Health & Medical

A twelve-year-old cat eating like she is twenty pounds and getting thinner every month is not aging gracefully. She is telling you something specific, and the most likely explanation is a small gland in her neck working overtime.

Hyperthyroidism is the most common hormonal disorder in older cats. It affects an estimated one in ten cats over the age of ten, and the numbers may be higher because early cases get written off as normal senior changes.

What goes wrong

The thyroid gland sits in the neck and produces hormones that set the body's metabolic rate. In hyperthyroid cats, one or both lobes develop benign nodular growths that produce thyroid hormone without regard to what the body actually needs.

The result is a metabolism running permanently at high speed. Every organ works harder. The cat burns through calories, the heart beats faster and harder, blood pressure rises, and over time this constant overdrive damages the heart and kidneys.

In roughly 98 percent of cases the growth is benign. Thyroid carcinoma exists but is rare. The cause is not fully understood — research has looked at dietary iodine, certain flame-retardant chemicals in household dust and furnishings, and canned food, without landing on one definitive answer.

The signs, and why they get missed

The classic combination is weight loss despite an increased appetite. A cat that is eating enthusiastically, sometimes demanding food constantly, while steadily losing condition. Owners often notice the spine and hips becoming prominent.

Increased thirst and urination are common. So is a coat that looks unkempt, greasy, or matted, because the cat stops grooming properly. Some become notably more vocal, especially at night, and more restless or irritable than they used to be.

Vomiting and diarrhoea appear in a fair number of cases. A rapid heart rate is almost always present, though owners cannot detect it at home.

Here is why the diagnosis gets delayed. Each of these individually looks like something else — the vocalising gets attributed to cognitive decline, the weight loss to age, the appetite to a healthy cat, the grumpiness to arthritis. Put together, the pattern is distinctive, but the pieces arrive one at a time over months.

A minority of cats present with what is called apathetic hyperthyroidism — lethargic, poor appetite, weak. Less common, more easily missed.

Getting a diagnosis

Diagnosis is usually straightforward. On examination, most vets can palpate an enlarged thyroid nodule in the neck. Heart rate is typically elevated, often above 220 beats per minute, and there may be a murmur or gallop rhythm.

A blood test measuring total T4 confirms it in most cases. Where clinical signs strongly suggest hyperthyroidism but T4 sits in the upper-normal range, additional testing — free T4 by equilibrium dialysis, or a repeat test in a few weeks — sorts it out. Concurrent illness can suppress T4 into the normal range, which is a known trap.

Full bloodwork, urinalysis, and blood pressure measurement should accompany the diagnosis, because hyperthyroidism travels with other problems. Kidney disease in particular is important to identify early.

Four treatment routes

Radioactive iodine therapy is the gold standard. A single injection of radioiodine is taken up by overactive thyroid tissue and destroys it, leaving normal tissue intact. Cure rates exceed 95 percent, there is no ongoing medication, and there are no significant side effects. The drawbacks are cost, limited availability of licensed facilities, and a hospital stay of several days to a couple of weeks for radiation safety.

Oral medication — methimazole, or carbimazole in some countries — blocks hormone production. It is affordable, widely available, and works well. It also has to be given twice daily for life, requires periodic blood monitoring, and causes side effects in a minority of cats including vomiting, appetite loss, facial itching, and occasionally blood or liver abnormalities. A transdermal gel applied to the ear is available for cats who refuse tablets.

Surgical thyroidectomy removes the affected tissue. It can be curative but carries anaesthetic risk in an older cat with a strained heart, and there is a risk of damaging the nearby parathyroid glands. It is less commonly chosen now that radioiodine exists.

Dietary management using an iodine-restricted prescription food can control the condition in some cats. It only works if the cat eats nothing else at all — no treats, no other cat's food, no hunting. That makes it impractical in many multi-cat households, but it is a reasonable option for a cat who cannot tolerate medication.

The kidney complication

This part matters and surprises many owners. Hyperthyroidism increases blood flow through the kidneys, which can mask underlying chronic kidney disease. When you treat the thyroid, that artificial boost disappears and kidney disease that was hiding underneath becomes visible.

This is not the treatment causing kidney damage. The disease was already there. But it is the reason vets often start with a trial of oral medication even when radioiodine is the eventual plan — the medication is reversible, so if kidney values deteriorate badly, the dose can be adjusted or stopped while a management plan is worked out.

Outlook

Treated hyperthyroid cats generally do well. Many live years beyond diagnosis with good quality of life, particularly when the condition is caught before serious cardiac changes set in. The heart changes caused by hyperthyroidism often improve substantially once hormone levels normalise.

Untreated, the trajectory is poor — progressive weight loss, heart failure, hypertension leading to retinal detachment and blindness, and eventual organ failure.

The practical takeaway for anyone with a cat over eight: weigh them regularly and get annual bloodwork including a T4. Weight loss in a senior cat is never just aging, and this particular cause is very treatable when you find it early.