I have just added data on hypothyroidism and vegetarian diets from Adventist Health Study-2 to Disease Rates of Vegetarians and Vegans. I have reprinted it below except for the table showing the rates for each diet group which you can see by clicking on the link to the article above.
In 2013, rates of hypothyroidism in AHS2 were published (1). Rates were reported both cross-sectionally at baseline and prospectively after about 4-6 years of follow-up (exact number not specified).
For the cross-sectional arm, lacto-ovo vegetarians were more likely to have been treated for hypothyroidism in the previous year, although the statistical significance was not strong (1.09, 1.01-1.18). There were no other statistically significant findings, although vegans had a trend towards less hypothyroidism in the prospective arm (0.78, 0.59-1.03).
According to the authors, “While vegan diets are associated with lower body weight, which may protect against hypothyroidism, the lower risk among vegans existed even after controlling for [body mass index] and potential demographic confounders.” It would have been interesting to see what the results were without adjusting for body mass index, but that information wasn’t reported.
Interestingly, adding salt to foods more often was associated with increased hypothyroidism among the entire population. Whether the salt was iodized was not determined (it likely was since most table salt in the U.S. is iodized).
1. Tonstad S, Nathan E, Oda K, Fraser G. Vegan diets and hypothyroidism. Nutrients. 2013 Nov 20;5(11):4642-52. | link