The main physiological functions of thyroid hormones are: (1) Promote metabolism, increase oxygen consumption in most tissues, and increase heat production. (2) Promote growth and development, which is essential for the development of long bones, brain and reproductive organs, especially in infancy. Lack of thyroid hormone at this time can lead to cretinism. (3) Increases excitability of the central nervous system. In addition, it also strengthens and regulates the effects of other hormones, accelerates heart rate, strengthens cardiac contraction and increases cardiac output. The thyroid gland is an important organ of the endocrine system, which is distinctly different from other systems of the human body (such as the respiratory system, etc.), but is closely related to the nervous system, interacting and cooperating with each other, and is called the two major biological information systems. The endocrine system includes many endocrine glands, which can be stimulated by appropriate nerves to release efficient chemical substances from some cells of these glands, which are sent to the corresponding organs at a distance through blood circulation to play their regulatory role. The thyroid gland is the largest endocrine gland in the human endocrine system, and it is stimulated by nerves to secrete thyroid hormones, which act on the corresponding organs in the body to exert physiological effects. The thyroid gland matures during puberty and weighs 15 to 30 grams. The two lateral lobes each have a width of about 2 cm and a height of 4 to 5 cm, and the isthmus has a width of 2 cm and a height of 2 cm. The thyroid gland is slightly larger in women than in men. Under normal circumstances, the thyroid gland is neither visible nor palpable in the neck because it is very small and thin. If the thyroid can be felt in the neck, even if it is not visible, the thyroid is considered to be enlarged. This degree of enlargement is often physiological, especially in women during puberty, and is not usually the result of disease, but sometimes it can be pathological. The human thyroid gland weighs 20 to 30 g and is the largest endocrine gland in the body. It is located on both sides of the upper trachea, below the thyroid cartilage, and is divided into two lobes, which are connected by a narrower isthmus in the middle, in the shape of an “H”. The thyroid gland is composed of many follicles. Microscopically, the follicles consist of simple cuboidal glandular epithelial cells surrounded by a central follicular cavity. The epithelial cells are the site of thyroid hormone synthesis and release, and the follicular cavity is filled with a homogeneous gelatinous substance that is the thyroid hormone complex and the reservoir of thyroid hormones. Changes in follicular morphology reflect the functional status of the gland: when glandular activity is diminished, the glandular epithelium is flattened and the follicular lumen has increased storage; if activity is hyperactive, the follicular epithelium is columnar and the follicular lumen has decreased storage.