Introduction: The study aimed at determining the prevalence of thyroid disorders among a pregnant population and identifying the related risk factors of thyroid dysfunction in these subjects. Materials and Methods: Between February and July 2008, a nested case-control study was conducted on 608 pregnant women in Bandar Abbas, Iran. We obtained information about thyroid dysfunction in the subject and their family, diabetes mellitus type 1, prior gestational diabetes, hepatitis B and rubella vaccination, any previous miscarriages and fifteen symptoms of hypothyroidism. T4, T3, T3RU, FTI, TPO Abs were measured and thyroid examinations were carried out. Multiple logistic regression tests were used to estimate relative risk of thyroid dysfunction for the variables. The rate of hypothyroidism was compared in groups with significant related risk factors at high or lower risk. Results: Of the 608 pregnant women, 519(85.4%) were euthyroid, 72(11.8%) had sub clinical hypothyroidism, 3(0.5%) overt hypothyroidism and 14(2.3%) were hyperthyroid. There were strong correlations between personal history of thyroid dysfunction (OR:5.23 %95CI:2.11-12.95 P: 0.00), TPO Abs(OR:2.81 CI%95:1.83-4.32 P:0.00), 3 symptoms or more of hypothyroidism(OR:2.50 CI%95:1.48-4.22 P:0.001), goiter (OR:2.24 CI%95: 1.32-3.80 P:0.002), being in the 20-30 year age range (OR:1.87 %95CI:1.004-3.51 P:0.04) and being hypothyroid in pregnancy, respectively, but not with other risk factors, especially family history of thyroid disorder. Conclusion: This study showed thyroid disorders are common among pregnant population and are correlated with personal history of thyroid disorder, TPO Abs, 3 symptoms or more of hypothyroidism, goiter and age range between 20 to 30 years old, all risk factors which should be considered at screening, in clinical practice.