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Purpose: Down syndrome screening with first trimester combined test is recommended for all pregnancies. It is unclear, whether thyroid stimulating hormone (TSH) affects secretion of free beta subunit human chorionic gonadotropin (fß-hcg) and pregnancy-related plasma protein (PAPP-A). We aimed to investigate the effect of serum thyroid stimulating hormone (TSH) levels on the combined test results. Materials and Methods: First trimester TSH levels together with nuchal translucency (NT(fß-hcg and PAPPA results of 297 patients were included in this study. Patients were divided into four groups according to their TSH levels; Group 1 (<0.1 µL / ml), Group 2 (0.1-2.5 µlU / ml), Group 3 (2.5-3.5 µlU / ml) and Group 4 (> 3.5 µlU / ml). Groups were compared in terms of NT, PAPP-A and fß-hcg levels. Results: The mean PAPP-A (MoM) value was found to be 0.99 in group 1, 1.12 in group 2, 1.03 in group 3, and 0.95 in group 4. Mean fß-hcg (MoM) value was 1.36 in group 1, 1.29 in group 2, 1.05 in group 3, and 1.32 in group 4. The mean N T measurement was 1.27 ± 0.25, 1.33 ± 0.38, 1.34 ± 0.33 and 1.32 ± 0.37 mm, from group 1 to group 4, respectively. There was no statistically significant difference between groups in terms of PAPP-A, fß-hcg and NT. Conclusion: The present study have shown that first trimester TSH levels do not affect combined test parameters. Prenatal screening by using NT, fß-hCG and PAPP-A seems to be safe for pregnant women with hypothyroidism or hyperthyroidism. [ABSTRACT FROM AUTHOR]