The thyroid gland is the largest endocrine gland in the human body and is located in a thin layer under the thyroid cartilage immediately in front of the third and fourth cartilaginous ring of the trachea, consisting of two lobes and the isthmus, with an average weight of about 20-25 g. Behind the thyroid gland are the parathyroid glands and the recurrent laryngeal nerve. The thyroid gland is rich in blood supply and its main function is to synthesize thyroid hormones and regulate the body’s metabolism. In general, there are about 100-200 μg of inorganic iodine compounds in people’s daily food, which are absorbed into the blood circulation through the gastrointestinal tract and rapidly concentrated for the thyroid gland to take up. The iodinated tyrosine thyroid acid, through the action of oxidase, couples 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 to plasma thyroxine-binding globulin (TBG) to facilitate 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 shed 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 pituitary cells and TSH via the adenylate cyclase-cAMP system. In contrast, TSH is controlled by TRH secreted by the hypothalamus, thus forming the hypothalamic-pituitary-thyroid axis that regulates thyroid function. The main role of thyroxine is to promote substance and energy metabolism and to facilitate growth and developmental processes. (1) Effects on metabolism 1) Thermogenic effect: Thyroid hormone can increase the oxygen consumption rate and heat production of most tissues, especially heart, liver, skeletal muscle and kidney tissues, etc. In hyperthyroidism, heat production increases, basal metabolic rate increases, and patients like to be cool and are afraid of heat and sweat easily. 2) Effects on protein, sugar and fat metabolism a. Protein metabolism: When T3 and T4 secretion is insufficient, protein synthesis is reduced and muscle is weak, but the mucin between tissues is increased. It can combine a large amount of positive ions and water, causing mucus edema, and vice versa, accelerating protein decomposition, especially accelerating protein decomposition of skeletal muscle, reducing creatinine content and increasing uric acid content, and promoting protein decomposition of bone, which leads to elevated blood calcium and osteoporosis and increased urinary calcium excretion. b. Glucose metabolism: On the one hand, thyroid hormone promotes the absorption of sugar by small intestinal mucosa, enhances glycogenolysis and inhibits glycogen synthesis, and strengthens the glucose-raising effect of adrenaline, glucagon, cortisol and growth hormone. On the other hand, it can also strengthen the utilization of sugar by peripheral tissues, which can lower blood sugar. c. Fat metabolism: Thyroid hormones promote fatty acid oxidation and enhance the catecholamines and glucagon’s effect on fat decomposition. t3 and t4 both promote cholesterol synthesis and accelerate cholesterol degradation through the liver, but the rate of decomposition exceeds synthesis. (2) Effects on growth and development. In human mammals, thyroid hormone is an indispensable hormone for maintaining normal growth and development, especially for bone and brain development. Children with hypothyroidism exhibit cretinism characterized by mental retardation and short stature. (3) Effects on the nervous system, not only on the central nervous system, but also on the activity of the mature nervous system by differentiation. In hyperthyroidism, excitability increases, mainly manifesting as irritability, inattention, allergy, suspicion, sentimentality and moodiness, etc. In hypothyroidism, excitability decreases, memory loss, slow speech and action, apathy and indifference, and sleepiness. In addition, thyroid hormones have a significant effect on the activity of the cardiovascular system. T3 and T4 can increase the heart rate. T3 and T4 can increase the heart rate, increase cardiac contraction, increase cardiac output and cardiac work.