Trimester-Specific Reference Range for Thyroid Function Tests (TFTs) in Normal Pregnant Women at a Tertiary Care Centre.

Introduction: Gestational age, ethnicity, assay method, thyroid autoimmunity and iodine status of the community affect thyroid hormone levels in pregnancy, and there is a need to establish trimester-specific reference ranges for thyroid hormones across different regions of the world. There was no previous study regarding this from this part of the country. The aim of this study was to establish trimester-specific reference range for thyroid hormones during pregnancy in a tertiary care centre in Southern Odisha.

Methods: In this cross-sectional study a total of 676 (n = 180, 244 and 342 for first, second and third trimester, respectively) healthy pregnant women (18-40 years) carrying a singleton pregnancy were recruited after excluding women with history of chronic medical illness, personal or family history thyroid disorder, recurrent abortion, oligo/polyhydramnios, hyperemesis gravidarum, goitre, intake of drugs affecting thyroid hormones, anti-TPO antibody positivity (>60 IU/ml) and low maternal spot urinary iodine concentration (<150 μg/L). Serum free tri-iodothyronine (FT3), free tetra-iodothyronine (FT4) and thyroid stimulating hormone (TSH) were estimated for each trimester by chemiluminescence (CLIA) method and reference interval was expressed as 2.5^th and 97.5^th percentile.

Results: The reference interval for TSH was 0.12-4.10 μIU/ml, 0.55-3.97 μIU/ml and 0.46-4.31 μIU/ml for first, second and third trimester, respectively. The reference intervals for FT3 were 1.93-3.78 pg/ml, 2.06-3.55 pg/ml, 1.77-3.25 pg/ml and for FT4 were 0.78-1.50 ng/dl, 0.72-1.34 ng/dl, 0.70-1.31 ng/dl in the first, second and third trimester respectively.

Conclusion: The upper reference limit of TSH obtained from our study corroborates with that proposed by American Thyroid Association (2017).