Experts do not know why most thyroid nodules form. However, several factors are associated with the development of noncancerous thyroid nodules. They include:

• Overgrowth of normal thyroid gland tissue (goiter)
  
  
• Development of fluid-filled cysts
  
  
• Inflammation of the thyroid (thyroiditis), especially Hashimoto’s thyroiditis
  
  
• Benign or cancerous tumor
  
  
• Hypothyroidism
  
  
• Genetic defect (associated with some autonomous nodules)
  
  
• Lack of dietary iodine (rare in the United States)

The risk of developing thyroid nodules increases with age. Thyroid nodules also tend appear more often in women than in men.

Risk factors associated with developing cancerous nodules include:

• Certain genetic conditions (mutation in a gene called RET proto-oncogene, or hereditary disorders including Gardner’s syndrome, familial polyposis or Cowden’s disease)
  
  
• Family history of medullary thyroid carcinoma or multiple endocrine neoplasia type II
  
  
• History of radiation exposure to the head or neck
  
  
• Male gender
  
  
• Age (younger than 20 or older than 70)