TSH is thyroid stimulating hormone, a peptide hormone secreted by the pituitary gland, which promotes the secretion of thyroid hormones by the thyroid gland. There are several possibilities for high TSH: 1. Pituitary lesions, such as pituitary tumors or pituitary hyperplasia, increase TSH secretion by the pituitary gland, resulting in a secondary increase in T3 and T4, and pituitary hyperthyroidism, also known as secondary hyperthyroidism. Therefore, in pituitary hyperthyroidism, TSH is high and T3 and T4 are high. 2. In primary hypothyroidism, when the thyroid gland secretes less T3 and T4 due to various reasons, the inhibitory effect of T3 and T4 on TSH is weakened and TSH is elevated. Therefore, primary hypothyroidism is characterized by elevated TSH and reduced T3 and T4. 3. Subclinical hypothyroidism can be diagnosed if only TSH is elevated and T3 and T4 are normal in the thyroid function test. Because the sensitivity of TSH is higher than that of T3 and T4, the change of TSH precedes T3 and T4. Therefore, the determination of thyroid function needs to be combined with T3, T4 and TSH, and cannot be judged by TSH alone.