The hypothalamic-pituitary-thyroid axis is a major feedback loop in the regulation of thyroid function and plays a central role in regulating thyroid function. The thyrotropin-releasing hormone secreted by the hypothalamus stimulates the pituitary gland, which secretes thyrotropin, which in turn stimulates thyroid cell synthesis and secretion of thyroid hormone. Thyroid hormone can also inhibit the release of thyrotropic hormone from the pituitary gland through negative feedback regulation. Conversely, a decrease in thyroid hormone levels can also increase the secretion of both. Thyrotropic hormone released by the pituitary gland is the main substance that controls the growth of thyroid cells, as well as the synthesis and secretion of thyroid hormones. Thyrotropic hormone can accelerate the reabsorption of thyroglobulin, reduce the glial component, increase the size of thyroid cells, expand the diameter and number of blood vessels, and increase blood flow to the gland, which can cause goiter in the long run. The transfer of iodine by thyroid stimulating hormone is bidirectional, initially thyroglobulin breakdown increases and iodine leaks out of the thyroid cells, after a few hours the uptake of iodine by the thyroid begins to increase. The thyrotropic hormone released by the pituitary gland is pulsatile and has a circadian rhythm. The human pituitary gland releases thyrotropic hormone every 2-6 hours because the frequency of thyrotropic hormone pulse secretion decreases and the amplitude increases at night, so there is more thyrotropic hormone secretion at night than during the day, and thyrotropic hormone secretion is most active at 0-4 at night and decreases at 9-12 in the morning because the half-life of thyrotropic hormone decreases. Because the half-life of thyroid hormone is long, about 50 minutes, a single blood test can reflect the circulating levels of thyroid stimulating hormone.