The thyroid gland, located anterior and inferior to the larynx, consists of two lobes united by an isthmus. It synthesizes the thyroid hormones thyroxine or T4 and triiodothyronine or T3, which help control the basal metabolic rate in cells throughout the body, as well as the polypeptide hormone calcitonin.
The thyroid gland is covered by a fibrous capsule from which septa extend into the parenchyma, dividing it into lobules and carrying blood vessels, nerves, and lymphatics. Follicles are densely packed together, separated from one another only by sparse reticular connective tissue , although this stroma is very well vascularized with fenestrated capillaries for transfer of released hormone to the blood.
The thyroid gland is unique because it’s the only endocrine gland that stores its inactive hormones extracellularly in follicles. Each follicle is filled with a gel like mass called colloid which stains dark pink with hematoxylin and eosin. The colloid consists mostly of thyroglobulin and thyroid
hormones that are bound to the colloid.
The follicular epithelium is made of a single layer of simple cuboidal cells with round central nuclei and dark chromatin. These cells are responsible for secreting thyroid
hormones when needed.
Basally the cells are rich in rough endoplasmic reticulum and apically there are Golgi complexes, secretory granules, numerous phagosomes and lysosomes, and microvilli.
Another endocrine cell type, the parafollicular cell, or C cell, is also found inside the basal lamina of the follicular epithelium or as isolated clusters between follicles. They are responsible for secreting calcitonin. Parafollicular cells are usually somewhat larger than follicular cells. They have a smaller amount of rough endoplasmic reticulum, large Golgi complexes, and numerous small granules containing calcitonin.

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