From the October 1962 issues of Science magazine:

It is the purpose of this paper to present data acquired from observations on children, which define this minimal effective dosage of iodide require to suppress the avidity of the normal thyroid for radioactive iodine, and the time required to achieve such suppression.

The studies were carried out between 14 December 1961 and 14 April 1962 on children at the Wrentham (Massachusetts) State School, which is a hospital for longterm care of mentally defective children. We chose this population of children because it was desirable to secure children living under constant condition of environment, diet and iodide uptake. Their salt supply was found to be free of iodine...

Toxic effects of iodide from doses of this order of magnitude given over relatively short periods of time are extremely unlikely. We know from clinical experience that toxic effects of iodide are not observed with doses of 100 mg of iodide per day given to children over a course of years. Iodide goiter has been observed to occur only following daily does of several hundred milligrams of iodide administered for years, a fact which indicates that it is desirable to have a ceiling on the doses used for prophylactic purposes.