There is a pattern that veterinary clinics see almost every week: a cat in her teens who eats like a horse and still loses weight. Owners often read it as “she’s just getting old.” In most cases it is hyperthyroidism, the most common endocrine disease in cats — and one of the most treatable.
Why the Weight Loss Happens
In roughly 98 percent of cases the cause is a benign tumour (adenoma) on one or both thyroid glands in the neck. Those glands pump out far too much thyroxine (T4), and T4 sets the body’s metabolic rate. The cat burns calories faster than she can eat them, which is why an increased appetite and a shrinking body appear together. The excess hormone also reaches almost every organ system: the heart beats faster and harder, blood pressure can climb, and the kidneys and gut are affected over time. Most cases are diagnosed in cats over ten, and it is rare before the age of seven.
Signs Beyond the Scale
| Sign | Frequency reported by owners |
|---|---|
| Weight loss | About 92% |
| Increased appetite | About 61% |
| Drinking and urinating more | About 47% |
| Restlessness, hyperactivity, irritability | About 40% |
| Vomiting or diarrhoea | About 38–39% |
| Coat changes: patchy hair loss, greasy or dull coat, thin skin | About 36% |
Other clues owners describe include night yowling, heat intolerance, fast nail growth, and a racing heart — vets record a heart rate over 240 beats per minute in roughly half of cases, and a heart murmur in about 40 percent. Not every cat fits the picture: a minority present with a reduced appetite and lethargy instead, which is one reason a blood test matters more than a hunch.
How Vets Confirm It
An enlarged thyroid gland can be felt in the neck in around 90 percent of cases, but the diagnosis rests on bloodwork. A total T4 test confirms most cases; if the result is borderline, a free T4 by equilibrium dialysis is added. Because the hormone affects so much else, vets usually check kidney values, liver enzymes, and blood pressure at the same time, and they look at the heart when a murmur or arrhythmia is present.
The Four Treatment Options, Compared
| Option | What it does | Does it cure? | Trade-offs |
|---|---|---|---|
| Radioactive iodine (I-131) | One injection or capsule destroys overactive thyroid tissue anywhere in the neck | Yes — around 95% within about three months | Requires hospitalisation for radiation safety, and limited contact with people for one to two weeks afterwards |
| Anti-thyroid medication | Methimazole or carbimazole, given as a pill, liquid, or gel rubbed on the ear | No — controls while it is given | Usually twice daily for life; drug reactions such as facial itching, vomiting, liver or blood changes occur in up to a quarter of cats |
| Surgery (thyroidectomy) | Removes the abnormal gland or glands | Yes — over 90% when both glands are removed | Needs general anaesthesia, which is riskier in older cats, and the nearby parathyroid glands can be injured |
| Iodine-restricted diet | Starves the thyroid of the iodine it needs to make hormone | No — and relapse is complete if the diet stops | Only works as the cat’s sole food: no treats, no other cat’s bowl, no hunting; hard in multi-cat homes |
Which one is right depends on the cat’s age, kidney function, heart, other illnesses, and what an owner can realistically maintain. Radioiodine is generally considered the treatment of choice for otherwise healthy cats; medication and diet are common choices for older cats or those with kidney disease.
The Kidney Catch
Hyperthyroidism can hide chronic kidney disease. The extra hormone pushes more blood through the kidneys and inflates their apparent performance, so kidney values can look normal until the thyroid is treated — at which point reduced kidney function often becomes visible. That is not a reason to leave the thyroid untreated, but it is the reason vets monitor kidney values throughout, adjust doses, and schedule rechecks even when the cat looks fantastic.
The practical takeaway is short. Do not write off weight loss in a senior cat as normal ageing, and do not skip monitoring once treatment starts. A simple T4 test is inexpensive, and hyperthyroidism is one of the conditions where a cat can go on feeling well for years with consistent management. Never start, stop, or change a thyroid medication without your veterinarian.