The thyroid gland is a butterfly-shaped endocrine gland located inferior to the larynx and anterior to the trachea. The thyroid gland develops from the fusion of the median thyroid anlage with the two lateral thyroid anlages, which are derived from the pharyngeal pouches. The thyroid gland receives its arterial supply from the superior and inferior thyroid arteries and drains into the superior, middle, and inferior thyroid veins. The thyroid gland secrets thyroid hormones, which regulate body metabolism and growth, and calcitonin, which lowers serum calcium and phosphate through inhibition of osteoclasts. Hormone synthesis occurs in the epithelial lining of the thyroid follicles. The epithelial lining consists of follicular (thyroid epithelial) cells, which synthesize thyroid hormone, and parafollicular (C) cells, which synthesize calcitonin.
The parathyroid glands are four, oval-shaped endocrine glands located on the posterior surface of the thyroid gland. They are derived from the third and fourth pharyngeal pouches. The parathyroid glands receive their arterial supply from the inferior thyroid arteries and drain into the thyroid venous plexus. The parathyroid chief cells secrete parathyroid hormone, which maintains serum calcium and phosphate homeostasis and, furthermore, antagonizes the effect of calcitonin by increasing serum calcium and decreasing serum phosphate. The recurrent laryngeal nerves, parathyroid glands, sympathetic trunks, and the nerves of the carotid sheath are at risk of injury during thyroid surgery.
- Characteristics: butterfly-shaped, unpaired endocrine gland composed of two lobes
- Located anteriorly in the lower part of the neck
- Extends from C5–T1
- Surrounded by pretracheal fascia (along with pharynx, trachea, esophagus)
- Relations of the thyroid gland
- Anteriorly: strap muscles (i.e., sternohyoid, sternothyroid, thyrohyoid, and omohyoid muscles)
- Medially: the trachea, esophagus, recurrent laryngeal nerve (RLN), and the external branch of the superior laryngeal nerve
- Posteriorly: the parathyroid glands cricoid cartilage, lower thyroid cartilage, and the carotid sheath with its contents (i.e., internal jugular vein, vagus nerve, and common carotid artery)
- Function: produce thyroid hormones that are essential for regulating metabolism and growth (see )
Damage to the recurrent laryngeal nerves, parathyroid glands, sympathetic trunks, and even the nerves of the carotid sheath is possible during thyroidectomy because of the thyroid's location in the anterior neck.
- The thyroid gland is made up of a left lobe and a right lobe connected by an isthmus.
- An ascending pyramidal lobe is present in ∼ 50% of the population.
- The thyroid gland is encapsulated by:
Vasculature and innervation
|Venous drainage|| |
- Paratracheal nodes
- Deep cervical nodes
- Main component: thyroid follicles
- Interfollicular spaces are filled by reticular connective tissue, fenestrated capillaries (facilitate the release of hormones into the blood), lymphatic vessels, adipocytes, and sympathetic nerves.
Thyroid epithelial cell (= follicular cells)
- Function: lowers calcium in serum (see )
- Clinical significance
- Thyroglobulin, an iodine-free hormone precursor, is stored in the .
- Iodide is actively taken up by thyrocytes and transported into the follicular lumen.
- Here, thyroid peroxidase catalyzes the iodination of tyrosine residues of thyroglobulin, creating precursors monoiodotyrosine (MIT) and diiodotyrosine (DIT) and eventually the thyroid hormones.
- To release T3 and T4, the iodinated thyroglobulin must be taken up again by thyrocytes, where it is broken down by lysosomes, thus releasing attached T4 and T3.
- T4 and T3 are then transported out of the thyrocyte into the blood.
Detailed steps of thyroid hormone synthesis
1. Synthesis of (TG)
|Thyrocyte → follicular lumen|
2. Uptake of iodide
|Blood vessel → thyrocyte → follicular lumen|
3. Iodination of thyroglobulin
| ||In follicular lumen|
| ||In follicular lumen|
| ||Thyrocyte → fenestrated capillary network|
Transport and degradation
- Transport proteins
- Degradation (liver): sulfation/glucuronidation (biotransformation) → excretion via bile
In general, thyroid hormones increase the metabolic rate: oxygen and energy consumption as well as thermogenesis increase under their influence. See in the learning card for more information regarding their functionality.
|Skeletal muscle|| |
|Growth during childhood|
TSH levels are very sensitive to thyroid hormone dysfunction. If thyroid hormone levels are very high, TSH can fall below detection limits and if they are very low, TSH increases markedly. Therefore, serum TSH is an important parameter for assessing thyroid function and is usually the first step in thyroid diagnostics.
- The thyroid gland develops in the first trimester of pregnancy from the fusion of the median thyroid anlage with the two lateral thyroid anlages derived from the pharyngeal pouches
Median thyroid anlage
- An endodermal thickening in the floor of the primordial pharynx between the 1st and 2nd pharyngeal pouches
- This thickening becomes the thyroid diverticulum (see below)
- Differentiates into the follicular cells of the thyroid gland (i.e., the major portion of the lobes and isthmus of the thyroid gland)
- Lateral thyroid anlagen (ultimobranchial bodies)
- Thus, both follicular cells and C cells arise from pharyngeal endoderm
- Median thyroid anlage
- A thyroid gland precursor that originates from the floor of the primordial pharynx
- It is located initially at the middle of the floor of the pharynx, near the base of the tongue, i.e., foramen caecum (tongue).
- It descends down the neck, forming the thyroglossal duct, to settle into its adult anatomical position.
- The thyroglossal duct usually obliterates after the thyroid gland has descended.
- The foramen cecum of the tongue remains
During embryological thyroid migration, remnants of thyroid tissue can remain in the tongue (lingual thyroid) or along the migration path. Ectopic thyroid should always be considered when performing surgical procedures involving thyroid tissue.
- (e.g., de Quervain)
- Drug-induced: ,
- (e.g., )
- There are four, oval-shaped endocrine glands embedded in the posterior surface of the thyroid gland
- Function: secretion of parathyroid hormone (PTH) in response to low calcium serum levels
- Innervation: thyroid branches of the cervical ganglia
- Adipocytes (∼ 50%)
- Parathyroid cells (parathyroid chief cells)
- Oxyphil cells: red/pink cytoplasm; function not clear
- PTH increases serum calcium and decreases serum phosphate) (
- Superior parathyroid glands: derived from the fourth pharyngeal pouch
- Inferior parathyroid gland: derived from the third pharyngeal pouch
congenital T-cell immunodeficiency that is caused by microdeletion at chromosome 22 (22q11.2). The deletion leads to defective development of the third and fourth pharyngeal pouches, resulting in aplastic parathyroids and hypocalcemia due to PTH deficiency. is a