of these at home with your cat, they should be evaluated by a veterinarian. While the above clinical signs are expected in most cases of hyperthyroid cats, a small subset of cats develop the apathetic form of hyperthyroidism. This can cause the opposite clinical signs, including lethargy, decreased activity, decreased appetite, and weakness.

It is important to notice any change in your cat and reach a diagnosis as soon as possible, because leaving hyperthyroidism untreated can be detrimental to their health. Hyperthyroidism can result in elevated blood pressure in your cat, which can result in retinal detachment and sudden blindness, strokes, heart failure, and kidney failure. Untreated hyperthyroidism can also result in a heart disease called thyrotoxic cardiomyopathy. Again, if left untreated, this can result in heart failure. Fortunately, these changes in blood pressure and the heart can be reversible if diagnosed early enough and appropriately treated. In addition to performing a physical exam, your veterinarian will diagnose hyperthyroidism by performing bloodwork. Once the diagnosis is made, treatment can be started.

There are four possible treatment options for a cat with hyperthyroidism. Two of the treatments are reversible, meaning if treatment is stopped, the hyperthyroidism will return. The other two treatment options are irreversible and permanent.

The first reversible treatment option, and the most common treatment option, is a medication called methimazole. This is a medication that inhibits the production of thyroid hormones. It comes in an oral and transdermal formula, though the oral formulations are much more effective. The benefits of this option are that it is easy to administer at home, it does not require anesthesia or surgery, hospitalization is not needed, and it is relatively inexpensive compared to other medications. However, there are also downsides. This requires frequent follow-up and monitoring to ensure the dose is accurate. It is also not a definitive treatment, and therefore would require twice daily administration lifelong. Lastly, methimazole has potential adverse effects. These can include decreased appetite, vomiting, lethargy, increased itchiness, liver toxicity, and bone marrow toxicity. While these adverse effects are uncommon, they can occur and force us to consider alternative treatment options.

The second reversible treatment option is a prescription diet. This diet is restricted in an element called iodine. The thyroid gland is the only organ in the body that requires iodine. Therefore, the goal is to provide the thyroid glands with less iodine, so that they produce less thyroid hormone. While this is the easiest treatment option for owners, some cats may find this diet not very palatable. In addition, it can be difficult to feed this diet to your cat, especially if more than one pet is living in the household. Lastly, it can be difficult to appropriately control thyroid levels in many hyperthyroid cats with this diet alone. Because of these variables, this treatment option is not commonly used.

The first permanent treatment option is using radioactive iodine, which is also called I-131 therapy. While the word radioactive can sound dangerous and intimidating, this is the treatment of choice for cats with hyperthyroidism. Radioactive iodine is administered underneath the patient’s skin. It is then taken up by thyroid cells, which emit radiation that eventually destroys the cells. It is a very effective treatment option; more than 95% of cats are cured within three months. It also does not require surgery or anesthesia, and does not require lifelong administration of medications. However, the downside is that it is a higher up-front cost. Because of the precautions that need to be made after radioactive iodine is administered, the patient is typically hospitalized for a few weeks until discharge is deemed safe. Cats may also become hypothyroid and cannot produce enough thyroid hormone. This would result in the need for lifelong thyroid supplementation. However, this risk is low. Before considering radioactive iodine as a treatment, your veterinarian will first want to start one of the reversible treatment options to ensure a normal thyroid level can be achieved.

The last permanent treatment option is surgery to remove the thyroid gland. While this option does require anesthesia, it is a definitive fix. However, this treatment option is usually not pursued for a few reasons. The majority of cats with hyperthyroidism have bilateral disease, meaning both sides of the thyroid gland are affected. By taking out both sides of the thyroid gland, cats can become hypothyroid and even develop issues with calcium regulation. There are also many important nerves in this area, which can be damaged during the procedure. If you do want to consider this option, it is important to discuss with your veterinarian the necessary pre-operative diagnostics to determine if your cat is a good candidate.

While hyperthyroidism is largely a disease of cats, it is possible that dogs can develop it. Unlike cats, dogs that develop hyperthyroidism usually have a malignant tumor called a carcinoma of their thyroid gland. Not all thyroid carcinomas will cause hyperthyroidism in the dog. In fact, only 10% to 20% of thyroid carcinomas will produce excessive thyroid hormone. Surgery is the treatment of choice for these, though before considering surgery, it is important to confirm that the cancer has not spread elsewhere.

Feline hyperthyroidism is the most common endocrine disorder in the cat, but it also carries a very good prognosis with appropriate treatment and monitoring. If you notice any clinical signs associated with hyperthyroidism in your cat, it is important to have them immediately evaluated by your veterinarian.