There are several theories regarding substernal goiter, but the most popular is that the goiter represents an inferior growth from the cervical thyroid gland that is supplied by the vessels in the neck. The upright posture of human beings combined with normal swallowing and breathing mechanisms induces negative intrathoracic pressure. The weight of the enlarging mass allows the gland to grow and descend through the thoracic inlet into the substernal position. Inferiorly, there is no anatomic structure that can restrict the growth of the thyroid gland. This leads to the path of “least resistance”—toward the thoracic inlet.