Hyperthyroidism

Because thyroid hormone affects nearly every organ, hyperthyroidism produces a wide range of signs. Owners often notice changes gradually and initially assume their cat is "just getting old."

• Weight loss despite a ravenous appetite — the hallmark combination
• Increased thirst and urination
• Hyperactivity, restlessness, or yowling, especially at night
• A rapid, pounding heartbeat (sometimes audible to owners)
• Vomiting or diarrhea, often intermittent
• A dull, unkempt, or matted coat from self-grooming changes
• Muscle wasting, particularly along the spine and hindquarters
• High blood pressure — often detected on routine screening — which can damage the eyes and brain

A small percentage of cats present with the opposite pattern — weight loss, lethargy, reduced appetite — sometimes called "apathetic hyperthyroidism." These cats are easily missed without a blood test.

The root cause of most feline hyperthyroidism is a benign adenomatous hyperplasia (overgrowth) of one or both thyroid lobes. The exact trigger is not fully understood, but several factors are associated with higher risk:

• Age — most affected cats are over ten years old; the disease is very rare in cats under seven
• Diet — long-term consumption of canned fish-flavored foods and certain food packaging components have been studied as possible contributors
• Indoor lifestyle and living in North America or Europe — prevalence is higher in countries where the disease has been tracked longest
• Exposure to household chemicals (flame retardants in furniture, carpet) — an active area of research
• Iodine levels in the diet — both excess and deficiency may play a role

Thyroid carcinoma (cancer) causes fewer than 2% of feline hyperthyroidism cases. Hyperthyroidism in dogs, while rare, is more commonly caused by thyroid cancer and warrants a different diagnostic approach.

Diagnosis is typically straightforward in cats with classic signs. Our in-house laboratory can measure total thyroxine (T4) — the primary thyroid hormone — and return results the same day. In most cases a single elevated T4 alongside compatible signs is enough to confirm the diagnosis. Where signs are present but T4 is borderline, we add a free T4 by equilibrium dialysis (fT4-ED) for greater sensitivity. The work-up also includes:

• Complete blood count and chemistry panel — to assess kidney function before treatment, since hyperthyroidism can mask concurrent chronic kidney disease (CKD)
• Urinalysis
• Blood pressure measurement — hyperthyroidism frequently causes hypertension (high blood pressure)
• Chest X-ray and cardiac evaluation if a murmur or arrhythmia is detected; our cardiologist Dr. Lynette D'Urso is on-site for echocardiogram when needed

Four approaches are available, and each has advantages depending on the cat's age, overall health, and owner preference:

• Daily oral medication (methimazole) — blocks thyroid hormone production; works quickly, is reversible if kidney disease emerges, but requires lifelong twice-daily dosing. A transdermal gel rubbed inside the ear flap is available for cats that resist pilling.
• Radioactive iodine (I-131) — a single injection that selectively destroys abnormal thyroid tissue; considered the gold standard curative treatment, though it requires a brief period of isolation after therapy
• Surgical thyroidectomy — surgical removal of the affected thyroid lobe(s), performed by Dr. Schulman; curative and does not require isolation, but carries anesthetic risk in older cats
• Prescription iodine-restricted diet (Hill's y/d) — controls the disease through strict dietary iodine limitation; requires that the cat eat nothing else, which is a challenge in multi-pet households

Regardless of which treatment is chosen, a trial of medication first is often recommended to confirm kidney function remains stable when thyroid levels normalize — an important step before committing to permanent cure.

Any cat over seven years old losing weight — even with a good appetite — warrants a blood panel. Early treatment prevents heart and kidney complications that are much harder to manage once established. High blood pressure from hyperthyroidism can cause sudden blindness or stroke, both of which are emergencies. Our team at Dog & Cat Hospital Calabasas runs same-day thyroid testing and can walk you through all the treatment options at the same appointment. Book an appointment for your cat today.