Background: This paper presents the protocol and primary findings of pregnancy cohort population-based study in Isfahan, Iran.

Materials and methods: In this cohort, 418 pregnant and 438 nonpregnant women were enrolled. In the first phase, serum concentrations of thyroid-stimulating hormone (TSH), free thyroxine (FT4), free triiodothyronine (FT3), thyroid peroxidase antibody, and urinary iodine concentration (UIC) were measured. Furthermore, the thyroid ultrasound was also performed. According to the results of thyroid function tests in the first phase, local reference range for TSH, FT4, and FT3 in pregnant and nonpregnant women are determined. The 2.5^th and 97.5^th percentiles are determined as limits of the reference ranges. In the second phase, all pregnant women underwent prenatal care visits in each trimester and they followed for 7 days after delivery and the pregnancy outcomes data are reported.

Results: The mean ± standard deviation for TSH, FT4, FT3, and UIC in the first trimester of gestation was 1.84 ± 1.32 mIU/L, 1.01 ± 0.15 ng/dL, 4.50 ± 0.64 pmol/L, and 172.0 ± 90.29 μg/L, respectively. In nonpregnant women, these values for TSH, FT4, FT3, and UIC were 2.58 ± 1.77 mIU/L, 1.10 ± 0.21 ng/dL, 4.49 ± 0.57 pmol/L, and 190.0 ± 109.6 μg/L, respectively.

Conclusion: The results of the present study could contribute to establish a local thyroid function tests reference ranges in the first trimester of pregnancy. It could possibly be effective on making a local reference value to prevent of thyroid disease misdiagnosis during pregnancy and adverse pregnancy outcomes.