Thyroid disease is one of the silent epidemics of our time. Around one in every 100 people in Britain suffers from an underactive thyroid, and many more walk around with a general feeling of malaise without knowing why. In one survey in Birmingham, up to a sixth of people older than 55 had what is referred to as hypothyroidism without obvious symptoms (Clinical Endocrinol (Oxford), 1991; 34: 77-83). Similar numbers of old people in the States have unidentified low thyroid activity (Arch Intern Med, 1990; 150: 785-7).

Thyroids that overheat with too much activity are just as common. The shocking fact is that nearly half of all women and a quarter of all men in Great Britain and the US will die with evidence of an inflamed thyroid (J Pathol Baceteriol, 1962; 83: 255-64).Even in areas with adequate iodine, the overall incidence of thyroid problems is a staggering 1 per every 100 per year (Thryoidology, 1994; 6 : 49-54).

Just what is going on here? We know that most thyroid problems are autoimmune diseases, where the body begins attacking itself. No one is sure why this happens, but many medical pioneers are beginning to suspect allergies and environmental assaults.

However, much of the blame has to be laid at the door of yet another well intended but misguided effort at prevention. Ever since medicine discovered that iodine is necessary for the healthy function of the thyroid, it has recommended that salt be iodized (that is, enriched with iodine). This is regardless of whether an area has sufficient iodine in the food supply or the population consumes adequate amounts of fish, vegetables or sea vegetables which contain iodine.

This preventive measure has now turned the situation on its head, causing a massive incidence of thyroid disease.

In one study in Galicia, in the northwestern Iberian peninsula of Spain, where the population is obliged to use iodized salt, the incidence of overactive thyroid was much higher than usual, particularly among women (J Endocrinol Invest, 1994, 17:23-7).

In the case of chronic autoimmune thyroiditis, the highest prevalence occurs in countries with the highest intake of iodine, such as the US and Japan. Even in areas where iodine is deficient, iodine supplementation trebles the incidence of conditions which prefigure Graves' disease; the prevalence of thyroid antibodies rises to more than 30 per cent within five years (J Clin Endocrinol Metab, 1983, 57: 859-62).

Extra iodine doesn't just come in salt. Cough expectorants, antiseptics, drugs and even radiographic contrast agents can cause either thyroid hypo or hyperactivity (New Eng J Med, 1995; 333: 1001-2).

What's happening with iodine is similar to the situation with fluoride. We're getting bombarded with a substance that naturally occurs in adequate amounts in a well balanced diet. In employing a crude solution more appropriate for a famine, medicine unintentionally has created the very problem it sought to solve.