Aims: Investigation Thyroid dysfunction and autoimmunity in pregnant women with gestational diabetes mellitus. Background: This article was written to evaluate the thyroid function and anti-thyroid peroxidase (anti-TPO) antibodies in pregnant women with gestational diabetes mellitus (GDM). Objective: • More analysis on the levels of iodine intake in the pregnant women. • The geographical area, the type of climate, and other factors are likely to affect on thyroid disorders and GDM. • That thyroid tests be performed in pregnant women, especially those with GDM. • It seems necessary establish new criteria for both thyroid function and GDM according to race and geographical area. Method: A total of 252 women with GDM and 252 healthy pregnant women were enrolled. Thyroid tests, including TSH, FreeT3, Free T4, and anti-TPO were performed for all women at 24–28 weeks of gestation. Data analysis was then carried out using SPSS ver. 22. Result: There was a significant difference between the experimental group (38.4%) and the control group (14.06%) in terms of the prevalence of subclinical hypothyroidism (p= 0.016). The frequency of anti-TPO was higher in the experimental group than the control group and positive anti-TPO was observed in 18.6% of women with GDM and 10.3% of healthy pregnant women (P= 0.008). Conclusion: Thyroid disorders are observed in pregnant women with GDM more frequently than healthy individuals and it may be thus reasonable to perform thyroid tests routinely.