The function of the thyroid gland is mainly affected by the following four factors: First, and most classically, the hypothalamic-pituitary-thyroid axis, which is a very important axis in classical endocrinology, that is, the hypothalamus secretes thyrotropin-releasing hormone, also called TRH, which stimulates the pituitary gland to synthesize thyrotropic hormone, also known as TSH. This thyroid stimulating hormone stimulates the growth of thyroid cells and the synthesis and secretion of thyroid hormones, thereby affecting thyroid function. Secondly, deiodinase in pituitary and peripheral tissues can regulate the conversion of tetraiodothyronine to triiodothyronine, which is a biologically active thyroid hormone, while tetraiodothyronine is a hormone without biological activity, and only the conversion of T4 to T3 can play a normal physiological role in the body. The third is the thyroid self-regulation associated with iodine storage. The fourth is the effect of thyroid stimulating hormone receptor antibodies on the thyroid gland. Thyroid function is mainly influenced by the above four factors.