The lateral surface of the thyroid is covered by the sternothyroid muscle, and its attachment to the oblique line of the thyroid cartilage prevents the superior pole from extending superiorly under the thyrohyoid muscle.
Thus, the upward extension of a thyroid swelling (goitre) is prevented by the attachment of the sternothyroid to the thyroid cartilage. A goitre can therefore only grow to the front, back or middle but no higher.