The main function of the thyroid gland is the synthesis, storage and secretion of thyroxine. The function of the thyroid gland is interconnected with the activity of various organ systems of the body and the external environment. The main regulatory mechanisms include the hypothalamic-pituitary-thyroid axis control system and the thyroid gland’s own internal regulatory system. Thyroxine acts on almost all tissues of the body, regulates metabolism and growth and development in many ways, is the basic hormone that maintains the functional activity of the body, and has a wide range of biological effects. The biological effects of thyroid hormones are summarized in several aspects: 1. Promote growth and development Thyroxine is a key hormone for fetal and neonatal brain development. During the embryonic period, thyroxine can promote the proliferation and differentiation of neurons and the formation of synapses; promote the growth of glial cells and the formation of myelin sheaths, induce the synthesis of nerve growth factors and certain enzymes, and promote the development of neuronal skeleton. Thyroxine stimulates the development and maturation of ossification centers, accelerates cartilage ossification, and promotes the growth of long bones and teeth. Lack of thyroxine during embryonic and early childhood can lead to irreversible neurodevelopmental disorders, as well as delayed or stagnant skeletal growth and maturation, and obvious symptoms such as mental retardation, short stature, and underdeveloped teeth, called cretinism or cretinism. (2) Regulation of metabolism (1) Enhance energy metabolism: thyroxine can increase the basal oxygen consumption and heat production of most tissues in the body. The effect of thyroxine on heat production of different tissues is different, and the effect on heart is the most significant, but the effect on brain, gonads (testes), spleen and other tissues is not obvious. (2) Regulation of substance metabolism: thyroxine widely affects the anabolism and catabolism of substances, and its effect on metabolism is also very complex, often showing a two-way effect. Therefore, patients with hyperthyroidism often show an increase in body fat consumption, a decrease in overall lipid volume and a lower than normal blood cholesterol level, while patients with hypothyroidism show an increase in the proportion of body fat and an increase in blood cholesterol level that predisposes them to atherosclerosis. (3) Protein metabolism: Thyroxine also has a bi-directional effect on protein synthesis and catabolism. Under physiological conditions, it can promote the synthesis of structural and functional proteins, which is conducive to the growth and development of the body and the maintenance of various functional activities, showing a positive nitrogen balance; however, when thyroxine is secreted in excess, it promotes the breakdown of proteins, showing a negative nitrogen balance. (4) Effects on the nervous system: Thyroxine also has effects on the activities of the nervous system in mature adults, mainly in the form of excitatory effects. Patients with hyperthyroidism often show increased excitability of the central nervous system, such as agitation, restlessness, moodiness, insomnia and dreaminess, and distraction. In contrast, hypothyroid patients show reduced excitability of the central nervous system, memory loss, delayed speech and movement, indifference, and hypoactivity and drowsiness. (5) Effects on the heart: Increased heart rate, increased myocardial contractility, increased cardiac output and myocardial oxygen consumption. Tachycardia, arrhythmia and even heart failure may occur in patients with hyperthyroidism. (6) Effects on the digestive system: It can promote the movement of the digestive tract and the secretion of the digestive glands. In hyperthyroidism, appetite is increased, gastrointestinal motility is accelerated, intestinal absorption is reduced, and even stubborn malabsorption diarrhea occurs; in hypothyroidism, appetite is reduced, and bloating and constipation may occur due to reduced gastrointestinal motility.