What Is Hyperthyroidism in Cats?
Hyperthyroidism in cats is one of the most common endocrine (hormonal) diseases in middle-aged and older cats. It occurs when the thyroid gland — two small lobes located on either side of the windpipe in the neck — produces excessive amounts of thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3).
These hormones regulate metabolism throughout the body. When levels are too high, nearly every organ system is affected, causing a cat’s metabolism to run in overdrive. This leads to a characteristic cluster of symptoms including weight loss despite an increased appetite, hyperactivity, and increased heart rate.
Hyperthyroidism affects cats of all breeds but is most common in cats over 10 years of age. It is rarely seen in cats younger than 7 years. The good news is that hyperthyroidism is very treatable — often with excellent long-term outcomes — when diagnosed and managed appropriately.
Causes and Risk Factors
In the vast majority of cases (about 98%), hyperthyroidism in cats is caused by a benign (non-cancerous) overgrowth of thyroid tissue called adenomatous hyperplasia or a thyroid adenoma. Thyroid carcinoma (cancer) accounts for fewer than 2% of cases.
The exact cause of this abnormal thyroid growth is not fully understood, but several factors are thought to contribute:
- Diet: Long-term feeding of certain canned cat foods, particularly those containing soy, fish, or high iodine levels, has been investigated as a possible environmental trigger.
- BPA and canned food liners: Some studies suggest chemicals in the lining of canned foods (such as bisphenol A) may disrupt thyroid function.
- Iodine levels: Both excess and deficient dietary iodine have been explored, though findings are inconsistent.
- Genetics: No specific breed predisposition has been firmly established, though Siamese and Himalayan cats appear slightly less likely to be affected.
- Age: The greatest risk factor — most affected cats are over 10 years old.
Signs and Symptoms of Hyperthyroidism in Cats
Hyperthyroidism develops gradually, which means owners often don’t notice the early signs until the condition has progressed. Symptoms can include:
- Weight loss despite a normal or increased appetite — the classic sign
- Increased appetite or food-seeking behaviour
- Increased thirst and urination
- Hyperactivity, restlessness, or vocalisation (especially at night)
- Vomiting and/or diarrhoea
- A poor, unkempt coat — reduced grooming or over-grooming
- Rapid or irregular heart rate (palpable when you hold the cat)
- Muscle weakness — difficulty jumping, wobbly back end
- A palpable thyroid lump in the neck (goitre) — found in about 80% of cases on examination
Some cats with hyperthyroidism have concurrent kidney disease. When thyroid levels are treated and normalise, underlying kidney disease can become more apparent — this is something your vet will monitor closely.
Diagnosis and Treatment Options
Diagnosis is straightforward: a blood test measuring total T4 levels will be elevated in most hyperthyroid cats. Occasionally a cat has symptoms but normal T4 — further testing (free T4, T3 suppression test, thyroid scintigraphy) may be needed.
Treatment options include:
1. Medication (Methimazole/Felimazole): This is the most common initial approach. Methimazole blocks thyroid hormone production and is given once or twice daily. It does not cure the condition but controls it effectively. Side effects (vomiting, facial itching, blood disorders) occur in 10–20% of cats. A transdermal gel form applied to the ear flap is available for cats that resist pills.
2. Radioactive Iodine (I-131): Considered the gold standard cure. A single injection of radioactive iodine concentrates in thyroid tissue and destroys the overactive cells while sparing the parathyroid glands. It cures 95%+ of cats with one treatment. Cats must be hospitalised for several days until radiation levels drop. Not all clinics offer this treatment.
3. Surgical Thyroidectomy: Surgical removal of the affected thyroid lobe(s) can be curative but carries anaesthetic risk in older cats. Rarely performed when radioactive iodine is available.
4. Prescription Iodine-Restricted Diet: Hill’s y/d diet limits iodine intake, reducing thyroid hormone production. Effective only if strictly fed — even occasional treats or other food can render it ineffective. Not suitable for multi-cat households feeding from shared bowls.
Long-Term Management and Monitoring
For cats on medication, blood tests every 3–6 months are essential to monitor T4 levels, kidney function, and complete blood count. Dosing adjustments are common as the disease progresses or if side effects develop.
After radioactive iodine treatment, most cats are cured and require only annual wellness blood work. A small percentage may develop hypothyroidism (low thyroid levels) post-treatment, requiring supplementation.
Concurrent conditions are common in older cats with hyperthyroidism — including kidney disease, hypertension, and heart disease (hypertrophic cardiomyopathy). Your vet will assess blood pressure, cardiac function, and kidney values as part of ongoing monitoring. With appropriate management, many hyperthyroid cats live comfortably for several years after diagnosis.