The thyroid gland is the largest endocrine gland in the human body. It is located under the thyroid cartilage and close to the front of the third and fourth cartilage rings of the trachea. It consists of two lobes and an isthmus. The average weight is about 20-25g, and it is slightly larger and heavier in women. There are 4 parathyroid glands and the recurrent laryngeal nerve behind the thyroid gland. The blood supply is provided by four arteries, so the thyroid gland has a rich blood supply. The gland is controlled by the sympathetic nerves and vagus nerves of the cervical sympathetic ganglia. The main function of the thyroid gland is to synthesize thyroid hormones and regulate the body's metabolism. The average person's daily food contains about 100-200μg of inorganic iodine compounds, which are absorbed into the blood circulation through the gastrointestinal tract and quickly absorbed and concentrated by the thyroid gland. The iodine stored in the gland is about 1/5 of the whole body. After iodide enters the cell, it is acted upon by oxidase to produce active iodine that quickly combines with the tyrosine group on the thyroglobulin molecule in the colloid cavity to form monoiodotyrosine (MIT) and diiodotyrosine (DIT). Iodinated tyrosine, through the action of oxidase, causes MIT and DIT to couple to form thyroxine (T4), and MID and DIT to couple to form triiodothyronine (T3), which are stored in the colloid cavity. After the synthesized thyroxine (T4) and triiodothyronine (T3) are secreted into the blood circulation, they mainly bind to thyroxine-binding globulin (TBG) in plasma to facilitate transport and regulate the concentration of thyroxine in the blood. Thyroxine (T4) is deiodinated in peripheral tissues to form biologically active T3 and biologically inactive rT3. The removed iodine can be reused. Therefore, in hyperthyroidism, blood T4, T3 and rT3 are all increased, while in hypothyroidism, all three are lower than normal values. The secretion of thyroxine is regulated by pituitary cells and TSH through the adenylate cyclase-cAMP system. TSH is controlled by TRH secreted by the hypothalamus, thus forming the hypothalamus-pituitary-thyroid axis to regulate thyroid function.