[Written by a Veterinarian] Is your cat eating but still losing weight? It might be hyperthyroidism. Symptoms to check and treatment options.

Have you been happy thinking, "My cat has become a senior, yet they've been eating really well and are full of energy lately!"?

In fact, for an elderly cat, "losing weight despite eating" is a classic sign of "hyperthyroidism." In this article, we will explain the signs of "hyperthyroidism," which is common in cats over 7 years old, that you should not overlook, as well as the correct ways to manage it.

Losing weight despite eating? What is "hyperthyroidism" in cats?

The role of the thyroid gland

The thyroid gland is an organ located in the throat that secretes thyroid hormones necessary for maintaining a cat's life. The primary functions of thyroid hormones are as follows:

• Promoting energy production
• Regulating body temperature
• Improving heart function
• Activating overall metabolism

Hyperthyroidism occurs when excessive thyroid hormone secretion causes the body's metabolism to become abnormally active. As a result, some cats lose weight even though they are eating, seem incredibly energetic despite being old, or suddenly become aggressive in temperament.

Do not overlook! Early symptoms often mistaken for "increased vitality"

This disease is also sometimes called a "happy disease," as owners often mistake the increased activity in the early stages for the cat being "rejuvenated." However, this is pathological, and as the condition progresses, it puts a significant burden on the body. Check for the following signs:

• Appetite increases abnormally (e.g., stealing food) but the cat fails to gain weight or begins losing it: Due to a decrease in muscle mass.
• Drinks a lot of water and urinates more (polyuria/polydipsia): Because increased renal blood flow leads to increased urine production.
• Restlessness, pacing, howling at night, and aggressive behavior: Due to heightened sensitivity caused by accelerated metabolism.
• Poor coat condition (scruffy) or hair loss: Due to hair metabolism.
• Thickened or overgrown claws: Due to accelerated nail growth.

Life-threatening risks as it progresses

While thyroid hormones play a vital role in maintaining life, excessive secretion causes abnormally high metabolism and increased stress on the heart. If left untreated, the cat essentially stays in a state of constant physical exertion, which places a heavy burden on the heart and kidneys, potentially leading to risks such as cardiomyopathy or underlying renal failure (where kidney function is technically normal but in a state of failure).

Why does it happen? Causes in elderly cats and diagnostic methods at the vet

Causes and common age of onset for hyperthyroidism

It is most common in elderly cats aged 12–13 and older, though it is said that approximately 10% of cats over 7 years old are affected. Owners often don't notice early symptoms, and it is frequently discovered during health checkups or vaccinations. It is said to be more common in female cats. In hyperthyroidism, the thyroid gland may become swollen, but most of these are benign conditions known as hyperplasia or adenomas; thyroid cancer is rare in cats.

While the specific cause of hyperthyroidism is unknown, iodine is involved in thyroid hormone function, so iodine in the diet may play a role. There are reports suggesting a link to canned cat food (aluminum cans), with risks said to be more than five times higher compared to cats fed dry food.

Diagnostic methods

Palpation: The veterinarian feels the throat to check for thyroid swelling (lumps). However, as the thyroid can enlarge even in healthy cats, a diagnosis cannot be made by palpation alone.

Visual examination: Checking physique, muscle mass, coat quality, and claw condition. Hyperthyroidism is suspected if the cat is too thin, energetic, has a scruffy coat, or thick claws. If high blood pressure is present, bleeding in the eyes or retinal detachment may occur.

Auscultation: If cardiomyopathy has developed, heart murmurs or tachycardia may be detected. If cardiomyopathy is advanced, abnormal lung sounds may be heard due to respiratory complications.

Blood tests: In addition to blood cell counts and biochemical tests, levels of thyroid hormone (T4) in the blood are measured. However, levels may fall within the normal range during early-stage hyperthyroidism or if the cat has other diseases. In such cases, a free thyroxine (fT4) test is performed. Note that these tests may not be available for in-house analysis at all clinics.

Is there a cure? 3 treatment methods including medication, diet, and surgery, and home care

Hyperthyroidism is a condition you can manage for a long time with proper control. Treatment is generally chosen from the following three methods, based on the cat's temperament and the owner's lifestyle.

1. Medical therapy (antithyroid drugs)

Giving medication to suppress the synthesis of thyroid hormones daily is the most common method. These are administered as tablets, and feline-specific medications are often formulated to mask the bitter taste, making them easier to give.Medication must be continued for life.

Additionally, if there are comorbid conditions (cardiomyopathy, hypertension, kidney disease), treatment for those will also be required.

2. Dietary therapy (therapeutic diet)

This method involves feeding a specialized therapeutic diet that restricts "iodine," the building block of thyroid hormones.

In some cases, the condition can be managed through diet alone, but this requires strict adherence where "no other food whatsoever" can be consumed. Therefore, whether the cat will eat the therapeutic diet is key.

*In multi-cat households, you must take steps to ensure other cats do not eat the therapeutic diet.

3. Surgery

Surgery involves removing the enlarged thyroid gland. The advantage is that if successful, it can lead to a cure, eliminating the need for further clinic visits or medication. The disadvantages include high anesthesia risk due to the cat's typically advanced age, and the risk of fatal hypocalcemia if the parathyroid glands (which regulate calcium) attached to the thyroid are accidentally removed.

Not all animal hospitals perform thyroidectomies, so surgery is typically recommended when management through medication or diet becomes difficult.

Summary

"Losing weight despite eating" or "suddenly becoming energetic" might not be signs of aging, but symptoms of hyperthyroidism. If detected early and hormone levels are controlled with medication or diet, your cat can return to a peaceful daily life. Once your cat turns 7, keep a close watch on the balance between their weight and appetite, and ensure they receive regular health checkups.