Chapter 38: Thyroid and Parathyroid Glands

Introduction

Embryology and Anatomy

Embryology

1. The thyroid’s medial anlage arises as a ventral diverticulum from the endoderm of the first and second pharyngeal pouches at the foramen cecum.

2. This diverticulum forms at 4-weeks gestation and descends from the base of the tongue to its adult pretracheal position in the root of the neck through a midline anterior path, the thyroglossal duct tract, assuming its final adult position by 7-weeks gestation.

3. If thyroid migration is completely arrested, a lingual thyroid results without normal tissue in the orthotopic location.

4. If the inferior most portion of the thyroglossal duct tract is maintained, a pyramidal lobe is formed. Persistence of a remnant of thyroid tissue at any point along the rest of the thyroglossal duct tract can develop into a thyroglossal duct cyst that may present as a midline neck mass, frequently in close association with the hyoid bone.

5. The lateral thyroid primordia develop from the fourth and fifth pharyngeal pouches and join the medial anlage as it descends in the neck.

6. Parafollicular C cells arising as ultimobranchial bodies from the neural crest of the fourth pharyngeal pouch migrate and infiltrate the forming lateral thyroid lobes.

Anatomy

1. General

   1. The thyroid gland is composed of two lateral lobes connected by an isthmus, which rests at the level of the second to fourth tracheal cartilages.

   2. Each thyroid lobe measures approximately 4.5 cm high × 1.5 cm wide × 2 cm deep.

   3. The pyramidal lobe, a remnant of descent of the thyroid, is present in up to 40% of patients.

2. Fascia

   1. The cervical viscera—including trachea, larynx, and thyroid—are ensheathed by the middle layer (visceral) of the deep cervical fascia.

   2. It is important to distinguish between the true thyroid capsule and the loose areolar tissue between the true thyroid capsule and strap muscles (ie, the perithyroidal sheath).

      1. True thyroid capsule: tightly adherent to the thyroid parenchyma and continuous with fibrous septa that divide the gland’s parenchyma into lobules.

      2. Perithyroidal sheath: thin cobweb-like tissue encountered as the strap muscles are elevated from the ventral surface of the thyroid; typically easily lysed and occasionally associated with small bridging vessels extending from the undersurface of the strap muscles to the true thyroid capsule.

3. Ligament of Berry

   1. The thyroid is attached to the laryngotracheal complex through anterior and posterior suspensory ligaments, causing the thyroid to elevate with the larynx and trachea during deglutition.

   2. The anterior suspensory ligament arises from the anterior aspect of the first several tracheal rings and inserts on the undersurface of the thyroid isthmus.

   3. The posterior suspensory ligament (ligament of Berry) is a condensation of the middle layer of deep cervical fascia and connects the posterior thyroid to the cricoid cartilage as well as first two tracheal rings. ...