T3 and T4 are normal, but elevated TSH is subclinical hypothyroidism. Subclinical hypothyroidism is a common abnormality of thyroid function that can be seen in the course of various thyroiditis, and can also be a preclinical hypothyroid state. Some subclinical hypothyroidism can develop into hypothyroidism, some subclinical hypothyroidism may remain subclinical hypothyroidism for a long time, and some subclinical hypothyroidism patients will gradually return to normal. Therefore, if T3 and T4 are normal and TSH is elevated, as long as TSH is <10.0mU/L, it can be left untreated. When TSH is >10.0mU/L, levothyroxine sodium replacement therapy is required. However, if a pregnant woman or a woman with childbearing requirements has normal T3 and T4 and elevated TSH, regardless of the degree of TSH elevation, levothyroxine sodium replacement therapy is required to avoid thyroid hormone deficiency during pregnancy, which may affect fetal growth and neurological development.