Thyroid disorders during pregnancy is widespread and it is linked with adverse obstetric outcomes. Serum thyroid – stimulating hormone ( TSH ), also known as thyrotropin hormone is optimal indicator for monitoring and evaluating thyroid function. There are significant changes in thyroid physiology during gestation. Thus different TSH levels are recorded in pregnant women in different countries, Therefore. the aim of current study : To determine trimester (Tri) specific levels of TSH in the apparently healthy pregnant women in Mosul city. A cross – sectional study was conducted at Hospitals, Primary Health Centers and Outpatient clinics in Mosul city and collected from December 2020 to April 2021. Total enrolled pregnant women were 200 of age 18 – 32 years, 80 of them were excluded from study for meeting one or more of exclusion criteria . The remaining of 120 women were included in study, 40 women were assigned to each Tri. Thyrotropin and anti – thyroid peroxidase were measured for participants. The study results showed TSH levels increased over the Tri with very significantly higher TSH levels in the third in compared with first Tri. Accordingly the reference levels of TSH 0.33-2.79 mIU/L, 0.43-3.88 mIU/L, 0.57-3.39 mIU/L for the first, second and third Tri respectively. Our conclusion that the upper limit of TSH reference ranges for pregnant women in the first and second Tri was closer to that suggested by the American Thyroid Association( ATA) guidelines 2011 and some other countries.