Subclinical hypothyroidism is one of the most common thyroid diseases during pregnancy, and the American Thyroid Association (ATA) guidelines proposed two diagnostic criteria for subclinical hypothyroidism during the first trimester of pregnancy: (1) pregnancy-specific TSH reference range; and (2) serum TSH>2.5 mIU/L with normal serum FT4. In China, the prevalence rate of subclinical hypothyroidism was as high as 27.8%, and the diagnosis based on criterion 2 may result in overdiagnosis and overtreatment. Establishing a hospital or region-specific thyroid function reference index during pregnancy needs considering many factors, including screening of normal people, detection reagents and methods, genetics, iodine nutritional status, race, sampling method (sequential or cross section), and the specific number of gestational weeks in early pregnancy. In this paper, we will review the progress in the establishment of the reference value of specific thyroid function index, the influencing factors, and the rational diagnosis of subclinical hypothyroidism.