Hyperthyroidism in cats is one of the most common hormonal disorders affecting middle-aged and senior cats. The condition occurs when the thyroid gland becomes overactive and produces an excessive amount of thyroid hormone, called thyroxine (T4).
Thyroxine is responsible for maintaining the cat’s metabolism. When too much of it circulates in the bloodstream, the body’s metabolic rate accelerates, leading to symptoms such as weight loss despite increased appetite, restlessness, and heart complications.
Understanding the causes, signs, and treatment of hyperthyroidism in cats is essential for early detection and improving your cat’s quality of life.
What Is Hyperthyroidism in Cats?
Hyperthyroidism in cats is caused by an overproduction of thyroid hormones due to the enlargement or overactivity of the thyroid glands located in the neck. In most cases, this happens because of benign thyroid nodules or adenomas, which are non-cancerous growths of thyroid tissue.
The thyroid in cats regulates important body functions such as metabolism, temperature, and heart rate. When the gland becomes overactive, the excess thyroid hormone speeds up every process in the body — causing increased hunger, frequent urination, and dramatic weight loss.
This condition is particularly common among older cats, with an average age of diagnosis around 13 years. While kittens and young adult cats can develop thyroid issues, hyperthyroidism primarily affects senior cats.
What Are the Causes of Hyperthyroidism in Cats?
Most cases of feline hyperthyroidism result from benign tumours or thyroid nodules in the gland. These abnormal growths increase hormone production, leading to elevated thyroxine levels.
Less common causes include:
- Thyroid cancer (carcinoma), seen in a small percentage of cats.
- Environmental toxins, such as flame-retardant chemicals (PBDEs) found in household furniture or dust.
- Advanced age, particularly cats over 10 years old.
Although research continues, genetics do not appear to play a significant role in hyperthyroidism in cats.
Role of the Thyroid Gland in Cats
The thyroid gland produces the hormones thyroxine (T4) and triiodothyronine (T3). These hormones regulate several vital processes in a cat’s body, including:
- Metabolism and energy use
- Heart rate and blood pressure
- Temperature regulation
- Fat and protein metabolism
- Nervous system and muscle function
- Skin, coat, and reproductive health
When the thyroid gland becomes overactive, these processes speed up excessively, leading to the clinical signs seen in hyperthyroidism.
What Are the Signs of Hyperthyroidism in Cats?
The signs of hyperthyroidism in cats often develop gradually, making them easy to miss in the early stages. Pet parents may notice their cat eating more but continuing to lose weight.
Common Physical Signs of Hyperthyroidism in Cats
- Sudden or rapid weight loss despite increased appetite
- Excessive thirst and frequent urination (polyuria)
- Vomiting or occasional diarrhoea
- Rough, unkempt coat or hair loss
- Rapid heartbeat (tachycardia) or heart murmurs
- Difficulty breathing or shortness of breath
- Thickened nails or brittle claws
- Noticeable enlargement of the thyroid gland in the neck
Behavioral Changes Linked to Feline Hyperthyroidism
- Restlessness and increased activity levels
- Irritability or anxiety
- Increased vocalisation (meowing more often)
- Occasional episodes of weakness or lethargy
A small number of cats (fewer than 10%) may show the opposite signs — reduced appetite and lethargy instead of hyperactivity.
How Is Hyperthyroidism in Cats Diagnosed?
Diagnosis begins with a thorough physical examination and blood tests to measure the levels of thyroid hormones, primarily T4, T3, and TSH. Elevated T4 levels generally confirm hyperthyroidism, but occasionally, results may appear normal in cats with mild disease.
If the initial results are inconclusive, veterinarians may recommend additional diagnostic tests such as
- Free T4 by equilibrium dialysis (fT4-ED) for greater accuracy.
- Thyroid imaging or nuclear scintigraphy to visualize thyroid nodules.
- Ultrasound or X-ray to detect internal abnormalities or heart enlargement.
It is also essential to rule out other conditions with similar symptoms, including diabetes, chronic kidney disease, liver disorders, and cancer. Cats with both kidney disease and hyperthyroidism require specialized treatment plans to balance both conditions safely.
What Is the Treatment of Hyperthyroidism in Cats?
The treatment of hyperthyroidism in cats aims to reduce excess thyroid hormone production and control the symptoms caused by an overactive metabolism. Several approaches are available depending on your cat’s health, age, and the clinic’s facilities.
Managing Feline Hyperthyroidism with Medication (Methimazole)
Methimazole is an oral or topical medication that blocks thyroid hormone production. It helps control hormone levels but does not cure the disease. Most cats require this medication for life, along with regular blood tests to monitor progress and side effects.
Radioactive Iodine Therapy (I-131) as a Long-Term Treatment Option
Radioactive iodine therapy (I-131) is considered the gold standard treatment for hyperthyroidism in cats. It selectively destroys overactive thyroid tissue without surgery, curing most cats permanently. However, it is available only at specialized veterinary centers.
Surgical Treatment for Hyperthyroidism in Cats (Thyroidectomy)
Thyroidectomy involves the surgical removal of one or both thyroid glands. While effective, it requires anesthesia and advanced surgical expertise. There’s also a risk of developing hypothyroidism or low calcium levels after surgery.
Prescription Diets to Help Control Thyroid Hormone Levels in Cats
Special low-iodine diets (iodine-restricted food) help reduce thyroid hormone production. These diets may be used alone in mild cases or alongside medication. Like Methimazole, diet management controls but doesn’t cure hyperthyroidism.
What Is the Prognosis of Cats with Hyperthyroidism?
The prognosis for hyperthyroidism in cats depends on the cat’s age, underlying cause, and overall health. Most cats respond well to treatment, especially when diagnosed early.
Advanced Symptoms and Complications in Untreated Cases
If left untreated, hyperthyroidism can lead to:
- Severe weight loss and muscle wasting
- High blood pressure (hypertension)
- Heart disease (hypertrophic cardiomyopathy)
- Kidney failure or liver damage
Early diagnosis and treatment significantly improve life expectancy and quality of life. Cats treated with Methimazole or radioactive iodine often live several more healthy years.
What Is the Follow-Up Care for Cats with Hyperthyroidism?
After initiating treatment, your veterinarian will schedule regular check-ups and blood tests every 2–4 weeks initially to monitor hormone levels and treatment response. Once stabilized, monitoring every 3–6 months is recommended.
If your cat has undergone surgery or radioactive iodine therapy, long-term monitoring is necessary to detect potential hypothyroidism or low calcium levels. Cats on Methimazole will need lifelong medication and routine bloodwork to ensure the dosage remains safe and effective.
Consistent veterinary care and dietary management help maintain hormonal balance and prevent recurrence of symptoms.
Conclusion
Hyperthyroidism in cats is a manageable condition when detected early and treated under veterinary supervision. Recognizing signs like weight loss despite good appetite, hyperactivity, or rapid heartbeat can help you act quickly.
With timely diagnosis, appropriate therapy, whether Methimazole, I-131 treatment, surgery, or dietary control, and regular follow-ups, most cats go on to live healthy, comfortable lives.
If you notice any unusual weight changes or behavioural shifts in your cat, book an appointment with your nearest Vetic Pet Clinic for an expert thyroid evaluation and care.
