External structure and location
The thyroid gland (glandula thyroidea) is located in the anterior region of the neck. Anatomically, it consists of two large lateral lobes — the right (lobus dexter) and the left (lobus sinister). In the midline, these lobes are connected by the isthmus (isthmus glandulae thyroideae).
The isthmus is a variable structure. In some individuals, it may be completely absent, in which case the lateral lobes lie side by side, abutting each other without a tissue bridge.
Occasionally, accessory thyroid glands (glandulae thyroideae accessoriae) can be found in the neck region. In their structure, they are completely identical to the main gland. Accessory glands may be located isolated or connected to the main organ by a thin connective tissue strand.
Capsular apparatus and internal structure
Externally, the gland is covered by a thin connective tissue plate — the fibrous capsule (capsula fibrosa). From this capsule, septa extend into the organ, dividing the glandular parenchyma into individual lobules (lobuli).
The supporting framework of the organ is called the stroma (stroma). It is formed by thin layers of connective tissue that are abundantly supplied with blood vessels and nerve fibers.
Within the meshes of the stroma lie the structural and functional units of the gland, the follicles (folliculi glandulae thyroideae). They have a round or oval shape and are lined with a single-layer epithelium. The follicular cavity is filled with colloid, a viscous substance containing the protein compound thyroglobulin. Hormones are subsequently synthesized from this very substance. Parafollicular cells are localized between the follicles.
Topography and syntopy
The thyroid gland has a complex system of relationships with adjacent organs of the neck:
- Anterolateral surfaces are covered by muscles: sternothyroid, omohyoid, and the superior bellies of the omohyoid muscles.
- Posturomedial surfaces are adjacent to the trachea (the inferior poles of the lobes descend to the level of the 5th–6th tracheal rings), esophagus, pharynx, and in the upper parts, to the cricoid and thyroid cartilages.
- In the region where the anterolateral surfaces transition into posteromedial ones, the gland comes into contact with the neurovascular bundle of the neck, which includes the common carotid artery, internal jugular vein, and vagus nerve.
- The recurrent laryngeal nerve runs near the posteromedial surface, and tracheal lymph nodes are located there.
There is a slit-like space between the outer and inner capsules of the organ. It is filled with loose adipose tissue, through which the venous vessels of the thyroid gland pass, and where lymph nodes and parathyroid glands are located.
Functions and age-related changes
The size and weight of the gland are subject to significant individual variations. During puberty, the organ noticeably enlarges. In elderly and senile age, the functional tissue gradually atrophies, and the volume of connective tissue in it increases, causing the overall size of the gland to decrease.
The gland secretes three most important hormones:
- Thyroxine
- Triiodothyronine
- Thyrocalcitonin
These substances exert a global effect on the body. They regulate basal metabolism, heat production, oxidative processes at the cellular level, and ensure normal tissue growth and development. In addition, thyroid hormones take a direct part in calcium and phosphorus metabolism.
Briefly: answers to topical questions
Which arteries supply blood to the thyroid gland?
Blood supply to the thyroid gland is provided by the paired superior and inferior thyroid arteries and, in some cases, the unpaired thyroidea ima artery.
- Superior thyroid artery (a. thyroidea superior) is a branch of the external carotid artery, giving off rr. glandulares anterior, posterior, lateralis to the lobes of the gland.
- Inferior thyroid artery (a. thyroidea inferior) is a branch of the thyrocervical trunk of the subclavian artery, giving off rr. glandulares.
- Thyroidea ima artery (a. thyroidea ima) is found in 6–8% of cases, originates from the brachiocephalic trunk, aortic arch, or left subclavian artery, and approaches the lower edge of the isthmus.
Inside the gland, the branches of these arteries form anastomoses.
Through which veins does venous blood drain from the thyroid gland?
Venous outflow from the thyroid gland occurs through the venous plexus surrounding the organ via the following pathways:
- Superior thyroid veins (vv. thyroideae superiores) run along with the arteries of the same name and drain into the facial vein or directly into the internal jugular vein.
- Inferior thyroid veins (vv. thyroideae inferiores) are formed from the venous plexus on the anterior surface of the gland and the unpaired thyroid venous plexus (plexus thyroideus impar), draining into the right and left brachiocephalic veins.
- Thyroidea ima veins (vv. thyroideae imae) drain into the brachiocephalic vein.
Which nerves provide autonomic innervation to the thyroid gland?
Autonomic innervation of the thyroid gland is provided by sympathetic and parasympathetic nerve fibers.
- Sympathetic nerves are the superior and inferior thyroid nerves originating from the cervical ganglia of the sympathetic trunk (truncus sympathicus). The fibers reach the organ via periarterial plexuses along the thyroid arteries.
- Parasympathetic nerves are represented by branches of the vagus nerve (n. vagus). The superior laryngeal nerve (n. laryngeus superior) and recurrent laryngeal nerve (n. laryngeus recurrens) approach the gland.
To which regional lymph nodes does lymph drain from the thyroid gland?
Lymph drainage from the thyroid gland is carried out to several groups of regional lymph nodes of the neck.
- Pretracheal and paratracheal nodes receive lymphatic vessels from the lobes and isthmus of the gland.
- Tracheal nodes are located near the posteromedial surface of the lobes.
- Deep anterior cervical nodes (nodi lymphoidei cervicales anteriores profundi) form the main drainage pathway from the organ.
- Anterior superficial cervical nodes collect lymph predominantly from the isthmus of the thyroid gland.
Subsequently, the lymph is directed to the deep lymph nodes of the neck.
What function do the parafollicular cells of the thyroid gland perform?
Parafollicular cells (C-cells) of the thyroid gland perform an endocrine function, participating in the regulation of calcium and phosphorus metabolism in the body. They synthesize and secrete the peptide hormone calcitonin (thyrocalcitonin). This hormone exerts a hypocalcemic effect. Calcitonin is accumulated in dense granules inside C-cells and is released via exocytosis in response to an increase in the concentration of calcium ions in the blood, promoting bone tissue mineralization.
Class and exam questions
Are the right and left lobes always connected to each other?
No, not always. The isthmus of the thyroid gland may be absent, in which case the right and left lobes lie side by side without being connected by a tissue bridge.
What is located inside the thyroid follicles?
The follicles are filled with colloid. It contains thyroglobulin, a specific protein compound that serves as a matrix for the formation of thyroid hormones.
Where are the parathyroid glands located in relation to the thyroid?
They lie in the loose adipose tissue of the slit-like space located between the outer and inner capsules of the thyroid gland.
To which level of the trachea do the lower poles of the gland descend?
The lower parts of the right and left lobes usually reach the level of the fifth or sixth cartilaginous ring of the trachea.
What else to study on this topic
- Histological structure of follicles and parafollicular cells
- Topography of the neurovascular bundle of the neck relative to the gland
- Anatomy and role of accessory thyroid glands
- Biochemistry of thyroglobulin and the hormone synthesis process
- Age-related involution and proliferation of connective tissue in the stroma
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