Hyperthyroidism is fairly common in the feline population. Hypothyroidism, by contrast, is rare in cats (it is common in dogs) — the opposite condition. The whole thing manifests as overactivity of the thyroid gland, a disorder found especially in elderly cats. It is in fact a condition involving benign neoplastic forms with the development of functional nodules of thyroid tissue — true adenomas — yet one that can lead to serious illness.

Hyperthyroidism can in any case cause problems for our cat's health, leading to overproduction of thyroid hormone. Malignant forms, however, account for under 2 per cent of all affected cats. To understand more, and above all to know how to navigate a diagnosis of hyperthyroidism, we asked for the opinion of an expert: Dr Gianni Marinacci, veterinary surgeon.

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At what age does hyperthyroidism tend to appear in cats, and in what way?
This is a disease typical of elderly cats, often arising around the age of 11–12 years. Younger cats can also be affected, but this is substantially uncommon. The typical presentation is characterised by increased appetite alongside weight loss, an unkempt coat and a lack of self-grooming. Nocturnal activity often increases, with vocalisation and food theft if food is left unattended. Chronic gastrointestinal symptoms such as diarrhoea and vomiting may also appear. Some cats affected by hyperthyroidism, in the early stages of the disease, seem to revert to kitten-like behaviour and attitudes.

There are, however, other atypical manifestations of feline hyperthyroidism that make diagnosis less straightforward. In fact, there are as many as six subtypes of the disease, with symptoms ranging from polyphagia to anorexia, from a general increase in activity all the way to complete lethargy and immobility. Without forgetting, of course, that because the disease presents in elderly animals, various other conditions may complicate the picture: heart disease, neoplasia, or chronic kidney disease. Sometimes, aside from neoplasia, an excess of thyroxine (thyroid hormone) can induce renal fibrosis, hypertension (in around 10 per cent of cases), and cardiovascular disorders.

How is the diagnosis made?
In cases of typical disease, the diagnosis is clinically straightforward, and a total thyroxine (tT4) reading above the upper threshold is usually confirmation of the condition. There are, however, borderline cases in which making the diagnosis on the first attempt is far from obvious. It is sometimes possible to palpate one or more nodules in the neck region where the thyroid is located. In some rare cases these nodules are ectopic — that is, they are not found in their usual anatomical site but in other areas. They can in fact be located along the neck or even in the chest.

And the treatment?
The disease, however unserious it may appear (the cat eats more, moves more, and so on), absolutely must be treated and stopped; otherwise it can significantly shorten the patient's survival. Several therapies are available, and I will outline four of them in order of efficacy.

1. Radioiodine therapy with iodine-131: effective, but not widely available in Italy, where very few centres offer it. The regulations governing the practice of nuclear medicine are extensive and permits are not easy to obtain. Nevertheless, if the patient is suitable, it represents the best solution.
2. Surgery: my absolute favourite — remission of symptoms is achieved within 24–36 hours and normal thyroid function is restored. It would be advisable to identify the nodules by scintigraphy or CT scan in order to avoid early recurrence.
3. Medical therapy with methimazole and its metabolites: requires the daily administration of a drug that suppresses thyroid function. Many cats do well on this therapy for a long time, but side effects often appear — gastrointestinal, with chronic vomiting, as well as cutaneous and haematological, with anaemia. In the latter two cases it is often necessary to change the treatment approach under the guidance of the attending vet.
4. Dietary therapy with an iodine-free diet: if no iodine is supplied through the diet, the thyroid lacks the building blocks to produce thyroxine, and euthyroidism is reached within one to two months. The cat obviously cannot consume any other food or supplements containing even trace amounts of iodine; otherwise the entire rationale of the therapy collapses. This approach is effective, but cats often refuse the specific food after a while.

Naturally, in choosing which therapy to pursue, the opinion of the attending vet is paramount — they alone are in a position to determine exactly the treatment and the manner in which the chosen therapy is carried out.