External Structure and Location
The thyroid gland (glandula thyroidea) is situated in the anterior neck. Anatomically, it consists of two large lateral lobes — the right lobe (lobus dexter) and the left lobe (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 such cases, the lateral lobes lie adjacent to each other without a connecting tissue bridge.
Occasionally, accessory thyroid glands (glandulae thyroideae accessoriae) can be found in the neck region. Structurally, they are identical to the main gland. Accessory glands may be located in isolation or connected to the main organ by a slender connective tissue strand.
Capsular Apparatus and Internal Structure
Externally, the gland is covered by a thin connective tissue layer — the fibrous capsule (capsula fibrosa). Septa extend inward from this capsule, dividing the glandular parenchyma into individual lobules (lobuli).
The supporting framework of the organ is called the stroma (stroma). It is formed by delicate connective tissue septa rich in blood vessels and nerve fibers.
Within the meshes of the stroma lie the structural and functional units of the gland — the thyroid follicles (folliculi glandulae thyroideae). They are rounded or oval and lined by a single layer of epithelium. The follicular lumen is filled with colloid, a viscous substance containing the protein thyroglobulin, which serves as the precursor for hormone synthesis. Parafollicular cells are located between the follicles.
Topography and Syntopy
The thyroid gland has complex topographical relationships with adjacent neck structures:
- Anterolateral surfaces are covered by muscles: sternothyroid, sternohyoid, and the superior belly of the omohyoid.
- Posteromedial surfaces are adjacent to the trachea (the inferior poles of the lobes descend to the level of the 5th to 6th tracheal rings), esophagus, pharynx, and superiorly to the cricoid and thyroid cartilages.
- At the transition from the anterolateral to the posteromedial surfaces, the gland contacts the carotid sheath of the neck, which contains the common carotid artery, internal jugular vein, and vagus nerve.
- The recurrent laryngeal nerve courses near the posteromedial surface, alongside tracheal lymph nodes.
A slit-like space exists between the true (fibrous) and false (fascial) capsules of the organ. It is filled with loose adipose tissue traversed by thyroid veins, lymph nodes, and the parathyroid glands.
Functions and Age-Related Changes
The size and weight of the gland undergo significant individual variations. During puberty, the organ noticeably enlarges. In elderly and senile age, the functional tissue gradually atrophies and is replaced by connective tissue, causing the overall size of the gland to decrease.
寿司 gland secretes three major hormones:
- Thyroxine ($T_4$)
- Triiodothyronine ($T_3$)
- Thyrocalcitonin (Calcitonin)
These substances exert systemic effects on the body. They regulate basal metabolism, thermogenesis, cellular oxidation, and ensure normal tissue growth and development. Additionally, thyroid hormones directly participate in calcium and phosphorus metabolism.