The thyroid gland is a butterfly-shaped endocrine organ that is positioned in the midline of the anterior aspect of the neck. It is found immediately below the cricoid cartilage of the larynx. The thyroid gland is the largest endocrine gland, weighing between 0.53 - 0.88 ounces (15 - 25 grams). However, it usually weighs 0.71 ounces (20 grams). In women, the gland can increase in size and weight during pregnancy and menstruation.

The thyroid gland overlies the trachea (Read more!) and wraps around it. Anteriorly, it is covered by the sternohyoid muscle and sternothyroid muscle. Usually, the thyroid gland extends from the level of the fifth cervical vertebra (C5) (Read more!) to the first thoracic vertebra (T1) (Read more!). Its superior portion also overlies some aspects of the cricoid cartilage and thyroid cartilage of the larynx.

The thyroid gland is entirely surrounded by the pretracheal layer of the deep cervical fascia. The pretracheal fascia also condensates into the posterior suspensory ligament of the thyroid gland (Berry's ligament), which attaches the posterior aspect of the gland to the first two tracheal cartilages and the cricoid cartilage of the larynx. This connection elevates the gland and its related structures during swallowing.

The thyroid gland belongs to the endocrine system, and it is responsible for producing thyroid hormones (T3, T4) and calcitonin. The thyroid hormones regulate body metabolism, while calcitonin maintains calcium homeostasis. To sum up, the thyroid gland participates in the regulation of metabolism, growth and development of the body. Also, it is involved in the regulation of calcium serum concentration levels.