Serum thyrotropin is one of the main hormones produced by the anterior pituitary gland. It promotes the growth and development of the thyroid gland, regulates the synthesis and secretion of thyroxine, and encourages the thyroid gland to synthesize and secrete thyroid hormones. However, if the thyroid gland has been destroyed, it cannot produce this effect. Because of the close negative feedback relationship between serum thyroid stimulating hormone levels and serum T4, serum thyroid stimulating hormone has become an important first-line indicator for the diagnosis of hyperthyroidism and hypothyroidism. Therefore, the combination of serum thyroid hormone and thyroid stimulating hormone can determine the cause of thyroid dysfunction and help develop an effective treatment plan. If elevated thyroid hormone is accompanied by decreased serum thyrotropic hormone, it is most often seen in primary hyperthyroidism caused by diseases of the thyroid gland itself, such as diffuse goiter and thyroid adenoma. If thyroid hormone is lowered with elevated thyroid stimulating hormone, it is most often due to primary hypothyroidism, such as over-treatment of primary hyperthyroidism or chronic thyroiditis.