Reference Range of Thyroid Hormones in Pregnancy: Customization Needed for Pregnant Women in India.

Background: The precise interpretation of thyroid function tests during pregnancy needs population-based trimester-specific data among pregnant women. This study was to determine trimester-specific reference range for free triiodothyronine (FT3), free thyroxine (FT4) and thyrotropin (TSH) among pregnant women in India.

Methods: In this prospective observational study, asymptomatic pregnant women with single fetus were enrolled from each trimester. Serum FT3, FT4, TSH and anti-thyroperoxidase (anti-TPO) antibodies were estimated using electrochemiluminescence technique.

Results: Among 918 pregnant women, 82 women were excluded due to the presence of anti-TPO antibodies (76) and overt hypothyroidism (6). Among the remaining 836 women, 279 (33.3%) were in first trimester, 309 (36.9%) in second and 248 (29.6%) in third trimester. The 5th and 95th percentile values for each trimester were used as reference ranges. For all three hormones, the reference ranges for the first, second and third trimesters were: FT3 (1.59-3.64, 1.60-3.50 and 1.44-3.28 pg/dl), FT4 (0.64-1.12, 0.64-1.05 and 0.60-1.01 ng/dl) and TSH (0.21-4.95, 0.23-4.90 and 0.14-4.59 µIU/ml). The mean and median values for TSH between each trimester showed no statistically significant difference. No specific trend was seen for FT3 and TSH with advancing gestation. FT4 showed a decreasing trend with advancing trimester (P value: first versus second = 0.01, first versus third = 0.00003 and second versus third = 0.004). The reference range (N = 836) irrespective of trimester for thyroid hormones was: FT3 (1.53-3.46 pg/dl), FT4 (0.63-1.08 ng/dl) and TSH (0.19-4.72 µIU/ml).

Conclusion: The trimester-specific reference levels of TSH among pregnant women from India are significantly higher than 4 mIU/L (American Thyroid Association 2017).