Trimester-Specific Reference Intervals Of Thyroid Status In Pregnancy – A Cross-Sectional Study

Abstract

Background: Thyroid dysfunction during pregnancy is associated with significant maternal and fetal complications. Accurate trimester-specific reference ranges are essential for appropriate diagnosis, risk stratification, and timely management.
Objectives: To determine the prevalence of thyroid dysfunction (euthyroid, subclinical hypothyroidism, and overt hypothyroidism) in pregnant women, identify associated maternal risk factors, compare maternal and neonatal outcomes across thyroid function groups, and derive trimester-specific reference intervals for thyroid-stimulating hormone.
Methods: This hospital-based cross-sectional study included 300 antenatal women attending a tertiary care center over a six-month period. Serum thyroid-stimulating hormone (TSH) and free thyroxine (FT4) levels were estimated using a chemiluminescent immunoassay. Participants were categorized as euthyroid, subclinical hypothyroid, or overt hypothyroid based on trimester-specific interpretation. Maternal and neonatal outcomes were recorded prospectively. Logistic regression analysis was used to identify independent risk factors. Trimester-specific reference intervals for TSH were calculated using the 2.5th–97.5th percentile method.
Results: The overall prevalence of hypothyroidism was 18.7%, including 13.3% subclinical and 5.4% overt hypothyroidism. Significant risk factors included anemia (OR 2.1, p=0.01), obesity (OR 1.8, p=0.03), and family history of thyroid disease (OR 2.5, p=0.004). Subclinical hypothyroidism was significantly associated with preeclampsia (p=0.02), gestational diabetes (p=0.03), and low birth weight (p=0.01). Trimester-specific TSH reference intervals were: first trimester 0.4–4.1 mIU/L, second trimester 0.6–4.5 mIU/L, third trimester 0.7–4.8 mIU/L.
Conclusion: Thyroid dysfunction is common in pregnancy and is associated with adverse outcomes. Population-specific trimester-based reference intervals improve diagnostic accuracy and clinical care.