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Background: Pregnancy is a physiological condition where early detection of any pathological change of thyroid hormone is very important to prevent complications. Population-specific thyroid hormone reference intervals are a key to this. Since Bangladesh is yet to develop such reference intervals, the aim of this study was to establish trimester-specific reference intervals addressing the physiological changes of thyroid hormone during pregnancy for Bangladeshi women. Materials and Methods: One thousand nine hundred and thirty-seven women with singleton pregnancy consuming iodinated salt were selected for this cross-sectional study. Participants having a history of thyroid disorder (TD), any illness influencing thyroid function, presence of goiter, recent history of consumption of thyroid status-altering medications, family history of TDs, and presence of thyroid antibodies were excluded from the study. 1038 pregnant women with thyroid-stimulating hormone (TSH) level within normal (nonpregnant) laboratory reference value (0.35–5.5 μIU/mL) were considered eligible for the study. Results: The mean age of the eligible cases was 28.06 ± 4.633 years ranging from 18 years to 42 years. The mean gestational ages of the study population while checking thyroid function were 8.83 ± 2.344 weeks, 19.99 ± 4.824 weeks, and 31.77 ± 2.123 weeks during 1st, 2nd, and 3rd trimesters, respectively. TSH level raised significantly from 1st trimester to 2nd trimester (1.85 ± 1.00 vs. 2.03 ± 1.02; P = 0.018) but the decline in 3rd trimester comparing to 2nd trimester (2.03 ± 1.02 vs. 1.90 ± 0.90; P = 0.071) was not statistically significant. The 2.5th–97.5th percentiles of TSH level of the study population were 0.440–4.050 μIU/mL, 0.531–4.570 μIU/mL, and 0.622–3.917 μIU/mL in 1st, 2nd, and 3rd trimesters, respectively. Conclusion: Reliance on nonpregnant reference intervals or any generalized gestational reference interval can hinder the desired progression of pregnancy influencing diagnosis and management for TDs in case of certain populations. Establishment of individual reference intervals for thyroid hormone in pregnancy bears great clinical importance for every population.