What is the most common cause of hypothyroidism?

Hashimoto's thyroiditis is the most common cause of hypothyroidism, particularly in areas where iodine levels are sufficient. This autoimmune disorder leads to the destruction of thyroid tissue due to lymphocytic infiltration and the presence of thyroid autoantibodies, which ultimately impairs the thyroid's ability to produce hormones.

Over time, the immune system's attack on the thyroid gland results in decreased levels of thyroxine (T4) and triiodothyronine (T3), leading to the clinical manifestations of hypothyroidism such as fatigue, weight gain, cold intolerance, and depression. The prevalence of Hashimoto’s thyroiditis makes it the leading cause of hypothyroidism in both adults and children in many populations.

While iodine deficiency is a known cause of hypothyroidism globally, especially in regions with low dietary iodine intake, it has become less common in many developed countries due to iodization of salt and increased awareness of iodine's importance. Thyroidectomy and congenital hypothyroidism are also recognized causes, but they are less prevalent compared to Hashimoto's thyroiditis in the general population. Therefore, Hashimoto's remains the predominant cause of hypothyroidism, particularly in countries where iodine is adequately supplied.