Reference Ranges and Determinants of Thyroid Function During Early Pregnancy: The SELMA Study.

Area of Science:

• Endocrinology and Reproductive Health
• Environmental Health Sciences
• Clinical Biochemistry

Background:

• Establishing accurate reference ranges for thyroid hormones during pregnancy is vital for clinical interpretation and research.
• Existing data are limited, especially for triiodothyronine, and often do not account for thyroid antibodies or human chorionic gonadotropin.

Purpose of the Study:

• To determine reference ranges for thyroid-stimulating hormone (TSH), free thyroxine (FT4), free triiodothyronine (FT3), total thyroxine (TT4), and total triiodothyronine (TT3).
• To identify and quantify the determinants of these thyroid hormones in pregnant individuals.

Main Methods:

• Utilized data from 2314 participants in the Swedish Environmental Longitudinal, Mother and child, Asthma and allergy study.
• Calculated reference ranges (2.5th–97.5th percentiles) after excluding women positive for thyroperoxidase antibodies (TPOAb) and/or thyroglobulin antibodies (TgAb).
• Assessed determinants including tobacco smoke exposure, body mass index (BMI), and gestational age.

Main Results:

• Established reference ranges: TSH (0.11–3.48 mU/L), FT4 (11.6–19.4 pmol/L), FT3 (3.72–5.92 pg/mL), TT4 (82.4–166.2 pmol/L), TT3 (1.28–2.92 nmol/L) after excluding TPOAb-positive women.
• Excluding TgAb-positive women did not substantially alter these ranges.
• Tobacco smoke exposure correlated with lower TSH and higher FT3/TT3; BMI and gestational age were key determinants for most measured thyroid hormones.

Conclusions:

• Excluding TgAb-positive women minimally impacts established clinical reference ranges for pregnancy thyroid hormones.
• Identified significant clinical determinants (BMI, gestational age, tobacco smoke) influencing thyroid function, potentially indicating endocrine-disrupting effects or impacts on thyroid hormone metabolism.