Breastfeeding mothers are invited to participate in an important research study at Boston University Medical Campus. This study will measure iodine levels in women's breastmilk and urine after taking a prenatal multivitamin and two iodine supplements, both of which are commonly available and over-the-counter. There will be two study visits (one to review the study and ask questions, and the second for the study itself, which will last approximately 8 hours). You will be able to bring your baby, and breastfeed during the visits, if needed. Subjects will be paid $100 for their participation and $20 for any parking or transportation costs. You may be eligible if: - you are currently breastfeeding; - at least 18 years old; - speak and understand English. The following women are not eligible: - you have a history of thyroid disease; - you take thyroid hormone or thyroid medications; - you have taken a prescription medication called amiodarone (for your heart) in the past 2 years; - you have had a radiologic study, such as a CT, that used contrast dye in the past three months. Interested participants may call (617) 638-8521 for more information.