Establishment of the trimester-specific reference range for free thyroxine index

Area of Science:

• Endocrinology
• Reproductive Medicine
• Clinical Chemistry

Background:

• Accurate thyroid status assessment during pregnancy is critical for maternal and fetal well-being.
• Free thyroxine (FT4) assays show significant variability, making total T4 (TT4) and FT4 index (FT4I) potentially more reliable in pregnant individuals.
• Established trimester-specific reference intervals for FT4I are lacking.

Purpose of the Study:

• To determine trimester-specific reference ranges for FT4I in healthy, iodine-sufficient pregnant Iranian women.
• To provide crucial data for accurate thyroid function assessment in pregnancy.

Main Methods:

• A longitudinal survey of 152 healthy pregnant women was conducted after excluding those with thyroid disorders or iodine deficiency.
• Serum thyrotropin (TSH), TT4, and FT4I were measured using immunoassay.
• Reference intervals were established using the 5th and 95th percentiles with a bootstrap procedure.

Main Results:

• Mean TSH, TT4, and FT4I values varied across trimesters.
• Trimester-specific reference intervals were established for TSH, TT4, and FT4I.
• A weak but significant negative correlation was found between TSH and FT4I in the first trimester only.

Conclusions:

• This study provides the first trimester-specific reference intervals for FT4I in a population of normal, iodine-sufficient pregnant Iranian women.
• These established ranges will aid in the accurate interpretation of thyroid function tests during pregnancy.
• The findings contribute to improved maternal and fetal health outcomes through better thyroid management.