When during pregnancy is it best for a woman to supplement iodine?

Iodine supplementation is critically important during pregnancy, and the optimal time to begin is before conception, ideally as part of preconception care. However, if supplementation has not been initiated prior to pregnancy, it should be started as soon as pregnancy is confirmed—typically in the first trimester—and continued throughout gestation and lactation.

This timing is essential because iodine is a key substrate for maternal thyroid hormone synthesis, which supports early fetal neurodevelopment—even before the fetal thyroid gland becomes functional (around week 12–14 of gestation). The fetus relies entirely on maternal thyroxine (T4) during the first trimester, and iodine deficiency during this window is associated with increased risks of impaired cognitive development, lower IQ, and neurologic deficits in the child.

The World Health Organization (WHO), International Council for Control of Iodine Deficiency Disorders (ICCIDD), and American Thyroid Association (ATA) all recommend a daily iodine intake of 250 µg for pregnant individuals. This is typically achieved through a prenatal multivitamin containing potassium iodide (150 µg) plus dietary sources (e.g., iodized salt, dairy, seafood), though many clinicians advise a dedicated iodine supplement (e.g., 150 µg) in addition to prenatal vitamins if dietary intake is uncertain or suboptimal.

It’s important to note that excessive iodine intake (>500–1100 µg/day, depending on guidelines) may disrupt thyroid function and should be avoided. Therefore, supplementation should be guided by clinical assessment and not self-prescribed at high doses. Routine urinary iodine concentration testing is not recommended for all pregnant individuals but may be considered in populations or individuals at high risk for deficiency (e.g., those avoiding iodized salt, dairy, or seafood; or residing in iodine-deficient regions).