The thyroid has an abundant blood supply (Fig. 3). The arterial supply to each thyroid lobe is twofold. The superior thyroid arteries arise from the external carotid artery on each side and descend several centimeters in the neck to reach the upper poles of each thyroid lobe, where they branch. The inferior thyroid arteries, each of which arises from the thyrocervical trunk of the subclavian artery, cross beneath the carotid sheath and enter the lower or midpart of the thyroid lobe. The thyroidea ima is sometimes present; it arises from the arch of the aorta and enters the thyroid in the midline. A venous plexus forms under the thyroid capsule. Each lobe is drained by the superior thyroid vein at the upper pole, which flows into the internal jugular vein; and by the middle thyroid vein at the middle part of the lobe, which enters either the internal jugular vein or the innominate vein. Arising from each lower pole is the inferior thyroid vein, which drains directly into the innominate vein.

Figure 3. Anatomy of the thyroid and parathyroid glands. A. Anterior view. B. Lateral view with the thyroid retracted anteriorly and medially to show the surgical landmarks (the head of the patient is to the left). (From Kaplan EL: Thyroid and parathyroid. In Schwartz SI (ed): Principles of Surgery, 5th ed., New York, McGraw-Hill, 1989, pp 1613-1685.)