Why is my senior cat losing weight?

The three most common causes are hyperthyroidism, chronic kidney disease, and diabetes mellitus. Hyperthyroidism typically causes weight loss with a normal or increased appetite, often along with a scruffy coat, vomiting, and nighttime vocalizing. Kidney disease tends to bring weight loss together with increased thirst and urination, and diabetes causes weight loss despite a ravenous appetite.

Other causes include dental disease and oral pain, which make eating hurt; inflammatory bowel disease and intestinal lymphoma, which reduce nutrient absorption; cancer; and chronic heart disease. Parasites, thyroid problems, and uncontrolled diabetes all accelerate muscle breakdown, so a cat can look thin along the spine while its belly stays round.

Because these conditions call for very different treatments, guessing is risky. Your veterinarian will run blood work including a thyroid panel, a urine test, and often blood pressure measurement, then may recommend imaging. Do not wait to see if it resolves on its own, and never put a sick senior cat on a homemade or restrictive diet without veterinary guidance. Bring a log of your cat's weight, appetite, water intake, and litter box output to the appointment; that information speeds up the diagnosis considerably.