The thyroid is a small gland, usually weighing 20-25g, and can be palpated if it exceeds 30g. It is slightly larger in women than in men and gradually shrinks in old age. The thyroid gland is located in front and on both sides of the lower anterior part of the neck (larynx and upper trachea). It is divided into two lobes, the right and left, connected by an isthmus in the middle, as in the H-shape. During development, the thyroid gland is initially located in the posterior part of the tongue, but is transferred to the front of the neck at birth. Sometimes it does not transfer properly and stays in the posterior part of the tongue or in the upper part of the neck (high thyroid); sometimes it transfers too quickly and appears in the chest or mediastinum (posterior sternal thyroid). Although the thyroid is small, it is the largest endocrine gland in the body. The thyroid gland takes iodine from food and produces hormones that are important for normal body function. The hormones produced by the thyroid gland are very important biologically active substances that act on all cells in the body and have a significant impact on protein metabolism, glucose metabolism, fat metabolism, water and salt metabolism, and vitamin metabolism.