The incidence of goitre among school-children in New Zealand has been determined by the examinations made by one of us (Hercus) and by the officers of the School Medical Service, of over 80,000 children. The proportion of men of military age who were unfitted for active service on account of goitre was ascertained during the examination of over 135,000 recruits by the Military Medical Boards in the years 1916–1918. Additional information has also been derived from hospital records, by the number of operations for goitre.
The amount of iodine has been determined in nearly five hundred samples of soil, obtained from all parts of New Zealand, together with samples of a number of waters from mineral springs and town supplies.
In order to ascertain whether any relation exists between the amount of iodine in the soil of any district (considered as a soil of plant food, and therefore indirectly of human food), we have divided the Dominion into thirty-three districts, each homogeneous as far as possible in its geological and geographical circumstances, and have ascertained the average amount of iodine in soil and average incidence of goitre among school-children within each district, utilising certain defined methods of soil-analysis and diagnosis of goitre. It is proved that the following relation is approximately true throughout: viz, percentage incidence of goitre among school-children is equal to 6 plus the quotient of 360 divided by the amount of iodine in ten million parts of soil, so long as this does not exceed 50, assuming defined methods of medical examination and chemical analysis. When the soil is abnormally rich in iodine the constant 6 must be diminished. Further (though the records are here less complete), the relation between the average amount of iodine in the soil and the percentage of military recruits medically rejected on account of goitre is of the same general nature, the latter figure being about one-twentieth of the incidence among school-children, though this proportion must also be diminished in regions in which iodine is particularly abundant.
Approximately, therefore, the incidence of the disease is inversely proportional to the amount of iodine in the soil. Further, where, though the incidence of goitre is low, the amount of iodine in the soil is abnormally low, we have proved that it is present in the water supply in greater amounts than usual, and the daily intake of iodine among the inhabitants of such a district is thereby maintained. Further, regions in which the average amount of soiliodine is low are also those in which goitre is most frequently seen amongst the domestic animals.
Though there are certain points as yet unexplained, the whole body of facts is too large, and the relationship too consistent, to be fortuitous. We believe, therefore, that the hypothesis that goitre is caused by a deficiency of iodine in the diet has been fully sustained by the present investigation. Reasons have been deduced also for believing that a relation holds between the incidence of goitre and the distribution of geological formations (as determining the nature of the soil) in New Zealand, Switzerland, and perhaps in other countries.
Further, a study has been made of the prophylactic treatment of schoolchildren in New Zealand, by administering small amounts of iodine weekly. This has hitherto had fairly good results. We believe that the best method of prophylaxis, considered on physiological grounds, as well as those of efficiency and economy, would be daily ingestion of minute amounts of iodine, obtained by the utilisation for all culinary and table purposes of an iodised salt, in which 1 part of potassium iodide had been added to 200,000 of sodium chloride—the method which has been attended by such satisfactory results in Switzerland. This would afford the minimum amount of 0·005 mg. per day, which seems necessary for the functional requirements of the thyroid gland, while it would be far too small to have any ill-effects in the production of hyperthyroidism.
A consideration also has been given to the source of the iodine in soils and natural waters in the light of our own analyses and those of previous investigators, especially Dr Th. v. Fellenberg, whose work has been of the greatest assistance to us.