What is the most common cause of hypothyroidism?

The most common cause of hypothyroidism is Hashimoto's thyroiditis, which is an autoimmune disorder. In this condition, the immune system mistakenly attacks the thyroid gland, leading to inflammation and decreased thyroid hormone production. Hashimoto's thyroiditis is prevalent in many populations and is often associated with other autoimmune diseases, making it a notably significant cause of hypothyroidism worldwide.

Iodine deficiency is another common cause of hypothyroidism, particularly in areas where iodine intake is low, but it is not as widespread in regions where iodized salt is used. Toxic adenomas and exophthalmos are related to other thyroid conditions. Toxic adenomas usually lead to hyperthyroidism rather than hypothyroidism, and exophthalmos, which is a symptom of hyperthyroidism such as Graves' disease, does not directly cause hypothyroidism. This makes Hashimoto's thyroiditis the correct and most common cause associated with hypothyroid conditions.