The thyroid gland is a small, butterfly-shaped gland located in the neck and covers the anterior surface of the trachea. The gland has two lateral lobes connected by a thin tissue mass called the isthmus. Internally, each lobe comprises many small spherical structures known as thyroid follicles, surrounded by a network of blood vessels.

The follicles have a central cavity lined by simple cuboidal to squamous epithelial cells called follicular cells. These cells produce the glycoprotein thyroglobulin and store it in the central colloid. Thyroglobulin acts as a precursor to triiodothyronine or T3 and thyroxine or T4, which are thyroid hormones essential for maintaining the body's metabolic rate and energy production. Additionally, the parafollicular or C cells scattered between the follicles are responsible for producing the hormone calcitonin, which regulates calcium metabolism.