(i) Heat production: Thyroid hormones accelerate the rate of cellular oxidation reactions in the body, thus releasing heat, called the thermogenic effect. (ii) Regulation of growth and development: The effect of thyroid hormones on growth and development is age-related. The younger the person is, the more pronounced the effect of thyroid hormone deficiency on stunted growth and development. Thyroid hormones have a stimulating effect on fetal development, especially on the growth and development of the nervous system and skeletal system. It not only promotes the growth and development of body organs and height, but also plays an important role in promoting the morphological differentiation of the fetus, i.e., promoting the development of simple structures to complex structures. The thyroid gland can secrete thyroid hormone at 11 weeks and needs sufficient thyroid hormone by 15 weeks, but maternal thyroid hormone does not easily pass through the placenta, so the fetus mainly relies on its own secretion of thyroid hormone to promote growth and development. When the fetus does not produce enough fetal thyroid hormone due to various reasons, it leads to congenital hypothyroidism, i.e. Cretinism, which is not only physically stunted but also brain stunted, manifesting as cretinism. Since a small amount of maternal thyroid hormone can pass through the placenta to the fetus, a larger dose of thyroid tablets can be given to mothers who have had a child with cretinism during pregnancy to prevent it. In humans, T4 is required during early childhood and it allows the GH it secretes to produce its maximum biological effect. In childhood, patients with thyroid hormone deficiency have infantile upper and lower proportions of their total body length, delayed ossification of bones and the appearance of bones such as carpal bones, impaired dental development, rough skin, and mucinous edema in adults. (iii) Effects on water and electrolyte metabolism: Some people with normal thyroid function and patients with mucinous edema were given larger doses of thyroid, and it was found that there was a loss of potassium and sodium in the urine (iv) Effects on protein metabolism: Protein catabolism is enhanced in hyperthyroidism (v) Effects on carbohydrate metabolism: 1. sugar absorption. 2, glycogen synthesis. 3, sugar utilization. (vi) Effects on fat metabolism: T4 accelerates cholesterol synthesis, decomposition and excretion in the bile. When hypothyroidism, blood cholesterol increases; hyperthyroidism makes cholesterol decomposition and excretion plus accelerated decomposition and excretion while blood concentration decreases (vii) Effect on vitamin metabolism The presence of T4 is required for the conversion of carotene into vitamin A in the body, which increases carotene in hypothyroid patients and makes the skin appear yellow. (viii) Effects on muscle metabolism: especially the heart muscle is susceptible to involvement. (ix) Cardiovascular system: thyroid hormones promote increased heart rate and increased myocardial contractility. (x) Effects on sympathetic nerves: Normal thyroid hormones are important for the development and functional regulation of the nervous system. Lack of thyroid hormone in the fetus and early childhood causes more severe damage to brain tissue than any other tissue. Cretinism caused by the lack of thyroid hormone during embryonic life is associated with impaired intellectual development and dementia, while lack of thyroid hormone during adulthood causes mucinous edema, and in severe cases, slow reactions and mental retardation, which can be completely restored to normal after thyroid hormone treatment, while cretinism shows little improvement. In hyperthyroidism, due to excessive thyroxine, neuroexcitability is increased, resulting in agitation, irritability, and muscle tremors. Due to the increased excitability of the vegetative nerves, there is increased gastrointestinal motility and excessive sweating. Many effects of thyroid hormone, especially on the cardiovascular system, are sensitive to catecholamine reactions. Thyroid hormone increases the number of catecholamine receptors in cardiac muscle cells, which enhances the role of catecholamine receptors. β-adrenergic receptor blockers can control certain symptoms of hyperthyroidism, while oxygen consumption is not controlled by them. (xi) Effects on the hematological system: Hypothyroid patients have reduced hematopoietic function, anemia due to reduced bone marrow hematopoietic activity, and a relative increase in lymphocytes. (xii) Lung: Hypoxia and hypercapnia enhance the excitability of the respiratory center when thyroid function is normal. (xiii) Endocrine system: Thyroid hormone increases the metabolism and clearance of various hormones and drugs, such as making steroid hormone clearance faster, thus causing an increase in compensatory generation. 40% of hypothyroid patients have hyperprolactinemia, which can be normalized after giving thyroid hormone treatment.