Iodine
Function
Iodine has only one metabolic function in the human body and that is to
participate in the
synthesis of the thyroid hormone called thyroxine. Thyroxine stimulates cell
oxidation and
regulates the basal metabolic rate. Thyroxine is believed to increase
oxygen uptake and the
rate of enzyme reactions that metabolize glucose.
Absorption and Excretion
Dietary iodine is absorbed by the small intestine, binds loosely to proteins,
and is transported via blood to the thyroid gland. Approximately 1/3 of the
iodine is taken up by the thyroid gland; the remainder is excreted in the urine
a few days after ingestion. Thyroid stimulating hormone, secreted by the
pituitary gland, regulates the uptake of iodine and subsequent synthesis of
thyroxine.
Clinical Conditions
A condition called hyperthyroidism results in an increase in the rate of
iodine uptake and increases the overall metabolic rate. Hypothyroidism
has the opposite affect, resulting in a decrease in the metabolic rate.
Goiter is a condition of iodine deficiency and is characterized by an
enlargement of the thyroid gland. With this condition, the thyroid gland cannot
produce adequate thyroxine to regulate thyroid stimulating hormone (TSH). TSH
output increases causing an increase in the uptake of iodine by the thyroid and
hyperplasia of the thyroid follicle cells. As a result, reserves of a
glycoprotein called thyroglobulin are used up and the gland increases in
size.
Fetal iodine deficiency can result in a condition called cretinism. Cretinism
is characterized by mental deficiency, deaf mutism, and spastic diplegia.
Thyroid hormone is involved in brain development, and therefore neonatal iodine
deficiency may result in abnormal brain development.
Recommended Intake
The RDA for iodine in adolescents and adults is 150 mcg/day. An additional 25
mcg/day is required during pregnancy, and 50 mcg/day during lactation. Infants
require 40-50 mcg/day.
Food Sources
Seafood, seaweed, and dairy products are high in iodine content. Iodized table
salt is a primary source in the diet. The iodine content in plant foods is
dependent on the iodine content of the soil.
Figure 6.1: Iodine Content of Selected Foods
Supplementation
Iodine supplementation is mainly used for the prevention and treatment of iodine
deficiency. Increased consumption of iodine has been associated with acne-like
skin eruptions. The tolerance for high levels of iodine is quite high; meaning
iodine toxicity is low. Excessive intake has been reported to be 2000 mcg/day,
but some coastal Japanese populations consuming large amounts of seaweed have
intakes of 50,000 to 80,000 mcg/day without ill effects.