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Feline hyperthyroidism
Hyperthyroidism is the most common hormonal disease in older cats. The thyroid gland, located in the neck, produces an excessive amount of hormones that speed up the entire metabolism. The cat loses weight even though it often eats more: th...
What is it?
Hyperthyroidism is the most common hormonal disease in older cats. The thyroid gland, located in the neck, produces an excessive amount of hormones that speed up the entire metabolism.
The cat loses weight even though it often eats more: this is the sign that most often alerts owners.
Commonly observed signs
These signs are neither exhaustive nor specific. Only a veterinarian can establish a diagnosis.
Related signs — each one links to its page:
Progressive weight loss despite a preserved or even increased appetite.
Thirst and urine volume greater than before.
Dull, poorly maintained coat, sometimes in patches.
Agitation, nocturnal meowing, unusual irritability in a previously calm cat.
Vomiting or loose stools in some cats.
In some cases, on the contrary, a marked depression with loss of appetite.
When to consult without delay
This list does not replace veterinary advice. It gathers situations that justify a prompt consultation.
Causes and risk factors
In the vast majority of cases, the excess hormones come from a benign transformation of the thyroid gland (adenoma), which begins to function without control.
A cancerous form exists but remains rare.
The exact causes of this transformation are not completely established; age is the best identified factor.
Advanced age: the disease mainly affects cats over ten years old.
No breed predisposition is clearly demonstrated; all older cats can be affected.
Diagnosis and care
Diagnosis is the veterinarian's responsibility, based on clinical examination and laboratory tests. Care is decided case by case.
The veterinarian begins with a complete clinical examination: palpation of the neck in search of an enlarged gland, weighing, taking heart rate and blood pressure.
Diagnosis is based on a blood test measuring thyroid hormones.
A renal and urinary assessment is carried out in parallel: hyperthyroidism can mask kidney disease which reveals itself once treatment has started.
A cardiac examination is often suggested, as the heart is strained by excess hormones.
Treatment
Several approaches exist and the choice depends on the age of the cat, its renal and cardiac condition, and the possibilities for monitoring.
Oral or cutaneous medical treatment helps slow down the production of hormones: it must be taken for life and monitored regularly by tests.
A specific diet low in iodine can be offered in certain situations, provided it is the cat's only source of nutrition.
Surgical removal of the gland and treatment with radioactive iodine resolve the problem permanently when they are accessible and the cat's condition allows it.
Treatment depends on the cause and the animal's condition: it must be determined by a veterinarian. No self-medication.
Prevention
There is no known way to prevent the onset of the disease.
Screening remains the best lever: an annual blood test from seven to eight years old allows hyperthyroidism to be identified before the cat weakens.
Weighing regularly at home helps spot weight loss that might go unnoticed by the naked eye.
Possible complications
The heart, stimulated for a long time, can thicken and function less well.
High blood pressure can damage the eyes, kidneys and brain; a detached retina sometimes causes sudden blindness.
Kidney disease can be unmasked as soon as treatment returns hormones to normal: it is not the treatment that caused it, it reveals its existence.
When to see a veterinarian
Your elderly cat is losing weight even though he is eating normally or more.
He drinks and urinates significantly more than before.
His behavior changes: agitation, meowing at night, unusual aggressiveness.
He breathes quickly when resting or becomes abnormally tired.
He bumps into objects or seems to no longer see: consult without delay.
Prognosis
With continued treatment and regular check-ups, most cats regain a stable weight and good quality of life for several years.
The prognosis depends mainly on the condition of the kidneys and heart at the time of diagnosis, hence the importance of not waiting.
Frequently asked questions
Mon chat mange beaucoup et maigrit quand même : est-ce grave ?
C'est une association qui doit toujours amener à consulter. Chez un chat âgé, elle évoque en premier lieu l'hyperthyroïdie ou le diabète, deux maladies qui se traitent d'autant mieux qu'elles sont prises tôt.
Le traitement est-il à vie ?
Le traitement médicamenteux, oui : il contrôle la production d'hormones sans supprimer la cause. La chirurgie et l'iode radioactif, quand ils sont possibles, permettent en revanche une correction durable.
Cette maladie est-elle contagieuse pour les autres chats ou pour moi ?
Non. C'est un dérèglement hormonal individuel : il ne se transmet ni aux autres animaux, ni à l'être humain.
À partir de quel âge faut-il dépister ?
Un bilan sanguin annuel à partir de sept à huit ans est une bonne pratique, car les premiers signes sont discrets et souvent mis sur le compte de la vieillesse.
References
AAFP — American Association of Feline Practitioners
Content reviewed by the TunisieVet editorial team · Updated 09 August 2026
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This information is provided for educational purposes only and is not a substitute for a veterinary consultation. If you have any concerns about your animal's health, consult a veterinarian.
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