Thyroid gland

Thyroid gland (lat. glandula thyreoidea) is an endocrine butterfly-shaped organ located on the front side of the neck. Functionally it has an impact on the metabolism regulation by the production of the triiodothyronine and thyroxine. These hormones affect the speed of the metabolic convertions, oxygen consumption, growth and development. The other hormone produced by the thyroid gland is calcitonine, which regulates calcium and phosphorus metabolism.

Anatomy
The gland consists of the two lobes (lobus dexter et sinister) in shape of the three-sided pyramid connected by the isthmus. In 1/4 of the cases also occures variational lobus pyramidalis − remnant of the thyroglossal duct stretching up from the thyroid isthmus.


 * Lobes: 5–8 cm length, 2–4 cm width, 1,5–2,5 cm thickness.

Weight is about 20–60 g (usually larger in women than in men).


 * Size and weight of the thyroid gland differ according to sex, age and geografical factor (averagely in population the thyroid gland's size grows with the increasing distance from the sea and the increasing height above sea level.

Surface is smooth and a bit bumpy, has the red-brown or red-violet color (depends on the blood vessel filling). It's covered by a fibrous capsule which consists of an inner and an outer layer.


 * Outer layer: capsula externa is associated with vagina carotica (common fibrous capsule for a. carotis communis, v. jugularis interna, n. vagus and radix superior ansae cervicalis) and lamina praetrachealis fasciae colli.
 * Inner layer: capsula propria extrudes septae and divides the gland into the lobules. Lobules consist of sacs, follicles separated by the soft fibrous tissue and capillary and lypmhatic networks.
 * Between both of the layers there are blood vessel networks.

Shapes of the gland are variable. The gland can be in shape of the letter H, U or A depending on the isthmus position. Bases of the lobes are round and rooting caudally, apexes are rooting cranially.

Position
The gland is lying around the front of the larynx and trachea, isthmus is situated between the 2. and the 4. tracheal ring. It is fixed to the trachea with the suspensory ligament of thyroid gland (Berry's ligament).

Position of the single anatomic surfaces of the lobes
Facies medialis: is attached to the larynx and trachea, related to the vagus nerve (nervus laryngeus recurrens). Facies anterolateralis: together with isthmus is covered by the infrahyoid neck muscles from lamina pretrachealis fasciae colli. Facies dorsalis: is related to vagina carotica, the parathyroid gland is partially immersed there.

Blood supply
Cranial part of the lobes is supplied by arteria thyroidea superior (5), the branch of the external carotid artery (4), caudal part is supplied by arteria thyroidea inferior (8), the branch of the thyrocervical trunk (7). Both arteries make a mutual anastomosis.

Non-constant a.thyroidea ima (9) from the brachiocephalic trunk (2) or the aortic arch (1) can be also present.

Veins connect together and form the network between the inner and the outer layer of the capsula. The blood from the upper pole of the gland is taken away by vv.thyroideae superiores (13) and vv.thyroideae mediae (14) to the internal jugular vein (12). The blood from the lower pole is taken away by vv.thyroideae inferiores (15) to the left brachiocephalic vein (11).

Lymphatic vessels lead to the subcapsular network and then to nodi lymphatici cervicales profundi.

Microscopic structure
Microscopically the thyroid gland consists of the round closed follicles. The wall of the follicles is lined by the simple cuboidal follicular cells, that can be changed to the squamous cells due to the hypothyreosis, while the hyperthyreosis manifests with the change to the columnar epithelium. Follicles are plentifully interlaced with the sinusoids. All the follicular cells are based on the basement membrane, their cytoplasm is basophilic and contains a lot of rough endoplasmic reticulum and secretory granules.

Follicles are filled with a colloid. It's a viscous homogeneous fluid which contains mainly glycoprotein thyroglobulin, which is the reason of PAS-positivity of the colloid. Thyroglobulin contains the precursor for the main thyroid gland hormones - thyroxine and triiodothyronine, both of them are synthetised in the colloid.

Besides the follicular cells there are also parafollicular cells (C cells) which produce calcitonine. Unlike the other thyroid gland parafollicular cells are derived from the neural crest.