By Dial A Vet Veterinary Team
An older cat who is eating more than ever but getting thinner is a classic picture of hyperthyroidism. It is the most common hormonal disease in cats over about ten years old, and because the early signs can look like a happy, energetic senior, it often goes unnoticed for months. Left untreated it strains the heart and raises blood pressure, which can damage the eyes and other organs. The good news is that it is very treatable. Hyperthyroidism also interacts with kidney disease in cats, so your vet will usually look at both together.
Key takeaways
- Hyperthyroidism is caused by an overactive thyroid gland, usually due to a benign growth.
- Typical signs are weight loss despite a big appetite, increased thirst, vomiting, restlessness, and a scruffy coat.
- It is diagnosed with a blood test for thyroid hormone, along with general blood, urine, and blood pressure checks.
- Treatment options include medication, a prescription iodine-restricted diet, radioactive iodine therapy, and surgery.
- Sudden blindness, breathing difficulty, or collapse in a hyperthyroid cat needs emergency care.
What is happening
The thyroid glands sit in the neck and produce hormones that control metabolism. In most hyperthyroid cats, a benign enlargement of one or both glands (adenomatous hyperplasia) produces too much hormone. Rarely, a malignant tumour is responsible. Excess thyroid hormone speeds up nearly every system in the body, burning calories faster, raising heart rate, and increasing blood pressure.
Signs of hyperthyroidism in cats
- Weight loss, often gradual, despite eating well or more than usual
- Increased appetite and begging for food
- Drinking and urinating more
- Vomiting or diarrhoea
- Restlessness, night-time yowling, or irritability
- An unkempt or greasy coat
- Fast heart rate or a new heart murmur
- Panting when stressed, which is unusual in cats
- A lump that a vet can feel in the neck
A small number of cats show the opposite pattern, with poor appetite and weakness, so any unexplained weight loss in an older cat is worth checking.
When it is an emergency
Go to an emergency veterinary hospital straight away if your cat:
- Is breathing with an open mouth or with effort (see breathing problems in cats)
- Suddenly appears blind or has dilated pupils that do not respond to light, which can follow very high blood pressure
- Suddenly cannot use its back legs or cries out in pain
- Collapses, is very weak, or has a racing heartbeat
- Stops eating, vomits repeatedly, or becomes yellow
How vets diagnose it
A vet will weigh your cat, feel the neck for an enlarged thyroid, listen to the heart, and check blood pressure. The main test is a blood test for total thyroxine (T4), which is raised in most hyperthyroid cats. If results are borderline, a free T4 test or repeat test may be suggested. General blood and urine tests check kidney and liver function. Because hyperthyroidism increases blood flow to the kidneys, it can mask kidney disease, which may only become obvious once the thyroid is treated. Your vet may also recommend a heart ultrasound if a murmur or rhythm problem is found.
Treatment options, in overview
- Medication reduces thyroid hormone production. It needs to be given for life and requires regular blood tests to adjust treatment and check for side effects. Some formulations can be applied to the ear skin for cats that are hard to pill.
- Prescription iodine-restricted diet. This works only if the cat eats nothing else, including treats, other pets' food, or hunted prey.
- Radioactive iodine therapy is often considered the gold standard, as a single treatment cures most cats. It is available at specialist centres in Australia and requires a short hospital stay.
- Surgery to remove the affected gland is an option for some cats, with careful monitoring of calcium levels afterwards.
The best option depends on your cat's age, other health problems, temperament, and your circumstances. Your vet may suggest a trial of medication first to see how the kidneys respond before choosing a permanent treatment.
Long-term management
- Attend recheck appointments and blood tests, especially in the first months of treatment.
- Monitor weight, appetite, thirst, and litter tray habits, and report changes.
- Keep an eye on blood pressure and kidney values, as these may need separate treatment.
- Report vomiting, itchy skin on the face, lethargy, or yellow gums after starting medication.
- Wear gloves when applying ear gel medication and wash your hands afterwards.
FAQ
Can hyperthyroidism in cats be cured?
Radioactive iodine therapy and surgery can cure most cats. Medication and diet control the disease but do not cure it.
Why does my cat have kidney problems now that the thyroid is treated?
Hyperthyroidism can hide underlying kidney disease by increasing blood flow through the kidneys. Your vet will monitor and manage both conditions.
Is hyperthyroidism painful?
It is not usually painful, but it makes cats restless and can cause serious complications if untreated.
Talk with a vet
If your older cat is losing weight, drinking more, or seems unusually restless, our vets can talk through the signs with you by video and help you plan testing. Book a consult at dialavet.com. If your cat is struggling to breathe, suddenly blind, or collapsing, go to an emergency vet hospital now.
References
- Carney HC, et al. 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. Journal of Feline Medicine and Surgery.
- Cornell Feline Health Center. Hyperthyroidism in Cats.
- International Cat Care. Hyperthyroidism in cats.
Disclaimer: Educational only. This page does not replace an examination or advice from your own veterinarian. If your cat is seriously unwell, contact a veterinary hospital promptly.
