The thyroid gland is an endocrine organ located on both sides of the human trachea and its main function is to secrete thyroxine. Thyroxine is involved in the metabolism of the body and is a very important organ that the body cannot do without. If thyroid hormone is not secreted sufficiently, the development of infants and children may be stagnant, commonly known as “cretinism”. The secretion and cell regeneration of the thyroid gland is regulated by thyroid stimulating hormone secreted by the hypothalamus, and the relationship between them is a “negative feedback” relationship. Just like the relationship between a thermal power plant and a coal yard, if the plant is out of coal, it will inform the coal yard to supply coal; if the plant has enough coal, it will inform the coal yard not to supply coal. This relationship between the cogeneration plant and the coal yard is the “negative feedback” relationship. It is reasonable to say that if one side of the thyroid gland is removed and one side is preserved, the physiological needs of the human body can be basically met, so why do we need to take synthetic thyroxine eugenol orally when only one side of the thyroid cancer has been removed? Most of the patients think that it is to supplement the shortage of thyroid hormone, which is biased. The main purpose of taking euthyroxine is to increase the level of thyroxine in the blood, so that the negative feedback can inhibit the release of thyroid stimulating hormone (equivalent to coal) in the hypothalamus (equivalent to coal in the coal field). The main purpose of oral euthyroxine is to increase the level of thyroxine in the blood and thus negatively inhibit the release of thyroid stimulating hormone (equivalent to coal in the coal field) in the hypothalamus (equivalent to coal in the coal field), thus inhibiting the proliferation of the remaining thyroid tissue and excessive cell proliferation, thus preventing the recurrence of thyroid cancer.