Results from any disorder that results in insufficient secretion of thyroid hormones from the thyroid gland

Aetiology

Congenital hypothyroidism

• Absent or under-developed thyroid gland
• Dyshormogenesis - genetic defects in the synthesis of thyroid hormones, resulting in hypothyroidism with goitre
• Iodine deficiency during pregnancy
• Maternal transmission of antithyroid drugs

Acquired hypothyroidism

Primary hypothyroidism:

Insufficient thyroid hormone production:

• Hashimoto's thyroiditis (chronic lymphocytic thyroiditis)
  • Most common cause of hypothyroidism in iodine-sufficient regions
  • Affects middle aged women (45-60 years)
  • Associated with family history of autoimmune thyroiditis or other autoimmune diseases
  • Associated with HLA - DR3 and DR5
• Iodine deficiency - most common cause worldwide, particularly in iodine-deficient regions
• Iatrogenic e.g. post-ablative therapy (e.g. radioiodine, surgery)
• Others: atrophic thyroiditis (autoimmune), de Quervain's thyroiditis and post-partum thyroiditis

Secondary hypothyroidism

• Pituitary disorders (e.g. pituitary adenoma) resulting in TSH deficiency

Tertiary hypothyroidism

• Hypothalamic disorders resulting in TRH deficiency

Pathophysiology

Hashimoto's thyroiditis