Trimester-Specific Reference Intervals for Thyroid Function Tests during Pregnancy: A Cross-Sectional Analysis

Abstract

BACKGROUND:

Pathological changes in maternal thyroid hormone (TH) levels during pregnancy can lead to adverse outcomes for both the mother and the baby. This underscores the significance of Thyroid Function Tests (TFTs) during pregnancy. However, the dynamic alterations in thyroid gland function and hormone levels throughout pregnancy warrant the establishment of trimester-specific reference intervals, distinct from those used for non-pregnant females.

Aim: This study was conducted in a teaching hospital in northern India to determine trimester-specific reference intervals for TFTs among pregnant women in the region.

MATERIALS & METHODS: This cross-sectional study was conducted at a tertiary care teaching hospital in northern India. A cohort of 600 pregnant women attending the Obstetrics Outpatient Department was considered. Among them, 145 healthy women (constituting 33.35% of the presented pregnant women) aged between 18 and 40, with uncomplicated singleton intrauterine pregnancies, who consumed iodised salt, and were in their first trimester, were included in the study. Women in the second trimester (100 participants, 16.7%) and third trimester (300 participants, 50%) were excluded. From the initial 200 first-trimester patients, individuals with a family history of thyroid disease (15%), abortion history (10%), goitre (0.5%), anti-thyroid peroxidase positivity, overt hypothyroidism (1%), hyperthyroidism, or cases of twin pregnancies (1%) were further excluded. Participants were monitored across successive trimesters to record observations and determine trimester-specific reference ranges.

RESULTS: Our study revealed a progressive decline in serum free T3 and serum free T4 levels during subsequent trimesters, accompanied by increased serum TSH levels. Notably, the reference values for pregnant women substantially differed from those of non-pregnant individuals.

CONCLUSION: Thyroid function testing protocols must be tailored to the unique needs of pregnant women, distinct from those of non-pregnant counterparts. Laboratories should exercise vigilance regarding reference ranges, assays, and screening guidelines to ensure optimal patient care.

KEYWORDS: Free thyroxine, pregnancy, thyroid function tests, trimester-specific reference intervals, physiological changes.