Historical Evidence of Benefits of
Iodized Salt in the United States

There are numerous reports in the literature that demonstrate effectiveness of iodized salt in controlling endemic goiter. Iodization of salt is known to be a safe, efficient, and preferred prophylactic method for endemic goiter in the U.S. (Ref. 1). In the early 1900's, goiter was prevalent in those states bordering the Great Lakes and in the northwestern region of the United States. Voluntary fortification of salt with iodine was introduced in 1924 and resulted in a virtual elimination of endemic goiter in the U.S. Some notable examples are as follows:

A. Studies in Michigan

The most extensive and systemic studies of the effects of iodized salt on the prevalence of goiter were conducted in the state of Michigan. In 1923 to 1924, the Michigan State Department of Health conducted a large scale survey of goiter in four counties selected to represent different geographic regions and soil conditions of the state. All school children up to the 8th grade in the four counties were examined for goiter. Of about 66,000 children examined, about 39% (range 26-64%) had visible enlargement of thyroid (Refs. 1-7). In the spring of 1924, a statewide campaign for goiter prevention was launched with the introduction of iodized salt containing 0.02% (later reduced to 0. 0 1 %) of sodium iodide. Three follow-up surveys were conducted in the same four counties in 1928 (only two counties surveyed), 1935, and 1951 to evaluate the program. A striking 70-75 % reduction of goiter was observed in the 1928 resurvey, merely four years after the introduction of iodized salt (Refs. 1, 3-6, 8). By 1951, goiter was practically eliminated among children in three of the four counties with the prevalence being 0.5% or less (Refs. 4, 6, 7).

Comparison of goiter prevalence by users vs. nonusers of iodized salt showed that the reduction of goiter was greatest among regular users of iodized salt, lower among irregular users, and least among nonusers. Nonusers in this study included those who used iodized salt in the past but did not use it during three years prior to the survey, which may explain the decrease in the prevalence among the nonusers (Ref. 1).

Calumet was a mining town and in 1932 the copper mines were closed, placing about two thirds of the families on relief. Only noniodized salt was provided to these families. The 1935 resurvey showed an increase in the prevalence of goiter among school children (Ref. 1).

B. Studies in Ohio

In 1925, the Ohio State Department of Health made a rather comprehensive study of the incidence of goiter throughout the state and planned to support the general use of iodized salt. Six counties were selected to represent the average conditions in Ohio, and approximately 60,000 school children were examined for goiter. The results of this survey were never published and the plan for the general use of iodized salt was abandoned because of a strong opposition to the general use of iodized salt by some leading goiter surgeons (Ref. 1). These surgeons were largely concerned with potential toxicity. However, some studies were still conducted in several cities and counties. Iodized salt came on the market in Cleveland in 1925 (Ref. 2) and Ohio families were encouraged by many health and educational agencies to begin the regular use of iodized salt (Ref. 9).

A study conducted between 1924 and 1936 showed that prior to the introduction of iodized salt, 31 % of school children had goiter. In 1936, only 7% of those who used iodized salt regularly had goiter while there was no change in the goiter prevalence among children who did not use iodized salt (Refs. 1, 5, 7).

Another study compared goiter prevalence among children in four counties in Ohio in 1925 and 1954. The prevalence decreased dramatically from 32% in 1925 to 4% in 1954 (Ref. 9).

C. Experience in West Virginia

Before 1900 endemic goiter was very rare in the Kanawha River valley in West Virginia, but soon after the turn of the century the incidence of simple goiter began to rise. A survey conducted in 1922 showed that 60% of school girls in Charleston and Huntington had enlarged thyroids. Until about 1900 all the table salt used in this valley came from salt wells in this region but around 1900 the crude dirty brown salt was replaced by pure fine white salt (noniodized) from Ohio and Michigan. There was no other change in food or water; the only change during this period was in the supply of salt. Chemical analysis showed that the crude salt contained iodine equivalent to 0.01 % potassium iodide (Refs. 8 and 10). The elimination of the natural source of iodine in the crude salt was followed by a dramatic increase in the incidence of endemic goiter. The experience in West Virginia provides indirect evidence of the benefits of iodized salt.

D. Potential adverse effects of iodized salt

When iodized salt was first introduced in the U.S. there was some concern about potential toxicity from the general use of iodized salt, notably an increase in hyperthyroidism. Studies reporting adverse effects of iodized salt showed the following:

Studies in Michigan and Ohio showed no case of hyperthyroidism among children using iodized salt regularly (Refs. 1, 3, 7, 10). Four children in Michigan who had nodular hyperplastic goiters with definite signs of toxicity had never used iodized salt or had used only noniodized salt since the depression (Ref. 1).

There was a report about an increase in toxic nodular goiter, number of total thyroidectomies, and yearly death rate during the three year period (1925-1927) after iodization of salt began in Michigan (Ref. 1 1). The increase was transient and both I number of thyroidectomies and death rate from goiter declined rapidly after 1927 despite continued use of iodized salt. By 1933, death rate from goiter was lower than the pre-iodization period. The report did not present other important information that might have played a bigger role in the increase such as whether toxic nodular goiter was present in these patients before the introduction of iodized salt, other iodine-containing compounds used or iodine therapies received by the patients, and any epidemics of disease or other conditions that might have contributed to the observed increase. It is hard to know what role iodized salt played in the reported adverse effects.

In 1927-1928 a study was conducted for the whole adult goiter population in several counties in Michigan where the majority of the population had been using iodized salt for four years. The results showed that the percentage of hyperthyroidism among adults with goiter was much smaller (4. 1 %) among users of iodized salt as compared to the nonusers of iodized salt before the onset (17-56%). Also, many adults with hyperthyroidism who used iodized salt had other conditions that seemed more important in the etiology of hyperthyroidism than the use of iodized salt such as pregnancy, severe throat infection, and severe nerve strain or shock which is often seen as a forerunner of this disease (Ref. 3).

References

1. Kimball OP. Prevention of goiter in Michigan and Ohio. JAMA 1937; 108:860-864.
2. Olin RM. Iodine deficiency and prevalence of simple goiter in Michigan. JAMA 1924;82:1328-1332.
3. Kimball OP. The efficiency and safety of the prevention of goiter. JAMA 1928;91:454-460.
4. Matovinovic J, Hayner NS, Epstein FH, Kjelsberg MO. Goiter and other thyroid diseases in Tecumseh, Michigan, studies in a total community. JAMA 1965:192(3):134-140.
5. Kimball OP. Endemic goiter -- a food deficiency disease. J Am Dietetic Assn 1949;25:112-115.
6. Altland JK, Brush BE. Goiter prevention in Michigan, results of thirty years' voluntary use of iodized salt. J Mich Med Soc 1952;51:985-989.
7. Brush BE, Altland JK. Goiter prevention with iodized salt: Results of a thirty-year study. J Clin Endocrinol Metab 1952;12:1380-1388.
8. Kimball OP. Iodized salt for the prophylaxis of endemic goiter. JAMA 1946; 130(2):80-81.
9. Hamwi GJ, Van Fossen AW, Whetstone RE, Williams 1. Endemic goiter in Ohio school children. Am J Pub Health 1955;45:1344-1348.
10. Kimball OP. Endemic goiter and public health. Am J Pub Health 1928;18:587-601.
11. McClure RD. Thyroid surgery as affected by the generalized use of iodized salt in an endemic goitre region -- preventive surgery. Ann Surg 1934;100:924-932.

Prepared by Youngmee K. Park
U.S. Food and Drug Administration
January 31, 1997

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