Purpose: The current study aims to establish reference ranges for thyroid hormones in normal pregnant women during their pregnancy period. Materials and Methods: A one-time cross-sectional survey was conducted on 490 normal pregnant women and 51 nonpregnant women (control). The serum thyroid stimulating hormone (TSH), free triiodothyronine (FT3), and free tetraiodothyronine (FT4) levels were measured. Results: The serum FT3 and FT4 levels in pregnant women decreased gradually from the first to the last three months of pregnancy (p < 0.01). The serum TSH level increased gradually during the whole pregnancy (p < 0.01), and was significantly lower than the control (p < 0.01) in the first three months. However, in the middle and last three months of pregnancy, TSH was higher than the control (p < 0.01). Conclusions: The thyroid hormone levels in normal pregnant women are different from those in non-pregnant women; significant differences exist among the three stages of pregnancy.