When thyroid hormone levels are measured in newborns, many have a slightly higher than normal T3 or free T3, which is due to the body’s adaptation to the external environment when the infant is suddenly switched from the mother’s body to the external environment. The body secretes a large amount of thyroid hormone so that the body can adapt to the external environment by adjusting to a high metabolic level as soon as possible. At the same time, many infants have high TSH because the body, especially in infants, has not yet perfected the negative feedback mediation. Normally, if T3 is high, the TSH level will be reduced by negative feedback, but this feedback mediation mechanism is not yet perfect in infants and children, so there may also be a situation where TSH is elevated.