Influence of iodine deficiency and iodine prophylaxis on thyroid cancer histotypes and incidence in endemic goiter area.

The aim of the study was to evaluate the correlation between thyroid cancer histotype and incidence rate (IR) and iodine nutrition level in two endemic goiter areas: the districts of Krakow and Nowy Sacz. The suspension of iodine prophylaxis in Poland in 1980 resulted in increased goiter prevalence in schoolchildren and adults and elevated TSH levels in newborns in the early 1990s. Since 1992 a rise in thyroid cancer IR was observed. Thyroid cancer IR in the Krakow population was 2.22 in 1986; 3.62 in 1995 and 6.02 in 2001; in Nowy Sacz: 1.52; 2.59 and 3.88 respectively. In 1986 papillary/follicular cancer ratio in both areas was about 1.0--the value typical of iodine deficient areas. After restoring the obligatory iodine prophylaxis in 1997, a significant decrease in elevated TSH concentration in newborns and urinary iodine concentration increase in schoolchildren were observed. A relative rise in the incidence of papillary thyroid cancer and decrease in follicular cancer, resulting in rise in papillary/follicular thyroid cancer ratio up to 5.9 in 2001 was also observed. Since 1999 no further thyroid cancer IR increase was noted. In conclusion, a significant increase in differentiated thyroid cancer IR was observed in association with the iodine prophylaxis suspension. Changes in thyroid cancer histotypes in 1986-2001 and a significant decrease in incremental rate of differentiated thyroid cancer probably reflect the influence of effective iodine prophylaxis. The significant difference between IR of thyroid cancer incidence in the districts of Krakow and Nowy Sacz may be related to differences in the exposure to radiation after the Chernobyl accident.