Iodine plays an important role in regulating the body's thyroid gland and thus, the body's production of energy, assists in encouraging metabolism, and helps the body burn excess fat. Travellers to remote areas often add iodine to local water supplies to remove impurities and make it safe for drinking.

Functions:

Iodine is needed to help form thyroid hormone, which controls the body's rate of metabolism.

Deficiencies:

A lack of iodine leads to goitre, recognisable as a visible lump in the neck. Iodine deficiency in pregnancy can lead to developmental problems with children. Iodine is toxic in large doses so it is best to obtain the small quantities of iodine needed from the food we eat.

Sources:

It is best obtained from seafood (it is especially high in seaweed), vegetable, dairy products and mushrooms. If you're feeling really herbiverous, iodine can also be obtained from Irish moss grown in iodine rich soils. Much simpler is to buy iodised salt. The iodine content of vegetables and grains is affected by the iodine content of the soil where they are grown. Mountainous areas (e.g. in Tasmania) are known to have soils low in iodine.

Interestingly:

Seaweed has been recorded as a treatment for goitre in China as far back as 4000 years ago.

RDI:

Adults: 120-150 mcg.

Source: Growcom's formulaforlife - choose good health & happiness.