Defining Trimester-Specific Reference Intervals for Thyroid Hormones: Insights from a Bulgarian Monocenter Study

Area of Science:

• Endocrinology
• Reproductive Medicine
• Clinical Chemistry

Background:

• Pregnancy alters thyroid function tests due to various interfering factors and individual variations.
• Established trimester-specific reference intervals for thyroid-stimulating hormone (TSH) are necessary for accurate assessment.
• Population-specific factors necessitate local establishment of TSH ranges for pregnant women.

Purpose of the Study:

• To establish first- and second-trimester-specific reference intervals for TSH and free thyroxine (FT4).
• To provide region-specific thyroid function test ranges for pregnant women.

Main Methods:

• Retrospective monocenter analysis of 497 pregnant and 250 non-pregnant women.
• Exclusion of participants with thyroid dysfunction, relevant family history, or interfering medications.
• Application of the percentile method with bootstrapping for TSH reference range determination, following IFCC recommendations.

Main Results:

• First-trimester TSH: 0.38-2.91 mIU/L; FT4: 12.18-19.48 pmol/L.
• Second-trimester TSH: 0.72-4.22 mIU/L; FT4: 9.64-17.39 pmol/L.
• Established reference range for non-pregnant controls aligns with laboratory standards.

Conclusions:

• Established TSH and FT4 intervals differ from current international guidelines (ATA, ETA, Endocrine Society).
• Adherence to these specific intervals can significantly impact the timely diagnosis and management of thyroid conditions in pregnancy.
• Local establishment of reference intervals is vital for accurate thyroid function assessment in pregnant populations.