Download 1.1. BASIC THYROID ANATOMY

1.1. BASIC THYROID ANATOMY
ROBERT J. AMDUR, MD AND ERNEST L. MAZZAFERRI, MD, MACP
The normal thyroid gland is located in the anterior neck at the level of the thoracic inlet
(Fig. 1). The majority of the gland consists of two lateral lobes connected anteriorly by
the isthmus. Approximately 50% of people have a pyramidal lobe, which is a remnant
of the distal end of the thyroglossal duct. There is variability in the superior extent of
the pyramidal lobe between the thyroid and hyoid bone. The pyramidal lobe is usually
located just to the left of midline.
In adults the average thyroid gland weighs 10 to 20 grams and measures approximately
5 × 5 cm in the superior-inferior and medial-lateral dimensions (Pankow 1985). Superiorly the lateral lobes of the thyroid usually extend to level of the middle of the thyroid
cartilage. Inferiorly the thyroid usually extends to the level of the sixth tracheal ring.
Laterally the thyroid lies just medial to the common carotid arteries. The thyroid wraps
around 75% of the circumference of the trachea and the most posterior aspects of the
lateral lobes may touch the esophagus (Fig. 2). The anterior surface of the thyroid is just
deep to the strap muscles of the neck.
The location of the thyroid gland relative to important structures in the neck explains
the presenting symptoms of locally advanced thyroid cancer, potential surgical complications, and the complexity of planning external beam radiotherapy. The main structures
of interest are the recurrent laryngeal nerve, the trachea, the esophagus, the sympathetic
trunk, the vagus and phrenic nerves and the carotid arteries. Figure 2 does not show
the parathyroid glands and spinal cord. The parathyroid glands lie close to the posterior
surface of the thyroid and vary in number and exact location. The parathyroid glands are
discussed in more detail in a later chapter that focuses on the thyroidectomy procedure.
4 1. Introduction
Central
compartment
lymph nodes
Level VI
Deep
jugular
lymph
nodes
Thyroid
cartilage
Pyramidal
lobe
Posterior
triangle
lymph nodes
Superior
mediastinal
lymph nodes
Figure 1. Anatomic location of the thyroid gland relative to the larynx, major vessles and draining
lymphatics. (Reproduced with permission from MedImmune, Inc 2002.)
The spinal cord is located in the midline, approximately 4 cm posterior to the thyroid
gland. This distance, and the intervening muscles of the floor of the neck and bone of
the vertebral column, makes it so that tumor rarely spreads directly from the thyroid
area to the spinal canal. The proximity of the thyroid gland to the spinal cord is a major
factor when planning external beam radiotherapy.
LYMPHATIC DRAINAGE
The thyroid gland has a dense lymphatic network characterized by interconnections that
drain each area of the gland in multiple different directions. The concept of a stepwise
progression of nodal metastasis from one nodal station to another determines the extent
of the neck dissection for thyroid cancers and the extent of the irradiated volume in
patients who receive external beam radiotherapy (Qubnain 2002).
According to the sixth edition of the Cancer Staging Handbook organized by the
American Joint Commission on Cancer, the first echelon nodal metastases from thyroid
cancer are the nodes of the central compartment of the neck, the nodes of the superior
1.1. Basic Thyroid Anatomy 5
Figure 2. Axial section through the neck at the level of the C-7. (Redrawn from Eycleshymer AC,
Schoemaker DM: A cross-section anatomy. New York, D. Appleton-Century, 1938:55.)
mediastinum, and the lateral cervical nodes (AJCC 2002). Figure 3 is a diagram of the
boundaries of level I-VII nodal stations. The central compartment nodes are level VI,
which is bounded by the hyoid bone superiorly, the suprasternal notch inferiorly, and
the carotid arteries laterally. The specific nodal groups that drain the thyroid in the
level VI compartment are the paralaryngeal, paratracheal, and prelaryngeal (Delphian)
nodes. The level VII nodes are those of the superior mediastinum that lie superior to
the innominate vein. The lateral cervical nodes include nodes in both level III and IV:
Bilateral metastases are common.
THE PAROTID DUCT (STENSEN’S DUCT)
Stensen’s duct, named after a Danish physician anatomist Niels Stensen (1638–1686), is
the excretory duct of the parotid gland. About 7 cm long, it courses anteriorly over the
masseter muscle and buccal fat pad, and then bends medially to pierce the buccinator
muscle, ending intraorally at the level of the second maxillary molar (Netter 1965).
Stensen’s duct may develop sialadenitis or obstruction following I-131 therapy.
6 1. Introduction
Figure 3. Schematic of the lymph node stations of the neck as described in the 6th edition of the AJCC
staging manual.
REFERENCES
American Joint Committee on Cancer. 2002. AJCC Cancer Staging Manual, 6th edn. New York: Springer
22:29 (nodal stations) and 90–97 (nodal drainage).
Netter, FH (ed). 1965. Anatomy of the Thyroid and Parathyroid Glands. The CIBA Collection of Medical
Illustrations. Endocrine System and Selected Metabolic Diseases. New York: CIBA 4:41–42.
Pankow, BG, J Michalak, and MK McGee. 1985. Adult human thyroid weight. Health Phys 49:1097–1103.
Qubnain, SW, et al. 2002. Distribution of lymph node micrometastases in PN0 well differentiated thyroid
carcinoma. Surgery 131:249.