Why Is My Senior Cat Eating So Much but Losing Weight?

Quick answer

If your older cat is eating like a horse but dropping weight, the most likely culprit is hyperthyroidism. This is a common, highly treatable hormonal condition where the thyroid gland produces too much thyroid hormone, sending your cat's metabolism into overdrive and burning calories faster than they can consume them.

*If your older cat has a ravenous appetite but is constantly losing weight, they *

:::key-facts

• Hyperthyroidism is caused by a benign (non-cancerous) tumor or enlargement of the thyroid gland in over 98% of cases.
• It primarily affects middle-aged and senior cats, typically those over the age of 10.
• Classic signs include progressive weight loss, a ravenous appetite, increased thirst, frequent urination, and sudden bursts of hyperactive energy.
• Left untreated, the condition puts immense strain on the heart, kidneys, and blood pressure, which can lead to life-threatening complications.
• Highly effective treatment options are available, including daily medication, radioactive iodine therapy, prescription diets, or surgery.
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Why it matters

The thyroid gland, located in your cat's neck, acts as the body's metabolic thermostat. It produces thyroid hormones (primarily T4 and T3) that regulate how quickly your cat burns energy, processes nutrients, and maintains cellular function. When a cat develops hyperthyroidism, one or both lobes of this gland grow abnormally and begin pumping out excessive amounts of these hormones.

This constant hormonal flood forces your cat's body into a state of hyper-metabolism. Imagine running a marathon 24 hours a day, seven days a week—that is what your cat's body is experiencing. To keep up with this extreme energy demand, their body begins breaking down its own fat stores and muscle tissue for fuel. This is why you will see your cat begging for food constantly, eating massive portions, yet looking increasingly bony and frail.

A top-down view of a thin cat. Noticeable spinal definition and a highly exaggerated waist tuck are classic signs of weight loss.

Beyond weight loss, this metabolic overdrive takes a heavy toll on internal organs. The heart is forced to beat much faster and pump harder to keep up with the metabolic demand. Over time, this can lead to high blood pressure (hypertension) and a specific form of heart disease called thyrotoxic cardiomyopathy, where the heart muscle becomes dangerously thickened. Additionally, the increased blood flow can mask underlying kidney disease, which often coexists in senior cats. Recognizing the signs early allows you to intervene before these secondary, life-threatening complications take hold.

:::ask-boo
"Can hyperthyroidism in cats cause sudden blindness?"
:::

What good looks like

A healthy senior cat should maintain a stable weight, have a soft, well-groomed coat, and exhibit balanced energy levels. While older cats naturally slow down, they should still spend a significant portion of their day resting peacefully, grooming themselves, and interacting calmly with their environment. Their appetite should be steady and predictable, and they should drink a moderate amount of water.

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A well-managed thyroid allows your cat to return to normal, calm behaviors like peaceful grooming and sunbathing.
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When hyperthyroidism is successfully diagnosed and managed, your cat's metabolism returns to its normal, balanced state. You will notice their frantic begging subsides to a normal appetite, and they will gradually regain their lost muscle mass and body weight. Their coat will lose its greasy, unkempt appearance and become soft and shiny again. Most importantly, their heart rate will stabilize, and their overall behavior will shift from anxious restlessness back to the calm, content demeanor of a happy senior pet.

Step-by-step

If you suspect your senior cat is showing signs of an overactive thyroid, taking structured, proactive steps will help you get them the right care quickly.

Step 1: Document and track their daily habits

Before heading to the clinic, spend a few days closely observing your cat. Note down how many times you refill the water bowl, how often you clean the litter box (looking for unusually large wet clumps), and how much food they are consuming. If possible, weigh your cat on a digital scale and write down the date and weight. This concrete information is incredibly valuable for your veterinarian.

Step 2: Schedule a senior veterinary consultation

Call your vet and explain that you have an older cat experiencing weight loss despite a very strong appetite. Request a comprehensive senior wellness exam. Because hyperthyroidism shares symptoms with other senior feline conditions—such as diabetes mellitus and chronic kidney disease—a thorough physical exam and diagnostic blood work are essential to pinpoint the exact cause.

Step 3: Understand the diagnostic process

During the visit, your vet will perform several key checks:

• Physical Palpation: The vet will gently feel your cat's neck to check for an enlarged thyroid gland, often referred to as a "thyroid slip."
• Blood Pressure Measurement: Because hypertension is a common side effect, checking blood pressure is highly recommended.
• Senior Blood Panel: This will include a Total T4 (thyroxine) test. In most hyperthyroid cats, the T4 level will be clearly elevated. The vet will also check kidney values, liver enzymes, and blood sugar levels to rule out other concurrent diseases.

Your veterinarian will gently feel your cat's neck to check for an enlarged thyroid gland, often called a 'thyroid slip'.

