Which condition is the most common cause of hypothyroidism?

Hashimoto thyroiditis is recognized as the most common cause of hypothyroidism. It is an autoimmune disorder where the immune system mistakenly attacks the thyroid gland, leading to inflammation and impairment of its ability to produce thyroid hormones. Over time, this damage can result in insufficient hormone levels, causing symptoms associated with hypothyroidism such as fatigue, weight gain, and sensitivity to cold.

In contrast, Graves' disease is an autoimmune condition that typically leads to hyperthyroidism, where the production of thyroid hormones is excessive. Thyroid adenomas are benign tumors of the thyroid that can also lead to hyperthyroidism or have no effect on hormone levels, while pituitary adenomas primarily affect hormone production from the pituitary gland, which may impact thyroid stimulation but do not directly cause hypothyroidism. Hashimoto thyroiditis is therefore the most relevant condition in the context of primary hypothyroidism, highlighting its significance in clinical practice.