Objective: Hydatidiform pregnancy occurs in 1:1000 pregnancies worldwide; incidence is higher in Asian countries. In approximately 5% of cases of hydatidiform mole, clinical hyperthyroidism is present. The aim of this study was to examine the relation between HCG level and thyroid function test. Methods: During 5 years (2009-2013) we included 146 cases of molar pregnancies into the study at Imam Reza teaching hospital. The demographic and clinical data as well as serum initial HCG level and thyroid function test (TSH T3 T4) were retrieved and entered into prepared proformas. p < 0.05 was considered significant. Result: The mean age of patients was 29.31 years; mean of gestational age was 11.71 weeks; mean of gravidity was 2.32; mean of serum BHCG was 3.88E4; mean of T4 was 11.07 and mean of T3 was 1.97. In this study significantly inverse relation was observed between B-HCG and TSH (p = 0.05). We also found a significantly direct correlation between B-HCG and T3 (p = 0.01) and T4 (p = 0.01). Conclusion: We concluded significantly meaningful relationship between BHCG and T3, T4, TSH.