:::pro-tip
Before your vet appointment, film a quick 30-second video of your cat's typical behavior at home, especially if they are unusually hyperactive, vocalizing loudly at night, or begging frantically for food. This helps your vet see their natural state.
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Step 4: Discuss and choose a treatment pathway

Once diagnosed, you and your vet will choose the best treatment option based on your cat's age, overall health, kidney function, and your lifestyle. The four primary treatment paths are:

1. Medical Management (Methimazole): Daily medication (given as an oral pill, a flavored liquid, or a transdermal gel rubbed inside the ear) blocks the production of thyroid hormone. This is a highly effective, reversible treatment, but it must be given daily for the rest of your cat's life.
2. Radioactive Iodine Therapy (I-131): Often considered the gold standard cure. Your cat receives an injection of radioactive iodine, which selectively targets and destroys the abnormal thyroid tissue while leaving healthy tissue intact. This requires a short stay at a specialized licensed facility but cures the condition in over 95% of cases without daily medication.
3. Prescription Dietary Therapy: Feeding a strictly controlled, ultra-low-iodine prescription wet and dry food. Because the thyroid gland needs iodine to produce hormones, starving it of iodine naturally lowers hormone levels. This option only works if your cat eats absolutely nothing else—no treats, no hunted prey, and no table scraps.
4. Surgical Thyroidectomy: Surgical removal of the affected thyroid lobe. While curative, it carries anesthetic risks for older cats and risks damaging the nearby parathyroid glands, which control calcium levels.

Signs something's wrong

While weight loss and a ravenous appetite are the hallmark signs of hyperthyroidism, the systemic effects of the disease can cause a wide range of subtle and overt symptoms. Keep a close eye out for these classic indicators:

• Unexplained Weight Loss: Your cat feels increasingly bony, with their spine, hips, and ribs becoming prominent, even though they are eating more than ever.
• Extreme Hunger (Polyphagia): Begging constantly, stealing food from counters, or crying loudly near their food bowl shortly after eating a full meal.
• Increased Thirst and Urination: Spending more time at the water bowl, drinking from unusual places (like dripping faucets), and producing very large, heavy urine clumps in the litter box.
• Hyperactivity and Restlessness: Pacing, an inability to settle down, or sudden bursts of kitten-like energy that seem unusual for an older cat.
• Nighttime Vocalization: Loud, distressed meowing or howling in the middle of the night, often caused by systemic hypertension or cognitive changes exacerbated by thyroid hormone levels.
• Poor Coat Appearance: A greasy, matted, unkempt, or dry coat. Some cats may stop grooming entirely, while others may over-groom and create bald patches.
• Gastrointestinal Issues: Frequent vomiting (often of undigested food eaten too quickly) or chronic, voluminous diarrhea.

:::video{src="https://storage.googleapis.com/decennium-global.appspot.com/knowledge_assets/care_guides/hyperthyroidism-in-older-cats-weight-loss-with-big-appetite/inline-4-1779984764674.mp4" poster="https://storage.googleapis.com/decennium-global.appspot.com/knowledge_assets/care_guides/hyperthyroidism-in-older-cats-weight-loss-with-big-appetite/inline-4-still-1779984627777.png" alt="Senior cat drinking water intently from a pet fountain."}
Increased thirst and frequent trips to the water bowl are common indicators of an overactive metabolism.
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:::ask-boo
"How do I know if my cat's weight loss is from hyperthyroidism or kidney disease?"
:::

When to call your vet

If you notice your senior cat losing weight despite a healthy or increased appetite, you should schedule a veterinary appointment within the week. While it is not an immediate midnight emergency, the longer the body remains in a hyper-metabolic state, the more damage is done to the heart and kidneys.

:::warning
If your senior cat suddenly collapses, struggles to breathe, pants heavily with an open mouth, or experiences sudden blindness (indicated by dilated pupils that do not shrink in bright light), this is a medical emergency. High blood pressure from untreated hyperthyroidism can cause sudden retinal detachment or acute heart failure. Seek emergency veterinary care immediately.
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Common mistakes

• Assuming "it's just old age": Many owners assume that slowing down, losing weight, or looking a bit scruffy are just natural parts of a cat growing old. Weight loss is always a sign of an underlying issue, not a normal aging process.
• Celebrating a "great appetite": It is easy to feel relieved when an older cat is eating enthusiastically. However, a ravenous appetite paired with weight loss is a classic red flag for disease, not a sign of youthfulness.
• Stopping medication once symptoms improve: Daily thyroid medications like methimazole manage the symptoms but do not cure the underlying tumor. If you stop giving the medication, your cat's thyroid levels will spike back up within days.
• Cheating on the prescription diet: If you choose dietary management, even a single treat, a crumb of bread, or a speck of normal cat food containing iodine will ruin the therapeutic effect and cause thyroid levels to rise again.
• Skipping follow-up blood work: When you start treating hyperthyroidism, your cat's blood flow changes, which can unmask hidden chronic kidney disease. Regular follow-up blood tests are vital to ensure the kidneys are coping well with the new, lower thyroid levels and to adjust medication doses safely.

Using pill pockets or flavored liquid suspensions can make daily thyroid medication stress-free for both you and your cat.

Quick FAQs

How long can a cat live with hyperthyroidism?

With early diagnosis and proper management, cats with hyperthyroidism can live normal, high-quality lives for many years. Many senior cats live comfortably into their late teens or early twenties once their thyroid levels are stabilized.

Is radioactive iodine therapy safe for older cats?

Yes, radioactive iodine therapy is highly safe and is considered the most effective cure. Because it does not require general anesthesia, it is often safer for senior cats than surgery. However, cats must have stable kidney function before undergoing this therapy, as the treatment can sometimes reveal pre-existing kidney issues.

Can I prevent my cat from getting hyperthyroidism?

There is no proven way to prevent hyperthyroidism, as the exact cause of the benign thyroid tumors remains unknown. The best defense is scheduling bi-annual veterinary checkups for cats over the age of seven, including routine blood work to catch rising thyroid levels before physical symptoms become severe.