Objective  To assess intertrimester and seasonal variations in urinary iodine concentration (UIC) among pregnant women.

Patients and measurements  Pregnant Tehranian women, with no history of thyroid ablation or consumption of thyroid‐affecting medication were selected consecutively from November 2004 to November 2006. Morning spot urine samples were collected from each participant during the first, second and third trimesters of pregnancy. Of 466 participants, 298 completed the study in all three trimesters of pregnancy. UIC < 150, 150–250, and > 250 µg/l indicated low, adequate, and high levels, respectively. UIC was measured using a modified Sandell–Kolthoff digestion method.

Results  The pregnant women were aged 25·0 ± 5·0 (range 16–48) years. Median UIC did not show seasonal fluctuations during pregnancy. However, median (range) UIC was 193 (19–840), 159 (16–640), and 141 (16–400) µg/l in the first, second and third trimesters of pregnancy, respectively (P < 0·0001). Frequency distributions of low, adequate and excessive UIC were 33·2, 31·2 and 35·6% in the first trimester, 46·0, 28·9 and 25·2% in the second trimester, and 53·4, 30·9 and 15·8% in the third trimester of pregnancy, respectively (P < 0·0001). Women with UIC < 150 were more frequently represented in the second vs. the first (P = 0·043; odds ratio 1·49, 95% confidence interval 1·012–2·213) and in the third vs. the first (P = 0·013; odds ratio 1·62, 95% confidence interval 1·108–2·379) trimesters of pregnancy than the 150–250 µg/l group were. UIC decreased throughout pregnancy in all seasons.

Conclusions  Iranian national salt iodization may prevent seasonal fluctuations of UIC but does not maintain median UIC within adequate and recommended ranges throughout pregnancy. Extra iodine supplementation during pregnancy is recommended.