Feline hyperthyroidism

Symptoms an owner can spot at home

• Weight loss despite a normal or increased appetite
• Increased thirst and urination
• Increased vocalization, restlessness, or irritability
• Unkempt or greasy coat
• Vomiting or intermittent diarrhea
• Rapid heart rate noted at vet visits

When to see a vet

• Any unexplained weight loss in a cat over 8, even if appetite is normal
• New-onset increased thirst or urination
• Sudden behavior change, pacing, yowling at night, aggression
• Rapid breathing or open-mouth breathing, possible secondary heart disease (HCM), an emergency

What it is

In feline hyperthyroidism, one or both thyroid lobes develop benign functional nodules that secrete excess thyroid hormone (T4 and T3). Excess thyroid hormone raises metabolic rate across nearly every body system, weight loss, increased food intake, tachycardia, hypertension, secondary cardiac changes, and accelerated decline of kidney function are the predictable consequences.

The Cornell Feline Health Center reports that roughly 10% of cats over 10 develop hyperthyroidism, making it the most common endocrine disease of older cats. Etiology is multifactorial, environmental, dietary, and possibly iodine-related, and is still under active study.

How vets diagnose it

A single total T4 blood test confirms most cases. Borderline cases get repeat T4, free T4 by equilibrium dialysis, or a T3 suppression test. Because hyperthyroidism can mask chronic kidney disease (the high cardiac output supports kidney filtration), vets typically run a full senior panel including kidney values, urinalysis, and blood pressure.

Treatment overview

Editorial overview, not a treatment plan. Four established options exist: radioactive iodine (I-131, curative, generally considered the gold standard), oral or transdermal antithyroid medication (lifelong management), prescription iodine-restricted diet (lifelong management, requires strict feeding compliance), and surgical thyroidectomy. Choice depends on the individual cat's age, concurrent kidney disease, comorbidities, and owner logistics. All four have good outcomes when started before significant cardiac or renal complications develop.

What owners can do

• Senior wellness exams every 6 months from age 8, with a baseline T4 by age 10.
• Weigh your cat monthly on a kitchen scale, a 200-300 g loss in a month is a real signal.
• Don't dismiss 'good appetite' as a sign of health in an older cat losing weight.
• If diagnosed, follow up bloodwork on the schedule your vet sets, kidney function often shifts after treatment and needs re-assessment.