Feline Hyperthyroidism: The Most Common Hormonal Disease in Senior Cats
What Is Feline Hyperthyroidism?
Hyperthyroidism — overproduction of thyroid hormones by the thyroid gland — is the most common endocrine disorder in cats and one of the most common diseases diagnosed in senior cats overall. It results almost always from a benign adenomatous growth (adenoma) of thyroid tissue, causing the gland to produce thyroid hormone autonomously and in excess. Unlike dogs, who predominantly develop hypothyroidism, cats almost exclusively develop hyperthyroidism.
Classic Signs
Because thyroid hormone drives metabolism, hyperthyroid cats are metabolically "revved up." Classic signs: weight loss despite increased or ravenous appetite (the hallmark), increased thirst and urination, hyperactivity or restlessness, vomiting (often immediately after eating, due to rapid gut motility), diarrhea, unkempt coat, and increased vocalization, especially at night. The heart is affected: elevated heart rate, hypertension, and hypertrophic cardiomyopathy are common consequences. A senior cat losing weight with a good appetite should be tested for hyperthyroidism.
Diagnosis and Masking
Diagnosis is confirmed by measuring total T4 (TT4) in the blood. Most hyperthyroid cats have elevated TT4, making diagnosis straightforward. However, concurrent illness (particularly kidney disease) can suppress TT4 into the normal range in a hyperthyroid cat, masking the diagnosis. If clinical signs are strong but TT4 is normal, free T4 or repeat testing may be needed. An important clinical concern: treating hyperthyroidism unmasks underlying kidney disease (high blood pressure was maintaining kidney perfusion), which is why kidney function must be evaluated before and after treatment initiation.
Treatment Options
Four treatment options exist: daily oral medication (methimazole — relatively inexpensive, reversible, but requires compliance and monitoring), transdermal ear gel (same drug in a skin-absorbed formulation, easier to administer), radioactive iodine (I-131) (curative in most cats, single treatment, no long-term medication, but requires brief hospitalization), and surgical thyroidectomy (less commonly used today given radioactive iodine availability). Most veterinarians recommend a trial of medication first to assess kidney function before committing to a curative treatment.