Understanding and Managing Feline Hyperthyroidism

What is Hyperthyroidism?

Hyperthyroidism is a common condition in older cats that occurs when the thyroid glands produce excessive thyroid hormones (T3 and T4). These hormones regulate many important body functions, and an excess can cause significant health issues in your cat.

Common Symptoms to Watch For

• Weight loss despite increased appetite
• Increased thirst and urination
• Restlessness or hyperactivity
• Vomiting or diarrhea
• Rapid heart rate
• Poor coat condition

Note: There is a correlation between hyperthyroidism and cardiac (heart) abnormalities. If a heart murmur or other cardiac abnormality is detected, further testing, including chest radiographs and echocardiogram, may be recommended.

Diagnosis

Hyperthyroidism is diagnosed through blood tests that measure thyroid hormone levels. We recommend a comprehensive workup including:

• Complete blood count
• Chemistry panel
• Urinalysis
• Total thyroid level

Related abnormalities often include elevated liver enzymes and increased red blood cell counts.

Treatment Options

Anti-Thyroid Medications (Methimazole)

This medication blocks the production of thyroid hormones and is the most commonly used treatment. It is available in pill form, liquid suspension, or topical transdermal ear gel. Potential side effects include facial itching, vomiting, loss of appetite, and mild lethargy. Regular blood tests are required to monitor thyroid hormone levels and to ensure the proper dose of medication.

Typical Monitoring Schedule:

• Initial check: 4 weeks after starting medication (bloodwork and blood pressure)
• Follow-up checks: 3 months, 6 months, and yearly
• After dose adjustments: Recheck thyroid level in 1 month

Radioactive Iodine Treatment (I-131)

Radioactive iodine therapy is highly efficacious, with the large majority of cats returning to normal thyroid function within 3 months of receiving a single treatment. A single injection destroys overactive thyroid tissue, often providing a permanent cure with minimal side effects.

This treatment requires:

• Referral to a specialty hospital
• A brief hospital stay
• Special handling due to the radioactive nature of the treatment
• Follow-up monitoring

The University of Tennessee Veterinary Medical Center and Animal Emergency Specialty Center are local facilities equipped to offer this treatment.

Thyroidectomy (Surgical Removal)

Surgical removal of one or both thyroid glands requires general anesthesia and an experienced surgeon. While effective, risks include:

• Hypothyroidism (too little thyroid hormone)
• Decreased calcium levels
• Other, less severe side effects

This option requires referral to a specialty hospital.

Dietary Management (Hill’s Prescription Diet y/d)

This specially formulated prescription diet reduces iodine intake, limiting thyroid hormone production. Requirements for success:

• The cat must eat ONLY this food (no treats or other foods)
• The cat must drink ONLY filtered water
• Best suited for single-cat households
• Annual monitoring required

Things to Watch For

Side Effects of Treatment

Watch for signs such as vomiting, loss of appetite, or behavior changes while your cat is receiving treatment.

Changes in Energy Level

Contact the clinic immediately if you notice drastic changes in energy, such as increased lethargy or difficulty eating.

Thyroid Storm

This rare but potentially life-threatening condition occurs when hyperthyroidism suddenly worsens. Symptoms include very high heart rate, panting, and severe agitation. This requires immediate veterinary care.

Litter Box Habits

Monitor for changes in urination frequency or amount, as excessive urination is a common symptom of hyperthyroidism.

Regular monitoring and open communication with your veterinary team are essential for successful management of your cat’s hyperthyroidism. With proper treatment, most cats with hyperthyroidism can lead comfortable, healthy lives.