Hyperthyroidism in Cats and Dogs: Dr. Moliya’s Guide to Thyroid Health in Pets

The thyroid gland, located in the neck near the trachea, plays a vital role in controlling metabolism through the secretion of thyroxine (T₄) and triiodothyronine (T₃).
When this gland becomes overactive, it results in hyperthyroidism — a condition where excessive thyroid hormones accelerate the body’s metabolism. Hyperthyroidism in cats and dogs affects metabolism, heart function, behaviour, and overall health, and early diagnosis significantly improves outcomes.

1. Feline Hyperthyroidism Hyperthyroidism in Cats (Feline Hyperthyroidism)

Hyperthyroidism in cats is very common in older felines and early identification helps prevent severe secondary complications.

Main Causes:

• Functional adenomatous hyperplasia (benign tumor) – accounts for about 98% of cases.
• Thyroid adenocarcinoma (malignant tumor) – rare (~2% of cases).
• No clear breed or genetic predisposition, but Siamese and Himalayan cats may have slightly lower incidence.

Excess thyroid hormones increase: In feline hyperthyroidism, excess thyroid hormones overstimulate metabolism and multiple body systems.

• Basal metabolic rate
• Heart rate and cardiac output
• Oxygen consumption
• Protein, carbohydrate, and fat metabolism

This leads to weight loss despite good appetite and widespread organ system effects.

Clinical Signs: Symptoms of Hyperthyroidism in Cats

 The symptoms of hyperthyroidism in cats affect metabolism, heart health, behaviour, digestion, and coat condition.

Diagnosis: Diagnosis of Hyperthyroidism in Cats

Diagnosing hyperthyroidism in cats involves physical examination, thyroid blood tests, and imaging when needed.

1. Clinical signs and palpable thyroid nodule in the neck.
2. Blood tests:
   • Elevated total T₄ (primary screening test)
   • If borderline, check free T₄ or T₃ suppression test
3. Ultrasound or scintigraphy: To locate functional thyroid tissue.
4. Blood pressure and ECG: To assess secondary cardiac changes.

 Treatment: Treatment for Hyperthyroidism in Cats

Treatment for hyperthyroidism in cats includes medication, surgery, iodine therapy, or dietary management depending on the cat’s condition.

1. Medical Management

• Methimazole (Tapazole): Blocks thyroid hormone synthesis.
  • Dose: 2.5–5 mg per cat twice daily (oral or transdermal gel).
  • Regular blood monitoring is required.

2. Surgical Thyroidectomy

• Removal of one or both thyroid lobes.
• Risk: Hypocalcemia (if parathyroid glands are damaged).
• Best for stable patients with normal renal function.

3. Radioactive Iodine Therapy (I¹³¹)

• Gold standard treatment.
• Selectively destroys overactive thyroid tissue while sparing normal cells.
• Highly effective; permanent cure in most cases.

4. Dietary Management

• Iodine-restricted diet (e.g., Hill’s y/d): Limits hormone production.
• Used when medication or surgery isn’t feasible.

• Canine Hyperthyroidism Hyperthyroidism in Dogs (Canine Hyperthyroidism)

 Hyperthyroidism in dogs is rare but typically more serious due to malignant thyroid tumors or excessive thyroid medication.

A rare condition in dogs characterized by excessive secretion of thyroid hormones due to a thyroid tumor (usually carcinoma) or iatrogenic causes (from overdose of thyroid supplements).

Main Causes: Causes of Hyperthyroidism in Dogs

Most cases of hyperthyroidism in dogs are caused by thyroid carcinoma, which can be invasive or metastatic.

1. Functional thyroid carcinoma (most common cause).
   • Usually malignant and invasive.
   • May metastasize to lungs or lymph nodes.
2. Iatrogenic hyperthyroidism – due to excessive L-thyroxine supplementation for treating hypothyroidism.

 Clinical Signs: Symptoms of Hyperthyroidism in Dogs

Symptoms of hyperthyroidism in dogs affect metabolic, cardiac, respiratory, neck, and neurological systems.

If untreated, hyperthyroidism in dogs progresses rapidly and leads to organ damage, hypertension, and neurological decline.

These symptoms will progress and become more profound. Over time, increased metabolism will lead to high blood pressure, heart disease, kidney disease, liver disease, respiratory issues, and skin disorders.

Late stages of the disease may result in additional symptoms:

Seizures

Lack of coordination

Muscle tremors

Muscle weakness and wastage

Dilated pupils

Blindness

Heart arrhythmias and murmurs

Diagnosis: Diagnosis of Hyperthyroidism in Dogs

Diagnosis of hyperthyroidism in dogs includes physical examination, thyroid hormone panels, imaging, and biopsy confirmation.

1. Palpation of thyroid mass in the neck.
2. Blood tests:
   • Elevated total and free T₄, T₃.
   • Decreased TSH (thyroid-stimulating hormone).
3. Imaging (ultrasound, CT, MRI): To assess size, invasion, and metastasis.
4. Fine needle aspiration or biopsy to confirm carcinoma.

Treatment: Treatment for Hyperthyroidism in Dogs

Treatment depends on tumour size and spread, and may include surgery, radioactive iodine, medical therapy, or chemotherapy.

1. Surgical Removal

• Preferred if the tumor is localized and non-invasive.
• May require partial or complete thyroidectomy.

2. Radioactive Iodine Therapy (I¹³¹)

• Effective for functional tumors with minimal metastasis.
• Safer than surgery for deep or vascular tumors.

3. Chemotherapy or Radiation Therapy

• For malignant or metastatic thyroid carcinomas.

4. Medical Management

• Methimazole or beta-blockers (e.g., propranolol) to manage symptoms temporarily.

Comparison Table: Feline vs. Canine Hyperthyroidism Feline vs. Canine Hyperthyroidism: Key Differences

Natural Support for Thyroid Health in Pets

Natural remedies for hyperthyroidism in cats and dogs may support overall wellness but should always complement veterinary treatment.

Hyperthyroidism is a disease of the endocrine system, affecting humans and other animal species. In general, endocrine disorders have been on the rise due to inferior diets, processed food, and chemicals in the environment. It is rare in dogs but is becoming more common in cats.  While there are conventional treatments for feline hyperthyroidism, natural treatments are also effective. A whole-food, species-appropriate diet boosts the immune system. Herbal supplementation can lower thyroid hormone levels without the side effects of conventional medication. Hyperthyroidism is a serious disease, but it can be controlled with proper care.