Thyroid Anatomy  
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Anatomy
T3, T4 Synthesis

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Biopsy
Thyroid Scan

Disease

Hyperthyroidism
Hypothyroidism
Thyroid Cancer
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Treatment

 

 Anatomy of the Thyroid

General

The thyroid gland is a highly vascularized organ located anteriorly in the neck, deep to the platysma, sternothyroid and sternohyoid muscles, and extending from the 5th cervical  (C5) to the 1st thoracic (T1) vertebrae.  The gland consists of two lobes (left and right) connected by a thin, median isthmus overlying the 2nd to 4th tracheal rings, typically forming an "H" or "U" shape.  Occasionally the isthmus is absent and the thyroid exists as two distinct lobes.  Embryologically, the thyroid gland develops as a thickening in the pharyngeal floor that elongates inferiorly as the thyroglossal duct, dividing into two lobes as it descends through the neck

Beneath the visceral layer of the pretracheal, deep cervical fascia, the thyroid gland is surrounded by a true inner capsule, which is thin and adheres closely to the gland.  The capsule sends projections into the thyroid forming septae and dividing it into lobes and lobules.  Dense connective tissue attachments secure the capsule of the thyroid to both the cricoid cartilage and the superior tracheal rings.

The lobules of the gland are composed of follicles, the structural unit of the thyroid.  Each follicle is lined by a simple layer of epithelium surrounding a colloid-filled core.  This colloid contains iodothyroglobulin, the precursor to thyroid hormones.

Thyroid Hormone Synthesis

Blood Supply and Nerves

Because the thyroid gland is a hormone secreting organ, it is highly vascularized.  It receives its blood supply from the superior and inferior thyroid arteries.  These arteries lie between the fibrous capsule and the pretracheal layer of deep cervical fascia.

The superior thyroid artery is the first branch of the external carotid artery and supplies the top half of the thyroid gland.  It divides into anterior and posterior branches supplying respective sides of the thyroid.  On the anterior side, the right and left branches anastomose with each other.  On the posterior side, the right and left branches anastomose with their respective inferior thyroid arteries. 

The inferior thyroid artery supplies the lower half of the thyroid and is the major branch of the thyrocervical trunk, which comes off the subclavian artery.  It too divides into several branches, supplying the inferior portion of the thyroid and anastomosing posteriorly with the superior thyroid branches.

There are three main veins that drain the venous plexus on the anterior surface of the thyroid.  They include the superior, middle, and inferior thyroid veins, and each drains its respective portion of the thyroid.  The superior and middle thyroid veins drain into the internal jugular veins, whereas the inferior thyroid vein drains into the brachiocephalic veins, behind the manubrium of the sternum.

Lymphatic drainage of the thyroid gland is quite extensive and flows multidirectionally.  Immediate drainage flows first to the periglandular nodes, then to the prelaryngeal (Delphian), pretracheal, and paratracheal nodes along the recurrent laryngeal nerve, and then to mediastinal lymph nodes.

The principal innervation of the thyroid gland is derived from the superior, middle, and inferior cervical sympathetic ganglia of the autonomic nervous system and parasympathetic fibers from the vagus nerves.  These nerves reach the thyroid gland by coursing with the blood vessels (superior and inferior thyroid periarterial plexuses).

Arteries Superior and inferior thyroid arteries
Veins Superior, middle, and inferior thyroid veins
Lymphatics Periglandular, prelaryngeal, pretracheal, and paratracheal lymph nodes
Nerves Sympathetic: Superior, middle, and inferior sympathetic ganglia
Parasympathetic:  Vagus nerves

Dissection Considerations

Cephalad to the superior pole of the thyroid gland, the external branch of the superior laryngeal nerve runs alongside the superior thyroid artery before turning medially to supply the cricothyroid muscle. High ligation of the superior thyroid artery during thyroidectomy places this nerve at risk of inadvertent injury, which would produce dysphonia by altering pitch regulation.

The cricothyroid artery is a potentially bothersome branch of the superior thyroid artery, which runs cephalad to the upper pole of the thyroid gland and runs toward the midline on the cricothyroid ligament. This vessel can be lacerated during emergent cricothyroidotomy.

The inferior thyroid artery is closely associated with the recurrent laryngeal nerve.  This nerve can be found after it emerges from the superior thoracic outlet, in a triangle bounded laterally by the common carotid artery, medially by the trachea, and superiorly by the thyroid lobe.  The relationship of the recurrent laryngeal nerve and the inferior thyroid artery is highly variable in that the nerve can lie deep to the artery, superficial to the artery, or between the branches of the artery, and be different on either side of the neck.  Consideration of this nerve and its branches must be given during dissection and thyroidectomy.