What is the basic normal anatomy and sonographic imaging appearance of the thyroid gland?
The thyroid gland is a butterfly-shaped organ located in the anterior neck, just below the thyroid cartilage and anterior to the trachea. It extends posteriorly to the esophagus and laterally to the carotid sheath, and consists of two lateral conically shaped lobes and the isthmus, a narrow band of midline tissue that connects the lateral lobes. The thyroid gland also has a pyramidal lobe, a superior extension of thyroidal tissue of variable length which is a vestigial remnant of the thyroglossal tract that arises from the isthmus and ascends toward the hyoid bone ( Figure 61-1 ). The normal thyroid gland has relatively homogeneous, bright parenchymal echogenicity with a smooth overlying capsule ( Figure 61-2 ). The dimensions of the thyroid gland vary with body habitus, but the general values for an adult are 15 to 20 mm in anteroposterior diameter, 20 to 30 mm in lateral width, and 40 to 60 mm in craniocaudal length, and the anteroposterior diameter of the isthmus is usually 6 mm. On ultrasonography (US) examination, the pyramidal lobe is quite small, but when hypertrophied, for example in the setting of Graves’ disease, it can be imaged and may be discontinuous from the isthmus.