Physiological Functions

The only function of iodine involves the synthesis of thyroid hormone. Approximately 60% of the total body pool of iodine is stored in the thyroid gland. The remainder is found in the blood, ovary, and muscle. Thyroid hormone is necessary for regulation of human growth and development.

Factors Affecting Availability

Iodine is absorbed intestinally from dietary sources or dermally from topical iodine applications or from iodine vapors produced as byproducts of industrial activity. Iodine vapor is also emitted from cleansing agents used commercially in sterilization processes and from fossil fuel combustion such as occurs in automobile engines. Currently, the most common source of exposure to iodine is from automobile exhaust. In the 1970's, the amount of iodine measured in the environment reached levels that were a cause for concern prompting the dairy industry to discontinue use of iodine-containing agents in sterilization of milking equipment to reduce the iodine content of milk.

Deficiency

Iodine deficiency was frequently observed in landlocked regions of the US at the beginning of the 20th century necessitating iodine fortification of salt, an inexpensive and widely used seasoning. The development of iodine deficiency is no longer a problem, since landlocked regions receive produce grown in coastal areas where soil is rich in iodine. Signs of iodine deficiency include hypothyroidism, lethargy, and weight gain. The clinical presentation of iodine deficiency is goiter. Goiter can also develop from high intakes of goitrogens, naturally occurring substances in foods which decrease iodine availability or interfere with its tissue utilization. Dietary sources of goitrogens include cabbage, turnips, rapeseed oil (canola oil), peanuts, cassava, and soybeans. Goitrogens are inactivated by heating, roasting or cooking.

Cretinism is a condition which develops in the fetus from iodine deficiency during pregnancy. This condition is characterized by mental retardation and dwarfism. Neonates are routinely screened for adequate thyroid hormone levels in developed countries and is being adopted in developing countries.

Toxicity

Chronic excessive intakes of iodine may compromise thyroid function and also contribute to development of goiter and hypothyroidism due to feedback inhibition of thyroid hormone synthesis. Grave's disease develops in response to an overactive thyroid and is not a condition associated with iodine toxicity.

The upper limit of safety established for iodine by the Food and Nutrition Board of the Institute of Medicine is approximately 1,100 mcg daily for adults. See table below for age- and gender specific guidelines.

Iodine Tolerable Upper Intake Levels

Life Stage	Iodine(mcg/d)
Infants
     0-6 months	N/A
     7-12 months	N/A
Children
     1-3 years	200
     4-8 years	300
Males, Females
     9-13 years	600
     14-18 years	900
     19-70 years	1100
     > 70 years	1100
Pregnancy
     < 18 years	900
     19-50 years 	1100
Lactation
     < 18 years	900
     19-50 years	1100

Requirements

The Daily Reference Intakes (DRI) for iodine are shown in the table below.

Daily Reference Intakes

Life Stage	Iodine mcg
Infants
 0-6 months		110
 7-12 months		130
Children
 1-3 years		90
 4-8 years		90
Males
 9-13 years		120
 14-18 years		150
 19-30 years		150
 31-50 years		150
 51-70 years		150
 > 70 years		150
Females	
 9-13 years		120
 14-18 years 		150
 19-30 years		150
 31-50 years		150
 51-70 years		150
 > 70 years		150
Pregnancy
 < 18 years		220
 19-30 years		220
 31-50 years		220
Lactation
 < 18 years		290
 19-30 years		290
 31-50 years		290

Dietary Sources

Although most foods do not contain iodine, one teaspoon of iodized salt consumed daily is more than sufficient to satisfy physiological requirements for this nutrient. Other dietary sources of iodine include drinking water, seafood (clams, lobster, oysters, sardines and ocean fish) and dairy products from feed additives as well as from disinfectants used on dairy farms. The iodine content of fruits and vegetables is dependent upon soil content. More detailed information on food sources of iodine is provided below.

Iodine Content of Food
Food	Iodine(mcg)
Salt, iodized, 1 tsp.	400
Haddock, 3 oz.	104 - 145
Bread, regular process, 1slice	35
Cheese, cottage, 2% fat, 1/2 cup	26 - 71
Shrimp, 3 oz.	21 - 37
Egg, 1	18 - 26
Cheese, cheddar, 1 oz.	5 - 23
Ground beef, 3 oz, cooked	8