Cat Hyperthyroidism: The 2026 Guide to Symptoms, Diagnosis & Treatment
Hyperthyroidism is the most common endocrine disorder in cats, primarily affecting senior felines over 10 years of age. This condition occurs when overactive thyroid glands produce excessive thyroid hormone, accelerating the body's metabolism and impacting nearly every organ system. While serious if left untreated, modern veterinary medicine offers excellent treatment outcomes, allowing most cats to regain their health and vitality.
Understanding Feline Hyperthyroidism
The Role of the Thyroid Glands
Cats have two small thyroid glands located in the neck. These glands produce thyroid hormones (T3 and T4), which are essential for regulating:
- Metabolic rate
- Heart function and rate
- Body temperature
- Digestive processes
- Skin and coat health
- Neurological function
What Causes Hyperthyroidism?
In hyperthyroid cats, one or both thyroid glands develop benign, non-cancerous tumors called adenomas. These tumors produce thyroid hormone autonomously, bypassing the body's normal regulatory feedback loops.
Key Fact: Over 98% of feline hyperthyroidism cases are caused by benign tumors. Malignant thyroid carcinoma is exceptionally rare.
Known Risk Factors
- Age: The average age of onset is 12-13 years. It is rare in cats under 8.
- Diet: Some studies suggest a potential link to fish-based, high-iodine diets and certain canned foods.
- Environment: Exposure to environmental chemicals like flame retardants (PBDEs) may be a contributing factor.
- Breed: No strong breed predisposition exists, though Siamese and Himalayan cats may have a slightly elevated risk.
Recognizing the Symptoms of Hyperthyroidism
Classic Clinical Signs
1. Weight Loss Despite Increased Appetite
- The most common and telling sign.
- Ravenous hunger paired with steady weight loss.
- May manifest as food-stealing, constant begging, or "always hungry" behavior.
2. Hyperactivity & Restlessness
- Inability to settle, pacing, or increased vocalization (especially at night).
- Excessive grooming.
3. Gastrointestinal Issues