The biological effects of thyroid hormones are mainly in the following three areas: (a) Promoting growth and development The most obvious role of thyroid hormones in promoting growth and development is during infancy, with the greatest impact during the first five months of life. It mainly promotes the growth and development of bones, brain and reproductive organs. Without thyroid hormone, the pituitary gland’s GH cannot function either. Moreover, in the absence of thyroid hormone, the pituitary gland also produces and secretes less GH. Therefore, congenital or early childhood deficiency of thyroid hormone causes cretinism. Patients with cretinism have short stature due to stagnant bone growth, and the ratio of upper and lower body lengths is abnormal, with the upper body taking up a larger proportion 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. (B) Effects on metabolism 1. Thermogenic effect: Thyroid hormone can increase the oxygen consumption rate of most tissues and increase the thermogenic effect. This thermogenic effect may be due to the fact that thyroid hormone can increase the synthesis of Na+-K+ pump on the cell membrane and can increase its vitality, the latter being an energy-consuming process. Thyroxine increases the basal metabolic rate, and 1mg of thyroxine can increase heat production by 4000 KJ. The basal metabolic rate of patients with hyperthyroidism can be increased by about 35%; while the basal metabolic rate of patients with hypofunction can be reduced by about 15%. 2, the role of the metabolism of the three major nutrients: its impact on the metabolism of the three major nutrients is very complex. In general, 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 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 myocardium to promote the release of Ca2+ from the sarcoplasmic reticulum, resulting in increased myocardial contractility and faster heart rate.