Content
- What is Hyperthyroidism?
- What are the Symptoms of Hyperthyroidism?
- Why does Hyperthyroidism Occur?
- How is Hyperthyroidism Diagnosed?
- How is Hyperthyroidism Treated?
- What is the Outlook for Affected Cats?
What is Hyperthyroidism?
Hyperthyroidism is a disease of the thyroid glands. There are two thyroid glands, located midway up the neck on either side of the windpipe (trachea). In the normal cat they are too small to feel. When one or both glands become overactive, too much thyroid hormone is released into the circulation, and the cat develops one or more of the symptoms of hyperthyroidism.
Hyperthyroidism is a disease of the old cat. More than 95% of cases are in cats over 10 years old, with an average age at diagnosis of 12–13 years.
What are the Symptoms of Hyperthyroidism?
Any one or more of the following may be seen in hyperthyroidism:
- Increased thirst
- Diarrhoea
- Heart disease¹
- Increased appetite
- Vomiting
- Eye abnormalities²
- Hyperactivity
- Unkempt coat
- Lethargy
- Weight loss
- Coughing/sneezing
- Goitre³
Note 1: Heart disease can include increased heart rate, rhythm abnormalities, high blood pressure, and congestive failure (fluid in lungs, abdomen, or legs).
Note 2: Eye abnormalities usually result from high blood pressure, including detached retina and bleeding into the eyeball.
Note 3: Goitre is enlargement of the thyroid gland, felt by running fingers along the neck on either side of the trachea.
The “typical” case is an old, thin cat with a good appetite, excessive drinking, an increased heart rate, and a goitre. Hyperactivity is more common than lethargy.
Why does Hyperthyroidism Occur?
In most cases, the disease is caused by a benign tumour in one or both glands. Rarely, it is due to a malignant tumour (thyroid carcinoma). The reason why the same tumour often appears in both glands almost simultaneously remains unclear.
How is Hyperthyroidism Diagnosed?
Diagnosis is usually straightforward based on symptoms, but blood testing for thyroid hormone levels is essential before treatment. High levels confirm the disease. Sometimes results fall in the high-normal range, but if symptoms are convincing, hyperthyroidism is still likely. In such cases, repeat testing or further diagnostics may be required.
Treatment of Hyperthyroidism
There are three treatment options:
- Medication
- Surgical removal of the thyroid glands
- Radioactive iodine therapy
Medication
Methimazole (Felimazole®) – the only licensed drug in the UK, effective but may cause vomiting, hair loss, skin rashes, reduced white blood cells, liver damage, jaundice, or agranulocytosis (low immune cells). Side-effects are most likely in the first 3 months.
Carbimazole (NeoMercazole®) – converted by the liver into methimazole, requires more frequent dosing but has fewer long-term side-effects. Often used for lifelong management once diagnosis is confirmed.
β-blockers (Propranolol, Atenolol) – reduce heart and respiratory rate, control hyperactivity, but do not treat hormone levels. Used short-term before surgery or radioactive iodine therapy.
Surgical Removal
A common and effective procedure, but carries higher anaesthetic risks since cats are usually elderly with heart or kidney disease. Medication beforehand improves safety. Possible complications include persistence of hyperthyroidism, kidney failure (masked until surgery), hypocalcaemia (low calcium if parathyroids are removed), hypothyroidism, or rare nerve damage.
Radioactive Iodine Therapy
This specialist treatment destroys diseased thyroid tissue while sparing normal cells. It requires hospitalisation (approx. 8 weeks), is expensive, and is currently available only in specialist centres such as the University of Glasgow. It is highly effective and rarely causes hypothyroidism.
Outlook for Affected Cats
Hyperthyroidism is common in older cats and very rewarding to treat. Most cats return to normal and live full lives after treatment. Even very old cats often do well with Felimazole® followed by surgery, provided there are no serious heart or kidney complications.
