Rationale: In regions of severe endemic goiter, the adverse effects of in utero iodine deficiency on neuromotor development are well established: randomized controlled trials of iodine supplements given to iodine deficient mothers before pregnancy or during early pregnancy improve motor and cognitive performance of their offspring. However, the potential adverse effects of mild-to-moderate iodine deficiency during pregnancy are unclear. Inadequate thyroid function in the fetus and newborn are the likely cause of brain damage in iodine deficiency.

Objective: To determine whether the daily oral administration of 200 µg iodine to pregnant women in areas of mild-to-moderate iodine deficiency improves maternal and newborn thyroid function, pregnancy outcome, birth weight, infant growth and cognitive performance.

Study design: Double-blind randomized controlled multicentre trial. Study population: Pregnant women (18-40 years) presenting at the clinic for their first prenatal visit will be recruited at two research sites, namely St. Martha's hospital in Bangalore, India and Ramathibodi Hospital, Mahidol University, Bangkok, Thailand. At each site, 400 women will be recruited.

Intervention: Half of the women will be randomized to iodine treatment (200 µg per day) and the other half to placebo throughout pregnancy.

Main study parameters/endpoints: Differences between group means in indicators of thyroid function, birth outcome, urinary iodine, breast milk iodine, infant growth, and psychomotor development.

Inclusion Criteria:

• Age 18-40 years;
• Gestational age: ≤ 14 weeks (as judged by the date of the last menstrual period);
• Single pregnancy;
• Non-lactating;
• Planned residence in the area for the duration of the study (3 years).

Exclusion Criteria:

• TSH levels outside the normal range
• History of serious medical conditions such as HIV, hypertension, diabetes, renal, hepatic or cardiovascular disease, thyroid disorders, mental disorders;
• Use of iodine supplement.