Low thyrotropin can be either hyperthyroid or hypothyroid. TSH, a peptide hormone secreted by the pituitary gland, promotes the secretion of thyroid hormones and is an important hormone in the body that regulates thyroid secretion.TSH is reduced in the following cases: 1. Pituitary lesions, when pituitary tumors or postpartum hemorrhage lead to hypopituitarism, the secretion of TSH is reduced, and secondary to this, the secretion of T3 and T4 by the thyroid gland is reduced, which leads to pituitary hypothyroidism . Laboratory tests show a decrease in TSH and a decrease in T3 and T4. 2. Primary hyperthyroidism, also known as thyroid hyperthyroidism. When there is excessive secretion of thyroid hormone for various reasons, the excessive thyroid hormone feeds back and inhibits TSH secretion by the pituitary gland, resulting in a decrease in TSH and thyrotoxic hyperthyroidism. In thyrotoxic hyperthyroidism, the main laboratory tests show a decrease in TSH and an increase in T3 and T4.3 If only TSH is decreased and T3 and T4 are normal, the diagnosis of subclinical hyperthyroidism can be made. Therefore, low thyroid stimulating hormone is most likely to be hyperthyroidism or hypothyroidism. The determination of thyroid function needs to be combined with T3, T4 and TSH for a comprehensive judgment.