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Objective: Subclinical hypothyroidism is the most frequent thyroid dysfunction in pregnancy with an incidence of 2-5% which can easily be overlooked due to the absence of clinical symptoms. Studies have shown that thyroid hormones could alter the level of free beta-human chorionic gonadotropin (ß-hCG) and pregnancy-associated plasma protein-A (PAPP-A) released from the placenta during early pregnancy. Since these two serum markers are the two of the parameters used in the first-trimester combined screening, we hypothesized that distorted risk calculation results could be obtained from pregnant women with subclinical hypothyroidism. Methods: We conducted a prospective and cohort study at a tertiary university hospital in northwest Turkey between February 2018 and June 2018, involving 250 pregnant women in their first trimester who were seeking care at the obstetrics outpatient clinic. After evaluation of thyroid functions, first-trimester screening was performed in all 250 singleton pregnancies with fetuses at 11+0 to 13+6 gestational weeks. Thyroid-stimulating hormone (TSH) values above 2,5 mIU/L were considered high. The correlation between the results and parameters of the first-trimester combined screening and thyroid hormone values was examined. Results: Among the participants, after analyzing the fT4 values, all 35 pregnant women with high TSH values were diagnosed with subclinical hypothyroidism. We observed a statistically weak negative correlation between TSH and ß-HCG, which was expected given the similarities in their alpha subunits. There was no statistically significant correlation between TSH and PAPP-A values, combined risk scores, and age risk scores of first-trimester screening. As a matter of course, combined risk scores of first-trimester screening were found to be statistically lower in younger mothers. Conclusion: Pregnant women with normal thyroid functions and subclinical hypothyroidism were investigated for alterations in first-trimester screening parameters and risk scores. As a result, no statistically significant correlation was found between subclinical hypothyroidism and first-trimester screening. [ABSTRACT FROM AUTHOR]