Veg Diets and Hypothyroidism in AHS-2

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).

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References

1. Tonstad S, Nathan E, Oda K, Fraser G. Vegan diets and hypothyroidism. Nutrients. 2013 Nov 20;5(11):4642-52. | link

10 Responses to “Veg Diets and Hypothyroidism in AHS-2”

  1. Dan Says:

    Thanks for this update,Jack!

    “Interestingly, more salt use 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).”

    I wonder if the majority of salt was from processed food. That tends not to be iodized; in fact, I think it never is. It is said that 80%+ of salt intake arrives in food at the dinner table without additional ‘salt-shaker’ contribution. None of that is iodized. Of course, it all depends on how they measured salt in their questionnaires.

  2. Jack Norris RD Says:

    Dan,

    They were assessing adding salt to foods, not sodium in processed foods. They say, “The frequency of adding salt to food was queried as once a week or less, 2 to 6 times per week, and once per day or more.” I will clarify this in the post and in the article.

  3. kate scott Says:

    This is interesting, and kind of reassuring. I see the authors provide no explanation for the link between higher salt intake and hypothyroidism. I gather that hypothyroidism is a kind of autoimmune disease. I am reminded of a study I remember seeing published in Nature earlier this year that found salt caused autoimmune problems in mice – not related to the thyroid in that instance, but still, one wonders if there will turn out to be a connection between salt and increased risk of autoimmunity.

  4. Ben Says:

    After switching to a vegan diet (3 1/2 years ago), I read your nutritional recommendation, and decided to add iodized salt on a daily basis. My TSH levels were normal (1.6 to 2.9) before I went vegan, even though I didn’t use any supplemental iodine at all. I was astonished when I saw that my TSH levels went up in the following blood exam, but as it wasn’t that high (5.0), my doctor said there’s nothing to worry about. I’ve got the same result after a year, and after two years it went up a little bit more (5.5).

    I didn’t know what to do, so I bought a supplement separately (that says, no salt anymore). But before taking the supplement, I’ve heard from another vegan that he also had a slightly elevated TSH (6.0) while using iodized salt, but that his levels came back to normal after stopping ANY intentional iodine intake (supplemental/iodized salt).

    So, I decided to give it a try, and eliminated the salt from my diet without taking the supplement I bought instead. And it worked! When I checked my TSH levels again (three months after I stopped using the salt), they came back to normal (3.0).

    I have always limited my intake to 1/2 tsp on a daily basis, so it shouldn’t be iodine overdose. So, how could it be possible? Maybe I’ve got more than enough from food alone?

  5. Jack Norris RD Says:

    Ben,

    Thanks for the information. That is very strange, but something to keep my eye on.

  6. Dan Says:

    OTOH, TSH is such a variable test, with fairly poor interassay reliability, that it is hard to know what to make of it

    It doesn’t make much sense to me, because iodized salt does not contain enough iodine to cause thyroid dysfunction. In fact, many sea-based cultures (eg Japanese) consume FAR MORE iodine than we do, and do not seem to have endemic hypothyroidism.

    I would want to rule out other dietary goitrogens before presuming it was iodized salt.

    Finally, depending on the lab and the reference range, a TSH of 5.5 could be considered normal (i.e. consistent with the range seen in 95% of the population).

  7. Ben Says:

    Dan, I’ve got the same results multiple times, not just once.
    And as my eating habits appear to be boring for most people (eating almost the same food every day), I could make the conclusion that the iodized salt was the culprit.

    I appear to eat a large amount of steamed cruciferous vegetables on a daily basis (a little bit more than 2 lbs), and it seems it has nothing to do with my TSH since it have gone down without changing anything (again, just the salt).

    I know that TSH levels of 5.5 aren’t considered “bad”, but still they can be considered as “subclinical hypothyroidism”. So I wanted them to be lower.

  8. Dan Says:

    I am not arguing with you. Is there a reason you are checking your TSH that frequently? I am just curious myself, having decided not to get any routine annual blood testing if I can avoid it…. (because of concerns about overdiagnosis, overtesting, false positives/false negatives, need for additional tests and referrals for asymptomatic non-disorder conditions that doctors just love to “treat”).

    Anyway, I do not think that the range of variability between 1.6 (the lowest TSH you had) and 5.5 (the highest TSH you had) is all that marked, but perhaps I am wrong. Did you get your thyroid hormones measured as well (T3, T4, etc)? I would be curious what a repeat follow-up TSH would show in you…

  9. Ben Says:

    Yes, I did get my T4 and T3 measured. They where always in the normal range (even though the T4 was a little low [11.6] when my TSH was at 5.5).

    I’m curious as well, and I’ll do a new blood check-up in the summer (I do it once-a-year, unless there’s a medical reason to do another one).

    There’s always a chance of over/misdiagnosis, that’s true. I guess that I take a little risk. :)

  10. Dan Says:

    Thank you, Ben.

    I still do not understand why you think a 1/2 tsp of iodized salt per day would cause a tendency towards subclinical hypothyroidism. This is far less iodine (and far less salt) than the average non-vegan would consume, and against the background of a vegan diet, it is certainly not an overdose in iodine consumption. My understanding is that too little iodine can lead to hypothyroidism, especially if there are dietary goitrogens or a hypofunctioning thyroid to begin with.

    I take 1/4 of a kelp tablet per day, but don’t measure my TSH (or do any other bloodwork anymore). I would do a TSH if I started having symptoms of hypothyroidism but most clinical practice guidelines recommend against checking TSH in healthy, asymptomatic persons — although I admit they do not take into account veganism. Yet according to Jack’s data, there may be some protection against hypothyroidism on a vegan diet ( odds ratio 0.78, 95% CI 0.59-1.03).

    If I was more worried about my diet, I’d probably get a full panel of all possible nutritional deficiencies and derangements that I could think of. Here in Canada, there is a tendency against ordering routine annual blood work at physical examinations, except for cholesterol, glucose and a few, selected parameters.

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