Reference Intervals for Thyroid-Stimulating Hormone and Free Thyroxine Established in Pregnant Women in Lhasa

Standard reference intervals derived from non-pregnant populations are unsuitable for evaluating thyroid hormone levels in pregnancy. Each geographical region should establish pregnancy-specific thyroid hormone reference intervals that consider local population characteristics, instrumentation, and analytical methods. This research aimed to assess variations in thyroid hormone concentrations during first, second, and third trimester stages among women residing in Lhasa, Xizang. Consequently, the study developed local reference intervals for thyroid-stimulating hormone (TSH), free triiodothyronine (FT3), and free thyroxine (FT4) to facilitate improved management and prevention of thyroid diseases among pregnant women in this area.

Adhering to the National Academy of Clinical Biochemistry (NACB) guidelines, the study enrolled 1,016 healthy pregnant women undergoing routine antenatal check-ups across different pregnancy stages from October 2023 to October 2024. Additionally, 120 non-pregnant healthy women who participated in health screenings dur-ing the identical period served as the control group. Measurements of FT3, FT4, and TSH were performed utilizing the Abbott Architect i2000 automated chemiluminescence immunoassay system. Established reference intervals for each pregnancy stage were represented as medians and corresponding 2.5th to 97.5th percentile values.

1) The derived reference intervals were: First trimester: FT3 (3.79 - 5.80 pmol/L), FT4 (10.42 - 16.43 pmol/L), TSH (0.40 - 3.41 mIU/L), second trimester: FT3 (3.36 - 5.80 pmol/L), FT4 (9.40 - 16.22 pmol/L), TSH (0.52 - 3.97 mIU/L), third trimester: FT3 (3.01 - 5.09 pmol/L), FT4 (9.52 - 15.05 pmol/L), TSH (0.69 - 4.43 mIU/L). 2) Significant differences in FT3, FT4, and TSH concentrations were identified among pregnancy stages and between pregnant participants and non-pregnant controls (p < 0.001). Specifically, FT3 and FT4 demonstrated a continuous decline with advancing pregnancy, while TSH exhibited a progressive increase.

For the first time, this study successfully established distinct reference intervals for TSH, FT3, and FT4 specific to healthy pregnant women in Lhasa, Xizang. These locally applicable intervals can enhance early detection accuracy for thyroid-related disorders in pregnant populations, thus mitigating risks associated with inappropriate application of general population-based reference standards.