molecule of T3 requires three molecules of iodine,
while formation of T4 requires four molecules of
iodine. The body contains between 20 and 30 mg of
iodine, 60% of which is stored in the thyroid. The
remainder is found in the blood, muscles, and ovaries.
Thyroid hormones are broken down in the liver and
some of the iodine is recycled. The rest is lost to the
body in urine.
Iodine is found in soil and in the ocean. The
amount of iodine varies widely by location. In moun-
tainous regions where heavy rain and snow cause ero-
sion or in low-lying regions where regular flooding
occurs, the soil is especially deficient in iodine. The
mountains of the Himalayas, Andes, and Alps are all
iodine-poor as is the Ganges river valley. The Interna-
tional Council for the Control of Iodine Deficiency
Disorders (ICCIDD) estimates that 38% of the
world’s population, or about 2.2 billion people, live
in areas where they are unlikely to get enough iodine
without supplementation.
Iodine deficiency disorders (IDDs) create serious
health problem. In the early 1900s, iodine deficiencywas common in interior regions of the United States
and Canada, as well as many other non-coastal
regions of the world. In the 1920s, the United States
began a voluntary program of adding iodine (in the
form of potassium iodide) to salt. Salt was chosen
because all races, cultures, and economic classes use
it, its consumption is not seasonal, and it is inexpen-
sive. Adding 77 mcg of iodine per gram of salt costs
about $0.04 per year per person in the United States.
About 50% of table salt sold in the United States
contains iodine. It is labeled ‘‘iodized salt.’’ All table
salt sold in Canada is iodized. In most other countries
iodine is added at lower concentrations ranging from
10–40 mcg/gram.Normal iodine requirements
The United States Institute of Medicine (IOM) of
the National Academy of Sciences has developed val-
ues calledDietary Reference Intakes(DRIs) forvita-
minsandminerals. The DRIs consist of three sets of
numbers. The Recommended Dietary Allowance
(RDA) defines the average daily amount of the
nutrient needed to meet the health needs of 97–98%
of the population. The Adequate Intake (AI) is an
estimate set when there is not enough information to
determine an RDA. The Tolerable Upper Intake Level
(UL) is the average maximum amount that can be
taken daily without risking negative side effects. The
DRIs are calculated for children, adult men, adult
women, pregnant women, andbreastfeedingwomen.
The IOM has not set UL levels for iodine in
children under one year old because of incomplete
scientific information. RDAs for iodine are measured
in micrograms (mcg). The following are the daily
RDAs and IAs for iodine for healthy individuals.
They are the same as the recommendations made by
the World Health Organization (WHO).Children 0–6 mos. 110 Not established
Children 7–12 mos. 130 Not established
Children 1–3 yrs. 90 200
Children 4–8 yrs. 90 300
Children 9–13 yrs. 120 600
Adolescents 14–18 yrs. 150 900
Adults 19 yrs. 150 1,100
Pregnant women 18 yrs. 220 900
Pregnant women 19 yrs. 220 1,100
Breastfeeding women 18 yrs. 290 900
Breastfeeding women 19 yrs. 290 1,100
Food Iodine (mcg)
Seaweed, dried, 1 oz. up to 18,000
Kelp, ¼ cup wet (amount is highly variable) 415
Salt, iodized, 1 tsp. 400
Haddock, 3 oz. 104–145
Cod, 3 oz. 99
Salt, iodized, 1 g 77
Milk, 1 cup 55–60
Turkey breast, cooked, 3 oz. 34
Cottage cheese, ½ cup 25–75
Shrimp, 3 oz. 21–37
Egg, 1 large 18–29
Processed fish sticks, 1 piece 17
Tuna, canned, 3 oz. 17
Ground beef, cooked, 3 oz. 8
mcg microgramRecommended Tolerable upper
Dietary Allowance intake level
Age (mcg) (mcg)Iodine(Illustration by GGS Information Services/Thomson Gale.)KEY TERMS
Hormone—A chemical messenger that is produced
by one type of cell and travels through the blood-
stream to change the metabolism of a different type
of cell.
Mineral—An inorganic substance found in the
earth that is necessary in small quantities for the
body to maintain a health. Examples: zinc, copper,
iron.
Pituitary gland—A small gland at the base of the
brain that produces many regulating hormones.Iodine