Thyroid disorders are among the most prevalent of medical conditions. Their manifestations vary considerably from area to area and are determined principally by the availability of iodine in the diet. The limitations of epidemiological studies of thyroid disorders
should therefore be borne in mind when considering the purported frequency of thyroid diseases in different communities (1).
Almost one-third of the world’s population live in areas of iodine deficiency and risk the consequences despite major national and international efforts to increase iodine intake, primarily through the voluntary or mandatory iodization of salt (2). The ideal dietary allowance of iodine recommended by the WHO is 150 μg iodine/day, which increases to 250 μg in pregnancy and 290 μg when lactating. The WHO estimates that two billion people, including 285 million school-age children still have iodine deficiency, defined as a urinary iodine excretion of less than 100 μg/l. This has substantial effects on growth and development and is the most common cause of preventable mental impairment worldwide. In areas where the daily iodine intake is below 50 μg, goitre is usually endemic, and when the daily intake falls below 25 μg, congenital hypothyroidism is seen. The prevalence of goitre in areas of severe iodine deficiency can be as high as 80%. Iodization programmes are of proven value in reducing goitre size and in preventing goitre development and cretinism in children. Goitrogens in the diet, such as thiocyanate in incompletely cooked cassava or thioglucosides in Brassica vegetables, can explain some of the differences in prevalence of endemic goitre in areas with similar degrees of iodine deficiency. Autonomy can develop in nodular goitres leading occasionally to hyperthyroidism, and iodization programmes can also induce hyperthyroidism, especially in those aged over 40 years with nodular goitres. Autoimmune thyroiditis or hypothyroidism has not been reported to complicate salt iodization programmes. Relatively little prevalence data exist for autoimmune thyroid disease in areas of iodine deficiency (3).
In iodine-replete areas, most people with thyroid disorders have autoimmune disease, ranging through primary atrophic hypothyroidism, Hashimoto’s thyroiditis, to hyperthyroidism caused by Graves’ disease. Cross-sectional studies in Europe, the USA, and Japan have determined the prevalence of hyperthyroidism, hypothyroidism, and the frequency and distribution of thyroid autoantibodies in different, mainly white, communities (1, 4–6). Recent US data have revealed differences in the frequency of thyroid dysfunction and serum antithyroid antibody concentrations in different ethnic groups (6), whereas studies from Europe have revealed the influence of dietary iodine intake on the epidemiology of thyroid dysfunction (7). Studies of incidence of autoimmune thyroid disease have only been conducted in a small number of developed countries (8–11). Following a review of the available epidemiological data, the value of screening adult populations for autoimmune thyroid disease will be considered.
Clinical and subclinical autoimmune thyroid disorders in systemic sclerosis