Decrease of incidence of toxic nodular goitre in a region of Switzerland after full correction of mild iodine deficiency.

In 1980 the iodide content of salt was increased in Switzerland from 7.5 to 15 mg/kg. This raised the mean urinary iodine excretion from 90 (indicating mild iodine deficiency) to 150 micrograms/g creatinine. We examined whether this public health measure was followed by a change of the incidence of hyperthyroidism in a defined catchment area of 109,000 persons. Except for a 27% rise in the first year of the new salt, the total incidence of hyperthyroidism declined steadily to reach 44% of the control level in 1988/89. This was due to a decrease mostly of toxic nodular goitre (minus 73%), less so of Graves' disease (minus 33%). We conclude that correction of mild iodine deficiency has beneficial effects on the incidence of hyperthyroidism, contrary to what is seen initially after correction of severe deficiency.