While hyperthyroidism (an overactive thyroid) is one of the most common hormonal disorders in older cats, true hypothyroidism — an underactive thyroid — is actually quite rare in felines. Understanding the difference matters, since the two conditions have opposite causes, symptoms, and treatments. This guide explains what hypothyroidism in cats looks like and when it’s most likely to occur.
What Is Hypothyroidism in Cats?
Hypothyroidism occurs when the thyroid gland doesn’t produce enough thyroid hormone, which regulates metabolism throughout the body. In cats, naturally occurring (spontaneous) hypothyroidism is uncommon in adults. The condition is seen far more often as a side effect of treatment for hyperthyroidism — including radioactive iodine therapy, surgical removal of the thyroid gland, or long-term use of anti-thyroid medication — where hormone production drops too low as a result of treatment. Congenital hypothyroidism, present from birth, is also seen occasionally in kittens and can affect growth and development.
How It Happens
Most cases of hypothyroidism in adult cats develop after treatment for hyperthyroidism overshoots and suppresses thyroid function too much. This is more common after radioactive iodine treatment or surgery than with medication, since dosing can be adjusted more easily with drugs like methimazole. Congenital hypothyroidism in kittens is caused by a developmental problem with the thyroid gland itself and is present from birth, though it may not be noticed until growth delays become apparent.
Signs and Symptoms
In adult cats, signs of hypothyroidism include lethargy, weight gain despite normal or reduced appetite, a dull or thinning coat, cold intolerance, and reduced activity levels. In kittens with congenital hypothyroidism, symptoms are more distinctive and include stunted growth, a disproportionately large head, short limbs, constipation, and delayed development. Because these symptoms can be subtle in adult cats and easily mistaken for normal aging, hypothyroidism is often under-recognized without targeted testing.
What Owners Should Do
If your cat has recently been treated for hyperthyroidism and starts showing signs of sluggishness, weight gain, or coat changes, mention it to your veterinarian so thyroid hormone levels (T4) can be tested. Diagnosis typically involves blood testing to measure thyroid hormone levels, sometimes alongside additional hormone panels for confirmation. Treatment usually involves daily oral thyroid hormone replacement medication, with regular blood testing to fine-tune the dose over time. Most cats respond well to treatment and regain normal energy and coat condition once hormone levels are stabilized.
Because hypothyroidism in cats is most often connected to prior hyperthyroidism treatment, staying on top of recommended follow-up bloodwork after any thyroid procedure is the best way to catch it early. If your cat needs ongoing monitoring, ask your veterinarian about a testing schedule and consider how pet insurance might help cover recurring lab work.