Feline hyperthyroidism is the most common endocrinopathy in cats. In the UK it is estimated that the incidence of hyperthyroidism is as high as 10% in geriatric cats. In 97-98% of cases the disease is caused by benign, adenomatous hyperplasia of the thyroid gland or by a single thyroid adenoma. In 2-3% of cases, the disease is caused by a malignant thyroid carcinoma. Clinical signs of hyperthyroidism vary with the severity. They typically include polyphagia and weight loss (most common) but also gastrointestinal signs, behavioural changes such as restlessness and anxiety, polyuria and polydipsia, tachypnoea or dyspnoea. In early stages or mild forms of the disease, clinical signs can be subtle and be missed by the owner. It is therefore important to screen all cats older than 8 years of age to allow diagnosis of subclinical cases.
Fortunately, diagnosing hyperthyroidism is relatively straightforward in around 90% of cases and is based on finding increased serum total thyroxine (T4) concentration. However in 10% of cats with mild disease or concurrent non-thyroidal illness, total T4 concentration can fall within the high end of the normal reference interval. In these cases repeating total T4 at a later date or measuring serum free T4 concentration might help diagnose the disease.
After diagnosing hyperthyroidism, all cats should be staged to identify possible comorbidities (concurrent diseases) or complications of hyperthyroidism such as secondary cardiomyopathies, congestive heart failure, urinary infections which will affect the choice of the most appropriate treatment modality.
There are four treatment modalities for feline hyperthyroidism. These are divided into definitive and non-definitive (palliative) modalities. The two definitive modalities include radioactive iodine (I131) and thyroidectomy. Non definitive modalities include anti-thyroid drugs (methimazole and carbimazole) and iodine-restricted diets (Hill's prescription diet Y/D).
Radioactive iodine (RAI) treatment is considered the gold standard treatment modality for feline hyperthyroidism. RAI therapy has a cure rate of around 95% after a single treatment. In addition, RAI is not invasive and does not carry anaesthetic or surgical risks.
The Feline Hyperthyroid Clinic (FHC) at Anderson Moores Veterinary Specialists offers treatment with RAI in cats suffering from hyperthyroidism. The clinic is run by a team of RCVS, European and American specialists in small animal internal medicine assisted by veterinary specialists in training and qualified veterinary nurses.