The goiter prevalence in 8 , 9 and 10 year-old children decreased respectively from 33.70 % ( 31/92 ) , 23.32 % ( 45/193 ) and 20.96 % ( 35/167 ) to 6.10 % ( 10/164 ) , 5.52 % ( 9/163 ) and 6.06 % ( 8/132 ) .