Anatomy and Structure of the Thyroid Gland
The organ is located in the anterior neck (regio infrahyoidea) and lies adjacent to the upper rings of the trachea. The thyroid gland consists of two lateral lobes and a connecting isthmus (isthmus), which may sometimes be absent. The upper poles of the lobes reach the middle of the thyroid cartilage laminae, while the lower poles fall short of the sternum by 1.5–2.0 cm. In one-third of individuals, a pyramidal lobe (lobus pyramidalis) extends upward from the isthmus or one of the lobes.
The gland is covered by two sheaths:
- Inner — the fibrous capsule (capsula fibrosa). It is thin and sends septa directly into the parenchyma of the organ.
- Outer — the fascial sheath, formed by the visceral layer of the pretracheal (fourth) cervical fascia. It gives off fixing ligaments to the thyroid and cricoid cartilages, as well as to the trachea.
Between these capsules lies a space filled with loose connective tissue, containing vascular and nerve plexuses as well as the parathyroid glands.
Syntopy and Relationship to Nerves
Anteriorly, the gland is covered by the sternothyroid and sternohyoid muscles, and the upper poles additionally by the omohyoid muscle (m. omohyoideus). Laterally, the organ is in contact with the fascial sheaths of the medial neurovascular bundles of the neck. Posteriorly and medially, the larynx, trachea, and esophagus are adjacent to the gland. Due to the physiological displacement of the esophagus to the left, the left lobe is in closer contact with it. Enlargement of the thyroid lobes may cause compression of these organs.
Of particular surgical significance is the recurrent laryngeal nerve (n. laryngeus recurrens). It runs in the tracheoesophageal groove posterior to the cricothyroid ligament, is surrounded by paratracheal lymph nodes, and lies strictly outside the fascial sheath of the gland. Its compression or involvement in an inflammatory process causes dysphonia (hoarseness).
Topography of the Parathyroid Glands
The parathyroid glands (glandulae parathyroideae) are located on the posteromedial surface of the lateral lobes of the thyroid gland. There are usually four of them (two on the right and two on the left):
- The superior glands are located at the level of the lower border of the cricoid cartilage.
- The inferior glands are located in the region of the lower third of the lateral lobe.
A landmark is the entry point of the inferior thyroid artery into the organ: the superior gland typically lies above this point, and the inferior gland below it. The main feature is that the parathyroid glands lie within the loose connective tissue between the fibrous and fascial capsules. Consequently, during subcapsular resection (when only the tissue inside the fibrous capsule is removed), they remain undamaged and retain their own blood supply.
Blood supply and venous drainage
The thyroid gland is richly vascularized by two pairs of main arteries:
- Superior thyroid arteries (a. thyroidea superior) — branches of the external carotid arteries.
- Inferior thyroid arteries (a. thyroidea inferior) — arise from the thyrocervical trunks of the subclavian arteries. They give off numerous branches to adjacent organs and form anastomoses. In surgery, it is important to consider that the inferior artery sometimes loops around the recurrent laryngeal nerve.
In 6–8% of cases, an unpaired lowest thyroid artery (a. thyroidea ima) is present, branching from the brachiocephalic trunk, aortic arch, or left subclavian artery. It runs in the pretracheal space and can be accidentally injured during lower tracheotomy.
The venous network is represented by robust plexuses. The superior thyroid veins accompany the corresponding arteries, draining into the internal jugular or facial vein. The inferior and lowest thyroid veins are formed from the unpaired plexus (plexus thyroideus impar) located anterior to the trachea, and drain blood into the brachiocephalic veins.
Briefly: answers to topical questions
Which regional lymph nodes receive lymphatic drainage from the thyroid gland?
Lymphatic drainage from the thyroid gland proceeds to the pretracheal and paratracheal lymph nodes, and subsequently to the deep cervical lymph nodes. Tracheal lymph nodes are located near the posteromedial surface of the lobes.
Which nerves form the nerve plexuses innervating the thyroid gland?
The nerve plexuses innervating the thyroid gland are formed by autonomic fibers of the sympathetic and parasympathetic nervous systems.
- Branches of the sympathetic trunk (truncus sympathicus) — originate predominantly from the cervical ganglia, forming periarterial plexuses along the thyroid arteries.
- Superior laryngeal nerve (n. laryngeus superior) — a parasympathetic branch of the vagus nerve (n. vagus).
- Recurrent laryngeal nerve (n. laryngeus recurrens) — a parasympathetic branch of the vagus nerve (n. vagus), running in the tracheoesophageal groove.
The parathyroid glands lie BETWEEN the capsules (fibrous and fascial). Therefore, subcapsular resection of the thyroid gland is safe for them — the glands physically remain outside the surgical field.
Class and exam questions
Where exactly does the recurrent laryngeal nerve run in relation to the gland?
It is located in the tracheoesophageal groove posterior to the cricothyroid ligament. It is important to remember that the nerve lies outside the outer (fascial) sheath of the thyroid gland.
What is the clinical significance of the a. thyroidea ima?
This unpaired lowest thyroid artery (present in 6–8% of cases) runs anterior to the trachea. During a lower tracheotomy, it can be injured if not retracted beforehand.
Why can swallowing difficulties occur when the thyroid gland enlarges?
Due to its topographical proximity, the gland is adjacent to the esophagus (especially the left lobe, as the esophagus is displaced to the left). Hypertrophy of the tissue can lead to compression of the esophagus and trachea.
What else to study on this topic
- Arterial anastomoses in the walls of neck organs and within the thyroid parenchyma
- Lymphatic drainage: pathway from paratracheal to deep cervical lymph nodes
- Syntopy with neck muscles: the role of the sternothyroid and omohyoid muscles
- Formation of the plexus thyroideus impar and its connection with the thymic venous system
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