Thyroid: Anatomy and Embyology

30/01/2020

THYROID ANATOMY

The thyroid is an endocrine gland located in the anterior median median of the neck. It consists of two lateral lobes united by a bridge, called isthmus. 

The thyroid is applied on the ventral surface of the larynx and the first two tracheal rings. It is partially covered by the sternocleidomastoid, sternothyroid and homoioid muscles, belonging to the subioid muscle group and from the middle cervical fascia that wraps them. 

For the characteristic oblique direction of the aforesaid muscles, the gland is no covered by along the midline. Here, just  two cervical fascia, medium and superficial, separate thyroid from the skin. 


neck anatomy
neck anatomy

Laterally, however, the gland is also covered by the two sternocleidomastoid muscles and more superficially from the bundles of the platysma muscle (you cannot see the platisma in the image above).

The drawing below shows the path of the thyroid gland during embryogenesis. Thyroid originates from the thyroglossal duct. It is possible that during migration some tissue residues in the path, then it can give rise to cysts of the thyroglossal duct which can manifest as congenital benign lesions without pathological consequences, if not of aesthetic impact.


These cysts can be removed by the surgeon.

On physical examination, in presence of a normal thyroid volume and in absence of nodular thyroid alteration, thyroid is not palpable. Thyroid palpation is executed from behind of the patient or alternatively with the patient in a supine position.

THYROID MICROSCOPIC ANATOMY

The thyroid gland is composed of a single-layer epithelial tissue that makes up the follicule, the structure basic anatomy of the thyroid. In the image below you can see how thyrocytes are disposed in a single layer around a central cavity called colloid so called because of the fact itself which contains colloid, a gelatinous substance which is more than 99% composed of thyroglobulin, the molecular precursor of thyroid hormones consisting of tyrosine molecules.


In addition to follicular cells or thyrocytes, the parenchyma is made up of another cell population: C cells or parafollicular. These cells have an endocrine function independent of the follicular thyroid function. Their product is an hormone called calcitonin, which participates in the regulation of phospho-calcium metabolism (which will be covered in other thyroid pills). The elevation of this hormone rarely has a physiological meaning. mostly it is pathological, being an indicator of malignant neoplasm of the thyroid gland, especially of medullary thyroid cancer. Calcitonin presents a diagnostic sensitivity is very high. When elevated calcitonin is present associated with a nodule or presumed neoplasm of the thyroid gland, it is almost certain that that neoplasm is a medullary thyroid cancer.