Iodine was discovered by Barnard Courtois in 1811. He isolated iodine from treating seaweed ash with sulphuric acid (H₂SO₄) while recovering sodium and potassium compounds. It was originally named from the Greek word "iodes" meaning "violet".

Iodine is a good example of a trace mineral whose deficiency creates a disease that is easily corrected by resupplying it in the diet. Goiter, an enlargement of the thyroid gland, develops when this important metabolic gland does not have enough iodine to manufacture hormones. As it increases its cell size to try to trap more iodine, the whole gland increases in size, creating a swelling in the neck. Without supplemental iodine, a hypothyroid condition results, likely leading to fatigue and sluggishness, weight gain, and coldness of the body; at this stage, the condition may be harder to treat with iodine alone and thyroid hormone supplementation may be needed.

Plants do not need iodine, but humans require it for the production of thyroid hormones that regulate the metabolic energy of the body and set the basal metabolic rate. Iodine in our food is dependent on the iodine found in the ground where the food is grown, in the food animals receive, as it influences the iodine content in the meat and eggs we consume.

Iodine is used in the production of hormones (such as thyroxine, thyroxin) by the thyroid gland, which in turn regulates the conversion of fat to energy, stabilizing our body weight as well as controlling our cholesterol levels. These hormones produced from the iodine are also needed to help form our bones, as well as keeping our skin, nails, hair and teeth in prime condition. Some indication also exists that iodine is helpful in preventing cancer of the breast and womb.

Iodine is not stored in the body, but various items in our diet do supply iodine, so a shortage does not happen overnight. When iodine is deficient the thyroid gland enlarges (referred to as a goiter) to maximize the amount of iodine to be extracted from the blood, and if this problem is not corrected, a shortage of this hormone in the body may lead to constipation, obesity, weakness, mental slowness as well as mental problems. Goiter is not always the cause of iodine deficiency, but can, in some cases, be caused by certain micro-organisms. Iodine is also thought to help protect the thyroid from the effects of radiation, and the Polish government handed out iodine tablets to their population after the explosion at Chernobyl. Other deficiencies include in newborns, cretinism — delayed growth and swollen features. Also may lead to slow mental reactions, dry hair, hardening of arteries, obesity, lethargy, intolerance to cold, and during pregnancy and infancy can produce mental and physical retardation. Deficiencies are rare and effects can be reversed with supplements. Fibrocystic disease of the breast is linked to iodine deficiency.

Iodine is rapidly eliminated from the body, so high intake or toxicity is not very likely, but if your diet is supplementedwith too much kelp or iodine you could have problems with acne or skin rashes. When iodine in the soil is very low, or if very little seafood is consumed, a person may want to check their iodine intake, or when breast feeding or pregnant as well as being on a sea-salt restricted diet.

If you have an under-active thyroid try and avoid large amounts of raw cabbage, peaches, pears, spinach and Brussels sprouts as they may block the absorption of iodine. Iodine is found in eggs, milk, sea fish and sea food, sea vegetables - such as kelp, asparagus, etc.