Thyroid function in pregnant women: iodine deficiency after iodine enrichment program

Significant changes in thyroid function occur during pregnancy which can complicate the interpretation of thyroid function tests. Therefore, normative gestational related reference ranges for thyroid hormones tests are required. The aim of this study was to determine the reference ranges for free triiodothyronine (FT3), free thyroxin (FT4) and thyroid stimulating hormone (TSH) in Iranian pregnant women.

This study was a cross-sectional observational study conducted in the Obstetrics and Gynecology department, Akbarabadi University Hospital. A single blood sample from 584 pregnant women was analyzed for thyroid function. Serum levels of TSH, FT4, FT3, total T4 (TT4), T3 resin uptake (T3RU) and anti-thyroid peroxidase antibody (TPO Ab) were measured. Urinary iodine was determined in some cases. Reference intervals based on 2.5th and 97.5th percentiles were calculated.

The composition of reference population comprising 584 women included 162 in first trimester and 422 in the third trimester. The 2.5th and 97.5th percentiles values were used to determine the reference ranges for FT3, FT4, TT4, T3RU and TSH. These values were T3 1.4 and 2.9 pmol/L, FT4 7.1 and 18 pmol/L, TT4 7.2 and 13.5 µg/dL and TSH 0.5 and 3.9 µg/L, respectively. The level of urinary iodine in 80.5% of the subjects was less than normal.

Serum levels of thyroid hormones are different in Iranian population that could be due to racial differences or differences in iodine intake.