Iodine is a pure element, whereas iodide is an anion attached to molecules made from iodine, including potassium iodide, sodium iodide and methyl iodide. Iodide has a negative charge of one. The pure element has the chemical symbol "I" on the periodic table of elements, and the anion has a minus sign to the upper right of the chemical symbol to designate iodide.
Iodine is used by the human body for thyroid function. The thyroid gland secretes hormones based upon iodine levels in the blood. When the body lacks iodine, the thyroid works harder. Lack of iodine can cause an enlarged thyroid and neck swelling.
Iodide is used for radiation emergencies. Potassium iodide tablets are approved by the Food and Drug Administration for people who have been over-exposed to radiation. These tablets reduce the risk of thyroid cancer after radiation exposure but are not intended to prevent the disease.
Iodine is crucial to fetal development in pregnant women, cognitive development in children and reduction of fibrocystic breast disease in adult women. Many multivitamins include iodine, and salt manufacturers add the nutrient to their product. Iodine is found naturally in kelp.
The upper safe limit for iodine intake is 1,110 micrograms per day in adults. An overdose of iodine may cause the same problems the mineral prevents in regular doses.