In areas where iodine - essential for the production of thyroxine - is lacking in the diet, the thyroid gland can be considerably enlarged, giving the swollen necks of goitre.
Thyroxine is critical to the regulation of metabolism and growth, throughout the animal kingdom. Among amphibians, for example, administering a thyroid blocking agent such as propylthiouracil can prevent tadpoles from metamorphosing into frogs; conversely, administering thyroxine will trigger off metamorphosis.
In humans, children born with thyroid hormone deficiency deficiency will not grow well, and brain development can be severely impaired, in the condition referred to as cretinism New-born children in many developed counties are now routinely tested at birth for thyroid hormone deficiency; this is done by analysis of a small spot of blood from the child (usually, the spot is also tested for phenylketonuria at the same time). A child so affected is easily treated by supplementation with synthetic thyroxine, which enables them to grow and develop normally.
Hypothyroidism is the condition whereby, for some reason there is inadequate thyroxine; sufferers tend to be tired, gain weight, feel mentally sluggish and feel the cold more thatn normal: treatment is by replacing the missing hormone by a simple tanblet of syntheic hormone.
Hyperthyroidism is the excess production of thyroxine - the sufferer has an increased heart rate and metabolism, and can be intolerant of heat. This can be treated by drugs that inhibit thyroxine production, or even removel of the thyroid in extrmem cases (once the thyroid is removed, the sufferer has to take replacement thyroxine for the rest of her (women appear to suffer thyroid disease more than men) life.
Because of the thyroid's selective uptake and extreme concentration of what is actually a quite rare element, it is extremely sensitive to the effects of various radioactive isotopes of iodine produced by nuclear fission. In the event of large accidental releases of such material into the environment, the uptake of the radioactive iodine by the thyroid can, in theory, be blocked by saturating the uptake mechanism with a large surplus of non-radioactive iodine i.e. iodide tablets. Biological researchers making compounds labelled with iodine isotopes do this, but in the wider world such countermeasures are not necessarily stockpiled before an accident, or distributed adequately after - one consequence of [Chernobyl disaster]? was an increase in thyroid cancers in the years following the accident.
More info on that here: http://news.bbc.co.uk/hi/english/sci/tech/newsid_1319000/1319386.stm
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