Iodine is a component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Both hormones are involved in energy metabolism, thermoregulation, protein synthesis, reproduction, growth, physical and mental development, neuromuscular function and red blood cell development.
Iodine is converted to iodide in the gut before absorption. Iodine in the blood is taken up by the thyroid gland. It is then bound to a thyroid protein called thyroglobulin. This is then turned into the hormones T4 and T3. T4 and T3 circulate bound to the proteins prealbumin and albumin. Iodine is excreted via the urine and to some extent in the bile.
The iodine content of food is dependent on the iodine found in the environment. Seafood and seaweed however are good sources. Meat, milk and eggs may also be good sources depending on the content found in the animal feed. Coastal areas tend to have soils with higher iodine content as opposed to inland areas. Fortification of salt with iodine has occurred in many developed countries. However, in many developing countries, iodine deficiency remains a problem.
Iodine deficiency results in goiter (an enlargement of the thyroid gland) due to over-stimulation by the hormones T4 and T3. Severe deficiency results in myxedema, a dry waxy-type swelling with deposits of mucoprotein under the skin. When iodine deficiency occurs in the womb, cretinism (retarded mental development), lowered metabolic rate and/or dwarfism may occur. Deficiencies are common in developing countries where no source of iodized salt is available or accessible.