Feline Hyperthyroidism

Hyperthyroidism is a common hormonal disease in older cats. It occurs when the thyroid glands produce excessive amounts of thyroid hormone (T4), which regulates the body’s metabolism.

Cats have two thyroid glands located in the neck. In hyperthyroidism, one or both glands become enlarged and produce too much thyroid hormone. Because thyroid hormone affects almost every cell in the body, this leads to overstimulation of many body systems.

The exact cause is unknown in most cases, but research suggests several possible contributing factors, including:

• Long-term consumption of tinned cat food, particularly from cans that are not fully lined
• Environmental chemicals or endocrine-disrupting compounds
• Dietary iodine imbalance
• Age-related changes in the thyroid gland.

Despite these associations, most cases do not have a clear identifiable cause.

Signs of hyperthyroidism usually develop gradually over weeks to months.

One of the most common signs is weight loss despite a ravenous appetite. Because hyperthyroidism typically occurs in older cats, early signs are sometimes mistaken for normal ageing.

Common symptoms include:

• Weight loss despite increased or ravenous appetite
• PU/PD (increased thirst and urination)
• Restlessness or hyperactivity
• Frequent vocalizing or meowing
• Behaviour changes
• Poor coat quality or an unkempt appearance
• Vomiting or diarrhoea
• Increased heart rate
• Panting or rapid breathing.

High blood pressure associated with hyperthyroidism can cause cats to appear restless and vocal, and over time may lead to complications affecting the eyes, brain, heart, and kidneys. Affected cats are often very thin despite eating large amounts of food and may seem constantly hungry.

Hyperthyroidism is usually diagnosed using a combination of:

• A blood test measuring thyroid hormone (T4)
• Clinical signs observed at home
• Physical examination findings.

In some cats, the veterinarian may be able to feel an enlarged thyroid gland during a physical exam, although this is not always detectable. In the very early stages, diagnosis can sometimes be more difficult because thyroid hormone levels may fluctuate. In these cases, repeat testing or additional blood tests may be recommended.

Hyperthyroidism is very treatable, and the prognosis is excellent in uncomplicated cases. Some cats are initially stabilized with medication for a few weeks before deciding on long-term treatment.

Radioactive Iodine Therapy (I-131)

This treatment is performed at specialist referral centers and is considered the gold standard treatment. A small dose of radioactive iodine is administered, which selectively destroys the abnormal thyroid tissue while leaving normal tissue largely unaffected. Over 90% of cats are cured with a single treatment and no ongoing medication is required in most cases.

Medical Management (Anti-thyroid Medication)

Medications that reduce thyroid hormone production include carbimazole or methimazole. This comes as tablets, liquid or an ointment that can be applied to the inside of the ear. Medical management is lifelong and requires regular blood tests to monitor thyroid levels and adjust dosage.

Prescription Diet (Hill’s y/d)

This diet works by restricting iodine intake, which reduces thyroid hormone production.

However, this treatment has significant limitations:

• It must be fed exclusively, with no other food or treats
• It can be difficult in multi-cat households
• It does not cure hyperthyroidism, and only manages the condition by controlling hormone production.

Cats undergoing treatment require regular monitoring of thyroid hormone levels and overall health.

Hyperthyroidism can mask underlying kidney disease, and treatment may unmask previously undetected kidney issues once thyroid levels return to normal.

With appropriate treatment and monitoring, many cats with hyperthyroidism can continue to live comfortable and good-quality lives for many years.