Epidemiology of thyroid diseases in iodine sufficiency.

Epidemiology of thyroid diseases in iodine-sufficient areas (ISA) deals with sporadic goiter, thyroid autoimmune diseases, and thyroid cancer. A comparison between the different studies performed is difficult because methods have changed over time and selection criteria and definitions such as prevalence or incidence were not used consistently by some authors. Sporadic goiter: in ISA, autoimmune processes play a major role in the development of sporadic goiter. In adults, sporadic diffuse goiter is most frequent in young women (16%), perhaps due to additional relative iodine deficiency especially in pregnancy, and declines with age (<10%). Sporadic nodular goiter increases from 5% in young women to 9% in older women. Autoimmune thyroid disease (AITD): thyroid autoantibodies (TAb) and histopathological lymphocytic infiltration of the thyroid is much more common in ISA (4.6% in women; 1.1% in men) than in iodine-deficient areas (IDA). The prevalence and incidence of hypothyroidism and hyperthyroidism varies, depending on whether overt and subclinical forms are included and whether newly or previously diagnosed dysfunction is considered. In an overview of the literature, the prevalence is 2 in 1000 for overt and 6 in 1000 for subclinical hyperthyroidism in ISA. The values for hypothyroidism are 5 in 1000 and 15 in 1000, respectively. Change from IDA to ISA: in former IDA, the percentage of hyperthyroidism increases up to 4 years after salt iodination. Whereas this effect is transient for Plummer's disease, a change from IDA to ISA seems to lead to a permanent increase in overt and subclinical Graves' disease. Thyroid cancer: most studies demonstrate that the histopathological types of thyroid cancer are different in IDA and ISA. There is a tendency toward an increase in differentiated and decrease of anaplastic cancer. The ratio of papillary to follicular thyroid cancer ranges from 6.5:1 to 3.4:1 in areas with high iodine intake, decreases 3.7:1 to 1.6:1 in areas with moderate iodine intake, and ranges from 1.7:1 to 0.19:1 in IDA.