Etiology of Feline Hyperthyroidism

Most cases of feline hyperthyroidism are caused by benign adenomas or adenomatous hyperplasia of the thyroid gland and frequently involve both lobes.

Malignant carcinomas of the thyroid are responsible for around 1-3% of all cases of feline hyperthyroidism and can involve both lobes.

The underlying pathology is important with regard to choice of treatment and prognosis.The cause of the changes in the thyroid gland is unknown. A range of possible underlying trigger factors have been suggested over the years with none proven. Theories include:

  • diet – high levels of iodine and goitrogenic compounds, such as phthalates
  • environmental factors – exposure to pesticides and fertilizers regularly
  • genetic mutation
  • abnormal immune and/or hormonal responses
adenomatous hyperplasia

Benign adenomatous hyperplasia seen on a fine needle aspirate from an enlarged thyroid gland
Photograph: Courtesy of Dr Hans Kooistra, Utrecht University

“Most cases of hyperthyroidism are caused by benign adenomas or adenomatous hyperplasia of the thyroid gland.”