Enlargement of the pituitary gland can be physiological or pathological. Physiological enlargement of the pituitary gland is mainly seen in women during pregnancy, especially in the middle and late stages of pregnancy. Pathological enlargement is mainly due to primary hypothyroidism and pituitary adenoma. In primary hypothyroidism, the pituitary gland enlarges because the thyroid gland does not produce enough thyroid hormone and the pituitary gland is asked to produce more thyrotropic hormone. More commonly, there is a pituitary microadenoma. If the pituitary gland is larger than three centimeters, it is a macroadenoma; if the pituitary gland is smaller than three centimeters and larger than one centimeter, it is called a macroadenoma. There is also a subset of pituitary microadenomas that are no more than one centimeter in height, but the pituitary MRI reveals a distinct pituitary tumor lesion.