Iodine is an element of the halogen group which forms black crystals and violet vapour. It is used in both medicine and photography.
Iodine is also an essential trace element and its importance is due to the fact that it forms part of the hormones thyroxine and triodothyronine. These hormones are vital for normal growth and both physical and mental development in animals and humans. A healthy human body contains around 15-20 mg of iodine, the majority of this is contained within the thyroid. In the early 1800s it was discovered that iodine was involved in the production of the thyroid hormones but it was not until many years later that iodine was used to treat thyroid gland problems and failures.
In many countries iodine deficiency was at one time widespread but around 50 years ago in many industrialised countries iodine was added to salt and the problem seems to have been reduced significantly.
The iodine content of plants varies widely depending on the iodine content of the soil in which they are grown. Small amounts may also be obtained from drinking water. Seafood is the richest source of iodine, thus fish such as cod, salmon and herring may be useful sources. In Britain cattle-feed has long been enriched with iodine which has meant that milk and other dairy products are important dietary sources.
Some sea vegetables concentrate iodides from seawater and are therefore a useful reservoir of combined iodine. This is relevant in the parts of the world where seaweeds are regarded as valuable foods for humans. One such example is southwest Wales where cooked seaweed known as laverbread is commonly eaten.
Iodine is readily absorbed by the body. Absorption occurs mainly in the small intestine, although it is also well absorbed from the stomach and into the blood. Iodine can also be absorbed through the skin if the body is exposed to any form of iodine gas.
After absorption the iodine is quickly dispersed throughout the extra-cellular fluid. About 30% of the iodine goes to the thyroid gland and small amounts are taken up by the salivary and gastric glands.
The remaining iodine is excreted via the kidneys into the urine. Iodine is not conserved in the blood or fluids because the body does not need to store it as it does some other nutrients such as iron.
Organically bound iodine is not very well absorbed by the body when compared to inorganically bound iodine which is rapidly and efficiently absorbed.
The thyroid is the only store of iodine in the body and the amount taken up is based on need. Thyroid Stimulating Hormone (TSH) controls the amount of iodine that is taken up. Once in the thyroid the iodine is incorporated into the thyroid hormones within a large protein called thyroglobulin. Thyroglobulin is then stored within the thyroid. The amount of thyroid hormones released is controlled by the pituitary gland. TSH is also involved.
In adults iodine deficiency can lead to hypothyroidism which often manifests itself in a variety of symptoms:
- Low energy levels
- Dry/scaly or yellowish skin
- Tingling and numbness in extremities
- Weight gain
- Personality changes
- Prolonged and heavy periods in women
A large swelling just below the Adams apple will also be visible. This is known as goitre and is actually the enlarged thyroid gland. The swelling is an attempt by the gland to produce more thyroid hormones.
More advanced hypothyroidism may worsen the above mentioned symptoms as well as leading to other conditions such as Raynauds syndrome, increased cholesterol and homocysteine levels, hypertension and a hyperactive or manic state.
If iodine deficiency persists in childhood it can lead to cretinism, with poor brain development and mental retardation.
With the consumption of foods grown all around the world, iodine deficiency has become a rare occurrence in Britain. Certain countries add iodine to table salt to avoid iodine deficiency. In countries which rely on local food, where iodine is lacking in the soil, iodine deficiency is still a problem. Children can suffer particularly in some developing countries if born to iodine deficient mothers.
Iodine interacts with selenium and possibly vanadium. It is also thought that low zinc intake can exacerbate the effect of low iodine intake.
It is worth noting that some otherwise healthy foods act as natural goitrogens in the diet. These foods can interfere with iodine metabolism and thus increase the risk of goitre occurring. They include:
- Cassava (used in tapioca)