Hyperthyroidism in cats: symptoms, treatments and what it costs
It is the most common hormonal disease in older cats, and the easiest to recognise if you know what to look for: a cat over ten that eats like never before, loses weight, is restless and yowls at night. It has good treatment, several in fact, and when well controlled it allows years of good life.
What it is
The thyroid gland, in the neck on either side of the trachea, produces the hormones that regulate metabolism. In feline hyperthyroidism, part of the gland enlarges and overproduces hormone, almost always because of a benign tumour (adenoma or adenomatous hyperplasia) in one or both lobes. Only in a small percentage of cases is it a malignant tumour. The excess hormone speeds up the whole body: the heart beats faster, calories are burnt at full speed, blood pressure rises and the cat lives in a state of constant hyperactivity. It is a disease of older cats: most cases appear from eight to ten years of age, and it is uncommon before then.
Typical symptoms
The most characteristic sign is weight loss with a big appetite: the cat eats more than ever, asks for food constantly and still loses weight and muscle. Often there is also increased thirst and urination, vomiting or diarrhoea, restlessness, hyperactivity or irritability, loud yowling, especially at night, and an unkempt or matted-looking coat. Some cats pant with stress or exertion, and others, on the contrary, are listless and have no appetite (the less common so-called apathetic form). Because the changes are gradual, many owners put them down to age: “he is getting old and eats a lot, but he is more active than ever”. In a twelve-year-old cat, that is not youth regained.
What happens if it is not treated
Excess thyroid hormone damages the body little by little. The heart muscle thickens and can progress to heart failure. High blood pressure can cause retinal detachment and sudden blindness, as well as damaging the kidneys and brain. Muscle loss weakens the cat, and malnutrition progresses despite eating. Well treated, on the other hand, hyperthyroidism has a good prognosis, and many cats live for years with a good quality of life. That is why feline guidelines recommend measuring thyroid hormone at check-ups for older cats, even before they show symptoms.
How it is diagnosed
When feeling the neck, the vet often notices a small lump in the thyroid area. The key test is measuring total T4 in the blood: if it is high and the cat has compatible symptoms, the diagnosis is made. In doubtful cases, especially early in the disease or in cats with another illness at the same time, T4 may come back in the upper part of the normal range; it is then repeated a few weeks later, or free T4 or other tests are measured. A full blood panel and urinalysis are also done to assess the kidneys, blood pressure is measured and, if there is a murmur or a fast rhythm, the heart is investigated. Some centres carry out thyroid scintigraphy.
The link with the kidneys
This is the most delicate point. Hyperthyroidism increases blood flow to the kidneys and makes them work “overtime”, which can hide chronic kidney disease that was already there. When the thyroid is treated and metabolism returns to normal, the kidneys go back to their real pace and the kidney disease shows up in the tests. It is not that treatment damages the kidneys, but that it reveals how they really were. That is why, before a definitive treatment, the vet often trials medication first and checks the kidneys a few weeks later. And that is why the two problems, both very common in older cats, are managed together.
The four treatments
There are four options, and the AAFP (American Association of Feline Practitioners) guidelines present them all as valid depending on the case. Antithyroid medication, as tablets, liquid or a gel applied to the skin of the ear, controls the disease but does not cure it: it is lifelong and needs regular blood checks. An iodine-restricted diet works only if the cat eats that food exclusively, with no treats or hunting. Surgery (thyroidectomy) can be curative, but carries the risks of anaesthesia in an older cat and of damaging the parathyroid glands. And radioactive iodine, which destroys only the diseased tissue with a single injection, is considered the treatment of choice because it cures the vast majority of cases, although it requires a licensed centre and a few days in isolation.
How to choose
There is no single best treatment for every cat. Radioactive iodine is curative and avoids years of tablets, but in Spain very few centres offer it, it means being apart from your cat for a few days and the initial outlay is high. Medication is the most widely used option: it is affordable and flexible, but it has to be given once or twice a day for life, and in a small proportion of cats it causes side effects such as vomiting, facial itching or blood changes, which the vet monitors. The diet is useful for cats that do not tolerate medication and only eat at home. Surgery makes sense in certain specific cases. Ask your vet these questions: which option they think is best for your cat, what each costs over five years and what monitoring it needs.
What it costs in Spain in 2026
As a guide: T4 measurement, €30 to €60; full blood tests with urinalysis and blood pressure for diagnosis, €150 to €300. With medication, treatment costs €20 to €50 a month, plus blood checks every few weeks at first and then every three to six months, €60 to €150 each. An iodine-restricted diet, €30 to €70 a month. Surgery, €600 to €1,500 including anaesthesia, blood tests and hospitalisation. Radioactive iodine, between the treatment, the preliminary tests and the days in hospital, usually comes to well over a thousand euros: ask the centre for a detailed estimate. Over five years, medication and radioactive iodine can end up costing about the same.
Living with a hyperthyroid cat
With treatment properly adjusted, within a few weeks the cat regains weight, calms down, stops vomiting and returns to its normal behaviour. If you use medication, consistency is what matters: give it every day, always at the same time, and do not skip check-ups, because the amount needed changes over time. If you use the ear gel, apply it wearing gloves and alternate ears. If you use the diet, that food and nothing else, for the other cats too if possible, or with separate feeding stations. And weigh your cat at home every month: the scales are the simplest way to tell whether treatment is working. The night-time yowling usually improves too, a relief for the whole household.
Prevention and early detection
Nobody knows for sure why it is so common in cats, and there is no proven prevention. Possible environmental factors have been studied, such as certain household chemicals or the composition of some diets, but without firm conclusions, so be wary of anyone selling a guaranteed way to avoid it. What does work is catching it early: check-ups at least once a year from seven or eight years old, and twice a year from eleven, with T4, blood tests, urinalysis and blood pressure; weighing the cat every month; and seeing the vet as soon as an older cat eats more, loses weight or is more restless than usual.
An older cat that eats a lot and loses weight, with thirst, vomiting or restlessness, probably has hyperthyroidism. It is diagnosed with a T4 test and treated with medication, diet, surgery or radioactive iodine. Well controlled, it allows years of good life, but the kidneys must be monitored.
Educational content from Michipedia, translated from the Spanish original. It does not replace the advice of a vet or a feline behaviour specialist. Every cat is an individual.