The thyroid gland is located around the upper part of the trachea in the neck. It has a very characteristic histological appearance. The thyroid gland is composed of numerous follicles enclosed within a connective tissue capsule and attached to neighbouring structures by an additional outer fascial sheath.

Each follicle consists of a layer of simple cuboidal epithelium which encloses a mass of glycoprotein known as thyroglobulin: this is sometimes referred to as the colloid. The thyroglobulin is produced by the cuboidal epithelial cells and deposited within the colloid where it is stored, (extracellularly), and iodinated. When the active hormones (thyroxine, triiodothyronine) are required, a small amount of thyroglobulin is endocytosed by the cuboidal epithelial cells. Within these cells, it is modified to form the active hormones which are then released into the connective tissue that surrounds the follicle. There, numerous fenestrated capillaries take up the hormone and transport it to its target sites.

In addition to thyroxine and tri-iodothyronine, the thyroid gland also produces the hormone calcitonin, a calcium lowering hormone. The calcitonin producing cells are known as "C" cells (clear cells), or parafollicular cells. These are large cells which are located in the walls of the thyroid follicles as widely spread but individual cells. They do not abut onto the central thyroglobulin core of the follicles, or deposit any product into the core. Calcitonin, is released directly into the connective tissue around the follicles and then passes into capillaries for distribution.