**2. Anatomy and embryology of thyroid gland**

Thyroid gland is located anteriorly in lower neck and extending between C5 to T1 vertebrae. A normal adult thyroid gland is approximately 40–60 mm longitudinal and 13–18 mm AP diameter in size [8]. Mean ultrasound volume is around 7–10 ml and weighing of 9–21 g.

Microscopically the gland is divided into lobes and lobules. Lobules are further subdivided into follicles. These follicles are made up of principal (follicular) cells which are a type of epithelial cell. These cells produce colloid (iodo-thyroglobulin).

Other types of epithelial cells seen in gland are parafollicular cells (c cell) which lie adjacent to follicles. C cell produce calcitonin.

Thyroid gland development starts by third to fourth week of gestation and originates from primitive pharynx and the neural crest cells. The development begins as a diverticulum at the dorsum of tongue. This diverticulum forms hypoglossal duct which passes from foramen caecum to infrahyoid region. The supra hyoid part of thyroglossal duct degenerates whereas the infra hyoid part develops into thyroid gland.

The gland is supplied by superior thyroid artery, inferior thyroid artery and thyroidea ima. Venous drainage is provided by superior, middle and inferior thyroid vein.

The lymphatic drainage courses to prelaryngeal, pretracheal and paratracheal lymph node.
