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.
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.
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.
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.
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.
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.