The most common symptoms of hyperthyroidism in cats are weight loss despite a normal or increased appetite, excessive thirst and urination, and hyperactivity. This is the single most frequent hormonal disorder in older cats, typically showing up after age 10. Because the signs develop gradually, many owners chalk them up to normal aging before realizing something else is going on.
Weight Loss With a Bigger Appetite
The hallmark combination is a cat that eats more than usual but keeps losing weight. Thyroid hormones control metabolism, and when the thyroid gland overproduces them, the body burns calories faster than the cat can take in. You might notice your cat begging for food, finishing meals quickly, or pestering you for extra feedings, all while their ribs and spine become more prominent. Some cats lose weight steadily over weeks or months, while others seem to drop it suddenly.
Vomiting and diarrhea are also common, partly because cats eat so rapidly and partly because the excess hormones speed up the entire digestive tract. Stools may be larger in volume than normal. These digestive symptoms alone can contribute to further weight loss.
Increased Thirst and Urination
Hyperthyroid cats often drink noticeably more water and produce larger volumes of urine. You may find yourself refilling the water bowl more frequently or cleaning the litter box more often. This happens because the overactive thyroid increases blood flow to the kidneys, which in turn filters and produces more urine. The cat drinks more to compensate.
This symptom is worth paying close attention to because it overlaps with other conditions common in older cats, including diabetes and kidney disease. A blood test can distinguish between them.
Behavioral Changes and Restlessness
Many hyperthyroid cats become noticeably more active, restless, or agitated. A previously calm cat may pace around the house, have trouble settling down, or seem wired and on edge. Excessive vocalization is another frequent sign, particularly at night. Owners often describe their cat yowling or crying loudly for no apparent reason, sometimes waking the household.
Some cats become more irritable or less tolerant of handling. Others seem anxious or easily startled. These changes reflect the stimulating effect of excess thyroid hormones on the nervous system. They can be dramatic enough that owners initially suspect a cognitive or neurological problem rather than a hormonal one.
Coat and Skin Changes
A hyperthyroid cat’s coat often looks unkempt, matted, or greasy. Cats that once groomed themselves fastidiously may stop keeping up with their fur. In some cases the coat becomes dry and dull instead. Rapid nail growth and thickened claws are another subtle clue. These changes happen because the body is diverting energy toward the ramped-up metabolism rather than maintaining skin and coat health. On their own, coat changes are easy to dismiss, but combined with weight loss or increased appetite, they become a meaningful part of the picture.
Heart and Breathing Problems
Excess thyroid hormones force the heart to work harder and beat faster. Rapid heart rate and heart murmurs are common findings when a vet examines a hyperthyroid cat. Some cats also breathe faster than normal or pant, which is unusual for cats at rest. Over time, the increased cardiac workload can cause the heart muscle to thicken, a condition called hypertrophic cardiomyopathy. If left untreated, this can progress to heart failure, with symptoms like labored breathing, open-mouth breathing, or lethargy.
You probably won’t detect a heart murmur at home, but you might notice your cat breathing rapidly while resting (more than 30 breaths per minute is a useful threshold to watch for) or seeming winded after minimal activity.
High Blood Pressure and Vision Loss
Hyperthyroidism can cause secondary high blood pressure, which damages sensitive organs. The eyes are the most vulnerable target. Elevated blood pressure can cause the retina to detach, leading to sudden blindness. Signs include dilated pupils that don’t respond to light, bumping into furniture, or a sudden reluctance to jump. Bleeding or swelling in the back of the eye may also occur, though this is typically only visible during a veterinary exam.
In some cases, blindness caused by high blood pressure can be reversed if treated very quickly, but in most cases it is permanent. This is one reason early detection of hyperthyroidism matters: catching the disease before blood pressure climbs high enough to damage the eyes can preserve your cat’s vision.
The Apathetic Form: When Symptoms Look Opposite
A small number of hyperthyroid cats present with what veterinarians call “apathetic” hyperthyroidism. Instead of hyperactivity and a ravenous appetite, these cats show lethargy, listlessness, and poor appetite. Weight loss remains the consistent sign. This form is rare, but it’s worth knowing about because it can look like kidney disease, cancer, or general decline rather than an overactive thyroid. A routine blood panel will catch it regardless of which symptoms appear.
The Hidden Link to Kidney Disease
One of the trickier aspects of feline hyperthyroidism is its relationship with the kidneys. The excess hormones increase blood flow through the kidneys, which temporarily makes them look healthier on blood tests than they actually are. Standard kidney markers like BUN and creatinine drop because the kidneys are filtering at an artificially high rate.
This means a cat can have both hyperthyroidism and early kidney disease, but the kidney problems stay hidden until the thyroid condition is treated. Roughly 30% of cats develop obvious kidney disease after their hyperthyroidism is brought under control, not because treatment caused it, but because the thyroid condition was masking it all along. Your vet will typically monitor kidney values closely during and after treatment to catch this early.
What a Vet Looks For
During a physical exam, a vet will feel for an enlarged thyroid gland in your cat’s neck. About 80% of hyperthyroid cats have a palpable thyroid nodule, though having a lump doesn’t automatically mean the cat is hyperthyroid, since roughly 20% of cats with a palpable neck mass have normal thyroid function. Diagnosis is confirmed with a blood test measuring thyroid hormone levels.
If your older cat is losing weight, drinking more water, acting restless, or vocalizing more than usual, a thyroid check is a straightforward first step. The condition is very treatable, and most cats respond well once it’s identified. The key is recognizing that these symptoms in an older cat aren’t just “getting old” and getting a blood panel done sooner rather than later.