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.