To determine iodine intake and urinary iodine excretion (UIE) in a group of pregnant Thai women and the concentration of thyroid-stimulating hormone (TSH) in their neonates.

A prospective cohort study.

Three districts of Songkhla, southern Thailand.

Two hundred and thirty-six pregnant women.

A quarter of the participants lacked knowledge of iodine and the prevention of iodine deficiency, although 70 % used iodized salt. Those who did not use iodized salt stated that they had no knowledge about iodine (57 %) and no iodized salt was sold in their village (36 %). The median iodine intake in the three districts was 205–240 μg/d, with 53–74 % of pregnant women having iodine intake <250 μg/d. The median UIE in the three districts was 51–106 μg/l, with 24–35 % having UIE < 50 μg/l. The mean neonatal TSH was 2·40 (sd 1·56) mU/l, with 8·9 % of neonates having TSH > 5 mU/l.

The studied women and their fetuses were at risk of mild iodine deficiency. About a quarter of the participants lacked knowledge of the importance of iodine. Education regarding the importance of iodine supplements and the promotion of iodized salt should be added to national health-care policies in order to prevent iodine-deficiency disorders, diseases that are subclinical but have long-term sequelae.