Epidemiology of Thyroid Disorders

Abstract

The manifestations of benign thyroid diseases vary considerably and are determined principally by the in the dietary availability of iodine which is an essential component of the thyroid hormones. 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. Almost one-third of the world’s population live in areas of iodine deficiency despite major national and international efforts to increase iodine intake, but iodine deficiency is an emerging issue in industrialised countries, previously thought of as iodine-sufficient. In iodine-replete areas, most persons with thyroid disorders have autoimmune disease, ranging from primary atrophic hypothyroidism to Hashimoto’s thyroiditis to thyrotoxicosis caused by Graves’ disease. The prevalence of spontaneous hypothyroidism is between 1 and 2%, and it is more common in older women and ten times more common in women than in men. The prevalence of hyperthyroidism in women is between 0.5 and 2% and is ten times more common in women than in men. Epidemiological studies suggest that 1% of men and 5% of women have thyroid nodules detected clinically and that the frequency increases with age and in iodine-deficient populations. In iodine-replete areas, congenital hypothyroidism affects about one newborn in 3500–4000 births, and the value of screening for congenital hypothyroidism in heel-prick blood specimens is unquestioned. Controversy exists as to whether healthy adults from screening for thyroid disease as the prevalence of unsuspected overt thyroid disease is low, but a substantial proportion of subjects tested will have evidence of thyroid dysfunction, with approximately 10% with subclinical hypothyroidism and 1% with subclinical hyperthyroidism. No appropriately powered prospective, randomised, controlled, double-blinded interventional trial for subclinical thyroid disease exists.