ABSTRACT
The thyroid located just below the larynx is composed of left and right lateral lobes that lie on either side of the trachea. Both lobes are connected in the midline by a mass of tissue known as an isthmus and the entire structure is in front of the trachea just inferior to cricoid cartilage. When present, there is an additional projection, which extends cephalad from its attachment at the isthmus and is known as the pyramidal lobe. This endocrine gland weighs approximately 25 g and is profused by approximately 80 to 120 ml of blood per minute. The thyroid gland has a unique configuration histologically. It is composed of spherical sacks known as thyroid follicles with the walls of each sack consisting of cells that project into the lumen of the follicle and another layer of cells that does not. Cells in contact with the lumen area are called follicular cells whereas those not in contact are called C cells or parafollicular cells. When actively secreting hormones, the cells take on a columnar appearance and when not in an active state, they appear cuboidal in shape.
