Introduction: Thyroid diseases, especially among women, are very common, and most women with severe hypo- or hyperthyroidism are faced with infertility, recurrent miscarriage and poor outcome pregnancy. This study aimed to determine the prevalence of thyroid dysfunction in pregnant women referring to clinics in Kermanshah, between 2011-2012. Materials and Methods: This cross-sectional study was conducted on a group of women with pregnancies under 16 weeks gestation, admitted for routine tests. Two-stage cluster sampling was conducted and 1200 samples were analyzed. Statiscal analysis was performed using the Chi 2 Correlation Test. Results: Of subjects, 248(20.7%) were diagnosed with thyroid dysfunction 194 patients (16.17%) with hyperthyroidism and 54 (4.5%) patients with hypothyroidism. The highest prevalences of hyperthyroidism (27.9 %) and hyporthyroidism (23.3%) were observed in pregnant women with their 4th gravidity. Significant relationships between variables such as family history of thyroid dysfunction, spontaneous abortion, infertility and preterm delivery in pregnant women with thyroid dysfunction were observed. Conclusions: Considering the high prevalence of thyroid dysfunction and relationships between some of the identified risk factors, it is recommended that screening tests for thyroid function in pregnant women at the first prenatal visit be conducted, at least in those with known risk factors of this condition.Introduction: Thyroid diseases, especially among women, are very common, and most women with severe hypo- or hyperthyroidism are faced with infertility, recurrent miscarriage and poor outcome pregnancy. This study aimed to determine the prevalence of thyroid dysfunction in pregnant women referring to clinics in Kermanshah, between 2011-2012. Materials and Methods: This cross-sectional study was conducted on a group of women with pregnancies under 16 weeks gestation, admitted for routine tests. Two-stage cluster sampling was conducted and 1200 samples were analyzed. Statiscal analysis was performed using the Chi 2 Correlation Test. Results: Of subjects, 248(20.7%) were diagnosed with thyroid dysfunction 194 patients (16.17%) with hyperthyroidism and 54 (4.5%) patients with hypothyroidism. The highest prevalences of hyperthyroidism (27.9 %) and hyporthyroidism (23.3%) were observed in pregnant women with their 4th gravidity. Significant relationships between variables such as family history of thyroid dysfunction, spontaneous abortion, infertility and preterm delivery in pregnant women with thyroid dysfunction were observed. Conclusions: Considering the high prevalence of thyroid dysfunction and relationships between some of the identified risk factors, it is recommended that screening tests for thyroid function in pregnant women at the first prenatal visit be conducted, at least in those with known risk factors of this condition.