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Medical Condition🐈 Catsendocrinecardiovascular

Feline Hyperthyroidism

🗣️ Pronounced:FEE-lyne hy-per-THY-royd-izm
In One Sentence (Plain English)

A common endocrine disorder of older cats caused by an overactive thyroid gland that overproduces thyroid hormones, accelerating metabolism.

Editorial Status: Verified from peer-reviewed veterinary sources (v1.0)
Updated: 2026-01-15

💬 In Plain English

Unlike dogs who get underactive thyroids, older cats frequently develop overactive thyroids, almost always caused by a benign (non-cancerous) tumor on one or both thyroid lobes in the neck. The excess thyroid hormone sends the cat’s metabolic engine into overdrive. Cats develop a voracious, screaming appetite yet lose dramatic amounts of weight. They often become hyperactive, vocalize at night, vomit frequently, and have a galloping fast heart rate.

🔬 Clinical & Technical Depth

For Vets & Technicians

Formal Clinical Definition

Autonomous hypersecretion of thyroxine (T4) and triiodothyronine (T3) by functional thyroid adenomatous hyperplasia (adenoma) in over 98% of feline cases, with thyroid carcinoma accounting for less than 2%.

Why Veterinarians Use It

Veterinarians screen for hyperthyroidism in any senior cat presenting with weight loss despite polyphagia, tachycardia, or a palpable thyroid nodule (“thyroid slip”).

Clinical Context Encountered

The most common endocrine pathology in domestic cats, diagnosed in over 10% of felines older than 10 years of age.

Clinical Reference Guide (Medical Condition)

Severity
Emergency StatusNon-Emergency Care
🚨 Red Flag Warnings (When to Contact an Emergency Vet)
  • •Thyroid storm is a rare life-threatening surge causing severe tachycardia, panting, hyperthermia, arrhythmias, and cardiac collapse requiring emergency stabilization.

❓ Questions to Ask Your Veterinarian

Take these actionable questions with you to your next clinic visit or appointment:

  • 1What was my cat’s Total T4 level, and was a Free T4 needed to confirm?
  • 2Did you palpate a thyroid nodule (“thyroid slip”) on the neck?
  • 3Has my cat’s kidney function (BUN, creatinine, SDMA, USG) and blood pressure been evaluated concurrently?
  • 4Which treatment option is best for my cat: oral methimazole, transdermal methimazole, radioactive iodine (I-131), or prescription y/d diet?

Academic & Clinical References