Which of the following statements regarding anatomic relationships of the thyroid gland are true?

b. The superior thyroid artery is usually the first branch of the external carotid artery

d. The parathyroid glands may lie within the pretracheal fascia

The thyroid gland is a vascular organ supplied by four main arteries: two superior and two inferior. The superior thyroid artery usually arises as the first branch of the external carotid artery just above the bifurcation of the common carotid artery. The superior thyroid artery descends medially on the surface of the inferior pharyngeal constrictor muscle to divide into an anterior and posterior branch at the apex of the thyroid lobe on its anteromedial surface. Its relation to the external branch of the superior laryngeal nerve is important during thyroid lobectomy. The inferior thyroid arteries usually arise from the thyrocervical trunks and ascend behind the carotid sheath before passing downward and medial to enter the thyroid gland at its middle portion. There are no arteries directly entering the lower poles from below with the exception of a thryoidea ima artery that may replace an absent inferior artery. Thyroidea ima arteries arise from either the innominate artery or aorta in 1% to 4% of individuals, entering the lower surface of the isthmus after coursing on the trachea.

The pretracheal fascia is referred to as the thyroid sheath and it varies in consistency and completeness among individuals. Posteromedially, it is usually condensed and firmly attaches the thyroid gland to the upper two or three tracheal rings and the cricoid cartilage. The superior parathyroid gland may lie between the sheath and the thyroid capsule, within the sheath, posterior to it in a potentially open plane, or occasionally anterior to the sheath. The inferior parathyroid gland likewise may be within the sheath, particularly when the gland is adherent to the lower pole of the thyroid.