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Subclinical hypothyroidism (SCH) in pregnancy is insidious or asymptomatic and easy to be ignored. It may lead to miscarriage, premature delivery, placental abruption, low birth weight, gestational diabetes and other adverse pregnancy outcomes. Although serum free thyroxine (FT4) levels were normal in women with SCH pregnancy, the utilization of FT4 in fetal or fetal tissues was decreased, suggesting that FT4 in the normal range is not sufficient to fully play its role. L-thyroxine (L-T4) is the first choice in the treatment of SCH. Because its structure is the same as human thyroid hormone, L-T4 can affect the growth, development and division of embryonic and neonatal cells by enhancing enzyme activity and promoting the synthesis of protease. At the same time, L-T4 also has a significant regulatory effect on body temperature regulation, energy metabolism, carbohydrate, protein and fat metabolism of pregnant women. Whether L-T4 treatment can reduce the abortion rate and other complications in pregnant women with negative thyroid peroxidase antibody (TPO-Ab) is still controversial. This article reviews the research progress of L-T4 in the treatment of TPO-Ab negative hypothyroidism in pregnancy. [ABSTRACT FROM AUTHOR]