Thyroid hormone is a hormone secreted by the thyroid gland, and its physiological effects are mainly in the following three areas: I. Promote growth and development The most obvious role of thyroid hormone in promoting growth and development is during infancy, with the greatest impact during the first five months after birth. It mainly promotes the growth and development of bones, brain and reproductive organs. Without thyroid hormone, growth hormone from the pituitary gland does not work either. Moreover, when thyroid hormone is deficient, the pituitary gland produces and secretes less growth hormone. Therefore, congenital or early childhood deficiency of thyroid hormone causes cretinism. In patients with cretinism, bone growth is stagnant and the stature is short, the ratio of upper and lower body length is abnormal, and the proportion of the upper body is higher than normal. In addition, the dendrites, axons, myelin sheaths and glial cells of nerve cells are impaired, the brain is underdeveloped and the intelligence is low, and their sexual organs are not mature and they do not have normal reproductive function. When newborns have low thyroid function, thyroid hormone should be supplemented in appropriate amounts within one year of age, which is also effective for the development of the central nervous system and the recovery of brain function. Later than this period, even if a large amount of T3 or T4 is supplemented later, the normal function cannot be restored and the treatment is often ineffective. Effects on metabolism 1. Thermogenic effect Thyroid hormone can increase the oxygen consumption rate of most tissues and increase thermogenic effect. The basal metabolic rate of patients with hyperthyroidism can be increased by about 35%, while the basal metabolic rate of patients with hypothyroidism can be reduced by about 15%. 2.The effect on the metabolism of three major nutrients Its effect on the metabolism of three major nutrients is very complex. Generally speaking, under normal conditions, thyroid hormone mainly promotes protein synthesis, especially protein synthesis of bone, skeletal muscle and liver, which is important for growth and development in early childhood. However, excessive secretion of thyroid hormone causes a large amount of protein breakdown, especially in skeletal muscle, resulting in wasting and weakness. In terms of glucose metabolism, thyroid hormone has the effect of promoting the absorption of sugar and the breakdown of liver glycogen. It also promotes the utilization of sugar by peripheral tissues. In short, it accelerates sugar and fat metabolism, especially promoting the process of sugar, fat and protein decomposition and oxidation in many tissues, thus increasing the body’s oxygen consumption and heat production. III. Other aspects In addition, thyroid hormone also has an important role in the activities of some organs. It is important for maintaining the excitability of the nervous system. Thyroid hormone can act directly on the heart muscle to promote the release of Ca2+ from the sarcoplasmic reticulum, resulting in increased contraction of the heart muscle and faster heart rate.