The thyroid gland (Latin: Glandula thyr(e)oidea; English: Thyroid) is a very important gland in vertebrates and belongs to the endocrine organs. In mammals it is located in the neck below the thyroid cartilage and on either side of the trachea. In humans, the thyroid gland is shaped like a butterfly and resembles a shield armor, hence the name. The thyroid gland controls the rate at which energy is used, makes proteins, and regulates the body’s sensitivity to other hormones. The thyroid gland relies on the production of thyroxine to regulate these responses, and has triiodothyronine (T3) and thyroxine, which may also be called tetraiodothyronine (T4). T3 and T4 are synthesized from iodine and tyramine. The thyroid gland also produces calcitonin, which regulates calcium balance in the body. 1. Morphological characteristics: The thyroid gland is shaped like an “H”, brownish red and divided into two lateral lobes, connected by an isthmus. The two lateral lobes are attached to the lower part of the larynx and the upper side of the trachea, up to the middle of the thyroid cartilage, down to the sixth tracheal cartilage, the isthmus is located in front of the second to fourth tracheal cartilage, some people are not developed. Sometimes a cone-like lobe protrudes upward from the isthmus, varying in length, with the longest reaching the hyoid bone, a vestige of embryonic development that often degenerates with age, making it more common in children than in adults. The thyroid gland is covered with a fibrous capsule, called the thyroid capsule, which extends into the glandular tissue and divides the gland into lobes of different sizes, and outside the capsule there is deep cervical fascia (pre-tracheal layer). 2. Anatomy: The thyroid gland is the largest endocrine gland in the body. It is brownish red, divided into two lobes, the middle is connected (called the isthmus), “H” shaped, about 20-30 grams. The thyroid gland is located on the anterior side of the upper part of the trachea in the lower part of the larynx and can move up and down with the larynx when swallowing. Although the iodine content of the gland is usually 25 to 50 times higher than that of the blood, 1/3 of the daily dietary iodine intake enters the thyroid gland, and 90% of the total body iodine content is concentrated in the thyroid gland. Thyroid hormone is a hormone secreted by the thyroid gland. 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 can sometimes be pathological. The thyroid gland is composed of many follicles. What is seen microscopically: the follicles consist of simple cuboidal glandular epithelial cells surrounded by a central follicular cavity. The adenosepithelial cells are the site of thyroid hormone synthesis and release, and the follicular cavity is filled with a homogeneous gelatinous substance that serves as a thyroid hormone complex and a reservoir for thyroid hormones. Changes in follicular morphology can reflect the functional state of the gland: when glandular activity is reduced, the glandular epithelium is flattened and the storage in the follicular lumen is increased; if activity is hyperactive, the follicular epithelium is columnar and the storage in the follicular lumen is reduced. 3. Main functions: The 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 during infancy. Lack of thyroid hormone at this time will suffer from 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 main function of the thyroid gland is to synthesize thyroid hormones and regulate the body’s metabolism, and there are about 100-200 μg of inorganic iodine compounds in the daily food of the average person, which are absorbed into the blood circulation through the gastrointestinal tract and rapidly concentrated for the thyroid gland. The iodinated tyrosine, through the action of oxidase, causes the coupling of MIT and DIT to form thyroxine (T4) and MID and DIT to form triiodothyronine (T3), which are stored in the glial cavity. The synthesized thyroxine (T4) and triiodothyronine (T3) are secreted into the circulation and bound primarily to plasma thyroxine-binding globulin (TBG) for transport and regulation of thyroxine concentrations in the blood. Thyroxine (T4) is deiodinated in peripheral tissues to form the biologically active T3 and the biologically inactive rT3, respectively, and the deiodinated iodine can be reused. Therefore, in hyperthyroidism, blood T4, T3 and rT3 are increased, while in hypothyroidism, all three are below normal. Thyroxine secretion is regulated by TSH secreted by pituitary cells through the adenylate cyclase-cAMP system. TSH, in turn, is controlled by TRH secreted by the hypothalamus, thus forming the hypothalamic-pituitary-thyroid axis that regulates thyroid function. When too much thyroid hormone is secreted, thyroid hormone in turn stimulates the hypothalamus and pituitary gland, inhibiting TRH secreted by the hypothalamus and TSH secreted by the pituitary gland, thus achieving the effect of reducing thyroid hormone secretion, this regulation is also called feedback regulation.