The vast majority of hypothyroidism in cats is caused by treatment for the opposite problem: hyperthyroidism. Roughly 1 in 5 cats treated for an overactive thyroid will develop abnormally low thyroid hormone levels afterward, and in cats taking the most common medication for hyperthyroidism, that number may reach as high as 50%. Spontaneous hypothyroidism, the kind that develops on its own without prior treatment, is extremely rare in cats, affecting only about 0.2% of the feline population.
Treatment for Hyperthyroidism Is the Leading Cause
Hyperthyroidism is one of the most common hormonal disorders in older cats. When it’s treated, whether by surgery to remove the thyroid gland, radioactive iodine therapy, or daily medication, there’s a real chance the treatment overcorrects and pushes thyroid hormone levels too low. Veterinarians call this iatrogenic hypothyroidism, meaning it’s medically induced rather than naturally occurring.
All three standard treatments carry this risk. Surgical removal of the thyroid gland and radioactive iodine therapy permanently destroy thyroid tissue, so if too much is eliminated, the cat’s body can no longer produce enough hormone on its own. Medication works differently: the drug methimazole blocks thyroid hormone production, and if the dose is too high or the cat is especially sensitive, hormone levels drop below normal. Methimazole-induced hypothyroidism is typically reversible by adjusting or stopping the medication, while post-surgical or post-radiation cases may require lifelong thyroid hormone supplementation.
One important wrinkle with any hyperthyroidism treatment is its effect on the kidneys. High thyroid hormone levels actually increase blood flow to the kidneys, which can mask underlying kidney disease. Once thyroid levels drop back to normal or below normal, that extra blood flow disappears, and kidney problems that were previously hidden can surface. This is why veterinarians monitor kidney function closely after treating hyperthyroidism, not just thyroid levels.
Methimazole Deserves Special Attention
Methimazole is the active ingredient in the most commonly prescribed hyperthyroidism medications for cats. It doesn’t destroy thyroid tissue; it blocks the chemical pathway the thyroid uses to manufacture hormones. That makes it reversible but also means the dose needs careful calibration. Too much, and the cat swings into hypothyroidism. Too little, and the hyperthyroidism isn’t controlled.
Because methimazole requires ongoing use, cats on this drug need regular blood work to check hormone levels and watch for side effects like fever, swollen lymph nodes, or anemia. If a cat develops signs of illness, a veterinarian may pause the medication and run additional tests. The good news is that hypothyroidism from methimazole almost always resolves once the dose is lowered or the drug is stopped.
Congenital Hypothyroidism in Kittens
A small number of kittens are born with hypothyroidism or develop it very early in life. This happens when the thyroid gland doesn’t form properly during development or when the gland is present but unable to produce hormones normally. Unlike the subtle, sometimes symptom-free hypothyroidism seen in adult cats after treatment, congenital hypothyroidism produces obvious and severe signs.
Affected kittens grow slowly and develop disproportionately short limbs and a broad, flattened head, a pattern called disproportionate dwarfism. They’re profoundly lethargic and mentally dull, often constipated, and have little interest in food. Other telltale signs include retained baby teeth, overgrown gums, a slow heart rate, and a kitten-like coat that never transitions to an adult texture. These signs typically become apparent within the first few months of life, and early treatment with supplemental thyroid hormone can significantly improve outcomes.
Spontaneous Hypothyroidism Is Exceptionally Rare
In dogs, hypothyroidism that develops on its own (usually from the immune system attacking the thyroid gland) is one of the most common hormonal disorders. In cats, it’s the opposite. A large study reviewing over 31,500 feline lab submissions found that only 0.2% of cats had lab results suggestive of spontaneous hypothyroidism. Published case reports are scarce, and many veterinarians may never encounter a naturally occurring case in their entire career.
Why cats are so resistant to spontaneous hypothyroidism while dogs are so prone to it isn’t fully understood. But the practical takeaway is clear: if your cat is diagnosed with hypothyroidism and hasn’t been treated for hyperthyroidism, it’s an unusual finding that warrants thorough investigation.
Pituitary Problems as a Secondary Cause
In rare cases, hypothyroidism in cats results not from a problem with the thyroid gland itself but from a failure higher up in the hormonal chain. The pituitary gland, a small structure at the base of the brain, sends signals that tell the thyroid how much hormone to produce. If the pituitary is damaged or compressed by a tumor, those signals stop and the thyroid essentially goes idle.
Inactive pituitary tumors are extremely rare in cats and typically occur in older animals. When they do develop, they tend to compress nearby brain structures including the hypothalamus, leading to deficiencies across multiple hormone systems, not just the thyroid. Cats with this condition may show a constellation of symptoms affecting energy, weight, reproductive function, and stress response, reflecting the broad role the pituitary plays in the body.
Signs to Watch For
The symptoms of hypothyroidism in adult cats are often subtle, especially in mild iatrogenic cases where a cat may show no obvious changes at all. When signs do appear, the most common include lethargy, weight gain or obesity, a dull or flaky coat, poor hair regrowth after shaving or grooming, decreased appetite, and a slower than normal heart rate. Some cats develop patchy hair loss on the ears, lower legs, or tail base, though the widespread symmetrical hair loss typical in hypothyroid dogs is not seen in cats.
Low body temperature is another clinical marker, though it’s not something most owners would notice without a thermometer. In many cases, hypothyroidism is caught through routine bloodwork rather than visible symptoms, particularly in cats being monitored after hyperthyroidism treatment. A diagnosis typically requires consistently low thyroid hormone levels paired with an elevated thyroid-stimulating hormone, confirmed on repeat testing, along with clinical signs that match. Treatment is only recommended when both the lab numbers and symptoms align.