Excess iodine intake: sources, assessment, and effects on thyroid function

[helvella]

A new paper on iodine and thyroid.

Ann N Y Acad Sci. 2019 Mar 20. doi: 10.1111/nyas.14041. [Epub ahead of print]

Excess iodine intake: sources, assessment, and effects on thyroid function.

Farebrother J1,2, Zimmermann MB1,3, Andersson M1,3,4.

Author information

1 Human Nutrition Laboratory, Institute of Food, Nutrition, and Health, ETH Zurich, Zurich, Switzerland.

2 Department of Women and Children's Health, King's College London, London, UK.

3 Iodine Global Network, Ottawa, Ontario, Canada.

4 Division of Gastroenterology and Nutrition, University Children's Hospital Zurich, Zurich, Switzerland.

Abstract

Iodine is essential for thyroid hormone synthesis. High iodine intakes are well tolerated by most healthy individuals, but in some people, excess iodine intakes may precipitate hyperthyroidism, hypothyroidism, goiter, and/or thyroid autoimmunity. Individuals with preexisting thyroid disease or those previously exposed to iodine deficiency may be more susceptible to thyroid disorders due to an increase in iodine intake, in some cases at intakes only slightly above physiological needs. Thyroid dysfunction due to excess iodine intake is usually mild and transient, but iodine-induced hyperthyroidism can be life-threatening in some individuals. At the population level, excess iodine intakes may arise from consumption of overiodized salt, drinking water, animal milk rich in iodine, certain seaweeds, iodine-containing dietary supplements, and from a combination of these sources. The median urinary iodine concentration (UIC) of a population reflects the total iodine intake from all sources and can accurately identify populations with excessive iodine intakes. Our review describes the association between excess iodine intake and thyroid function. We outline potential sources of excess iodine intake and the physiological responses and consequences of excess iodine intakes. We provide guidance on choice of biomarkers to assess iodine intake, with an emphasis on the UIC and thyroglobulin.

© 2019 New York Academy of Sciences.

KEYWORDS:

iodine; iodine excess; iodized salt; thyroglobulin; thyroid autoimmunity

PMID: 30891786

DOI: 10.1111/nyas.14041

ncbi.nlm.nih.gov/pubmed/308...

[helvella Administrator Thyroid UK]

Adding a reply so that this post no longer shows as having no replies.

[BirgitteG]

It's amazing that some researchers in that area apparently still haven't realized what a lot of other researchers have - that iodine is only dangerous to people that don't get enough selenium.

[helvella Administrator Thyroid UK in reply to BirgitteG]

That could encourage people who are taking selenium, or know they have what are usually regarded as adequate selenium levels, to take large amounts of iodine.

I am not at all convinced that the ONLY factor that determines the possibility of bad effects from large doses of iodine is the person's selenium level.

Have you the evidence that every case of iodine-induced hyperthyroidism has been in someone with inadequate selenium?

[BirgitteG in reply to helvella]

I don't know if such evidence exists. But there's a lot of evidence that you can take huge amounts of iodine without becoming hypothoid if you have adequate selenium levels. I really don't understand why researchers in the field can ignore that and stick to results of research that have ignored the selenium connection.

I'll try to find something about the iodine/hyperthyroid connection.

[helvella Administrator Thyroid UK in reply to BirgitteG]

If the evidence doesn't exist, or even if you just don't have the evidence, people could read what you posted, and take large amounts of iodine.

If we found that adequate selenium were NOT the only factor, then anyone doing so could end up dangerously ill.

This sort of claim really, really needs evidence that no-one with adequate selenium has ever suffered from large amounts of iodine. I doubt that is true, so I doubt that such evidence can be found. Of course, my doubts are not evidence, they are just caution.

There are few substances which can be ingested in massive multiples of daily requirement with impunity. Most nutrients cause issues in relatively small multiples of daily requirement. Ironically, selenium is one where the clearly toxic dose is usually considered to be around ten times typical daily requirement.

One nutrient which can be swallowed with little worry is B12. But a significant factor with that is most of us would not absorb more than a tiny proportion of the amount swallowed. Availability of Intrinsic Factor often appearing to be the limitation on our absorption.

[BirgitteG in reply to helvella]

I'm not quite sure what you're aiming at here - it might be a problem with my English.

[helvella Administrator Thyroid UK in reply to BirgitteG]

Imagine that in addition to the selenium, you also need sufficient of something else. (Let's call this other factor "unobtainium" - a play on words for something that you cannot obtain. A joke name.).

You measure your selenium and think you will have no problem. But you are actually low in unobtainium. When you take high doses of iodine you suffer - because you have not got enough unobtainium.

Nobody mentioned unobtainium. There might not even be a regular test for unobtainium. But following the idea that all you need is sufficient selenium, you have ended up making yourself very ill.

That is what worries me about your statement.

[BirgitteG in reply to helvella]

Life is dangerous, isn't it?

A lot of research has shown that if you get adequate amounts of selenium you are not harmed by iodine. That wouldn't be the conclusion if something else was needed, would it?