Objective: To evaluate the association between hypothyroidism and first-trimester spontaneous miscarriages and to explain the mechanism. Materials and Methods: Patients admitted between October and May 2011 with threatened miscarriage in the first trimester were analyzed and levels of progesterone and thyroid hormones as T3, T4, and thyroid-stimulating hormone (TSH) were estimated. Once hypothyroidism was diagnosed, patients were treated with sodium levothyroxine (LT4) as substitution and outcomes were observed. Results: Measurement of progesterone was useful for predicting the outcome of threatened miscarriage The results showed that progesterone (P) = 14.74 ng/ml is selected as predictive value to judge whether the fetal treatment was successfully or not. When serum P value is above 14.74 ng/ml before treatment, it may favour a miscarriage, if the serum P value is below 14.74 ng/ml, miscarriage is unlikely; its sensitivity and specificity are high. The risk for miscarriage in patients diagnosed with hypothyroidism in which LT4 substitution was similar to the level observed in the controls, and P between the two groups had no distinct difference. The mechanism explaining the risk of miscarriage increased by thyroid disorders remains unclear, which needs advanced research. Conclusion: Screening of thyroid disorders has important clinical significance in early pregnancy, and substitution of LT4 to those who are in the early pregnancy with hypothyroidism could reduce the risk of miscarriage.