Recommended Nutrient Intakes for Vitamin and Minerals

Iodine

Functions

Iodine is an essential trace element. Iodine is an integral component of 2 thyroid hormones, triiodothyronine (aka thyroxine) (T4) and 3,5,3'-triiodothyronine (T3). These hormones are important for cell processes and control the rate at which cells use oxygen i.e. metabolic rate. The thyroid hormones are also required for normal growth and development of tissues such as the central nervous system and maturation of the whole body.

Recommended Intakes

Table: Recommended Intakes for Iodine (mg/day)

Food Sources

Foods that are rich in iodine include foods of marine origin e.g. sea fish, shellfish, seameal, and seaweeds. The iodine content of plants and foods depends on the iodine content of the soils where they are grown.

The use of iodophors by the dairy industry as sanitisers have provided a residual source of iodine in milk, cheese and other dairy products. However, it is thought that this practice is decreasing in New Zealand.

Also food products containing the colouring erythrosine can contribute iodine.

Deficiency

With iodine deficiency the thyroid hormone levels decline. When thyroid hormones   secretion is inadequate, the basal metabolic rate is reduced and the general level of activity of individuals is reduced (hypothyroidism).

The regulation of thyroid hormones involves the thyroid, pituitary, the brain and peripheral tissues. The hypothalamus in the brain controls the release of thyroid-stimulating hormone (TSH) from the pituitary gland, through a feedback mechanism related to the level of T4. If the level of T4 decreases, the secretion of TSH is increased. If iodine deficiency persists the cells of the thyroid gland enlarge to trap as much iodine as possible. Sometimes the gland enlarges to such an extent that there is a visible lump in the neck, known as a simple goitre.

Iodine deficiency is the world's leading cause of preventable brain damage and mental retardation.

There are a wide spectrum of effects of iodine deficiency on growth and development. These are referred to as iodine deficiency disorders (IDD). Iodine deficiency can cause sluggishness and weight gain, and in pregnancy may impair the development of the foetus. This can cause irreversible mental and physical retardation known as cretinism.

The major cause of IDD is an inadequate intake of iodine from foods which are grown in areas where the iodine content of soils is low. In a minority of individuals goitre may be caused by exposure to antithyroid compounds found in foods (e.g. cabbage family) and drugs. These are known as goitrogens.

Toxicity

Excessive intakes of iodine can enlarge the thyroid gland, just as deficiency can. This condition can become severe and block the airways in infants and cause suffocation. It is thought that intakes in excess of 2000mg/day may cause toxic effects. It is unlikely that such intakes will be obtained from normal diets.