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Iodine for Hypothyroidism: Crucial Nutrient or Harmful Toxin?


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This article is part of a special report on Thyroid Disorders. To see the other articles in this series, click here. Note: This article was originally published in July 2010 and was updated in May 2018 to include the latest research. My original discussion of the research on iodine and selenium still stands, but I have updated my recommendations for iodine testing in light of new evidence and added a section with practical steps for correcting iodine status.

iodine and hypothyroidism
Striking a balance with regard to iodine intake is vital for optimal health due to the connection between iodine and hypothyroidism. istockphoto.com/Dmitry_Tsvetkov

In a previous article I showed why, when used alone, thyroid hormone replacement often fails. In this post I’ll explain why optimizing your iodine intake is so crucial and why both too little and too much iodine can be harmful.

Iodine and Hypothyroidism

Iodine deficiency is the most common cause of hypothyroidism worldwide. Once researchers realized this, health authorities around the world began adding iodine to table salt.

This strategy was effective in correcting iodine deficiency. But it had an unanticipated—and undesired—effect. In countries where iodine has been added to table salt, the rates of autoimmune thyroid disease have risen. The following is just a sample of studies around the world demonstrating this effect:

Why does this happen? Because increased iodine intake, especially in supplement form, can increase the autoimmune attack on the thyroid. Iodine reduces the activity of an enzyme called thyroid peroxidase (TPO). TPO is required for proper thyroid hormone production.

 Iodine deficiency is the most common cause of hypothyroidism worldwide. Optimizing your iodine intake is crucial, and both too little and too much iodine can be harmful.

On the other hand, restricting intake of iodine can reverse hypothyroidism. In one study, 78 percent of patients with Hashimoto’s regained normal thyroid function with iodine restriction alone.

Selenium Deficiency and Hashimoto’s

However—and this is a big “however”—it appears that iodine may only pose a problem for people with Hashimoto’s and other autoimmune thyroid diseases in the presence of concurrent selenium deficiency. One study in rats found that those given excess iodine only developed goiter if they weren’t consuming adequate selenium.

Other studies have shown that selenium protects against the effects of iodine toxicity and prevents the triggering and flaring of autoimmune disease that excess iodine without selenium can cause.

In my practice I always test for both iodine deficiency and Hashimoto’s when a patient presents with hypothyroid symptoms. If they are iodine deficient, I will start them on a trial of iodine and selenium together. In most cases, patients see a significant improvement. In a minority of cases, they cannot tolerate supplemental iodine even with adequate selenium intake.

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Testing for Iodine Status

In most population studies, iodine is tested using spot urine, which is convenient and easy and correlates fairly well with recent iodine intake. However, studies have shown that spot urine and even 24-hour urine collections have high variability due to significant day-to-day variations in iodine intake.

Some clinicians have advocated iodine challenge urine testing, where a patient takes a large dose of iodine, often 50 mg, and collects urine for 24 hours afterward. This is based on research showing that 90 percent of ingested iodine should be excreted in the urine when the patient has sufficient iodine intake in the diet (1). However, this testing has not been validated to my knowledge and has been heavily criticized by several iodine researchers (2).

In my clinic, I use a combination of three tests:

  • 24-hour urine iodine: to assess recent iodine intake
  • Serum thyroglobulin: high thyroglobulin indicates low iodine status, and levels above 40 mcg/L are suggestive of deficiency (4)
  • Hair iodine: to assess long-term iodine intake (3)

Correcting Iodine Deficiency

If iodine deficiency is suspected and there is no evidence of Hashimoto’s, supplementation with doses of 200 to 300 micrograms of iodine are safe and well tolerated. However, I would still recommend monitoring thyroid antibodies and other thyroid markers closely. Higher doses of iodine supplementation should only be used under medical supervision.

If iodine deficiency is suspected and you have Hashimoto’s, you can still try supplementing, but I would start with a very low dose, perhaps 100 micrograms of iodine, or simply increase intake of foods that contain iodine, such as sea vegetables, fish heads, or dairy.

You’ll also want to make sure that you are getting adequate dietary selenium to protect against any negative effects of excess iodine. The richest source of selenium is Brazil nuts, but it is also found in high quantities in seafood and organ meats.

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  1. I was always told that consuming iodine in food form (seaweed–Dulce flakes, kelp, others, added to vegetables or bitter greens) is preferable over supplemental form for everyone, whether hypo or Hashi, and that selenium supplement should be started the minute someone is aware there are “issues” with their thyroid.

  2. Chris,
    Thank you so much for having been addressing the thyroid issues for so long.
    How about the hyperthyroidism and (active) nodules? I remember you promissed quite some time ago you would address this for the unfortunate minority affected by it.

    Thanks again!

  3. Must admit I puzzle over the very different views on iodine, but I always come back to the Japanese diet containing much higher levels.

  4. Thanks for your thoughts. However….

    “even with adequate selenium intake.”

    But you give no indication of how you assess this “adequacy” of intake. I am very sceptical here. A significant proportion of people (and ESPECIALLY those with fatigue etc problems) have substantial mercury toxicity burdens (see Chapter 3 at http://www.pseudoexpertise.com), and the main effect of mercury is to completely zap the selenium – it is effectively “anti-selenium”.

    My own personal experience reflects this. In recovering from many years of very high exposure to dental mercury, I found that by far the most symptom-reducing factor was intake of selenium (as semet yeast) – with a latency of a couple of days. To get that symptom reduction I had to take far more than what others would deem “safe” let alone “adequate”. I mean AT LEAST 1000 mcg and up to 3000 mcg per day. I took that level for some years and didn’t die of se poisoning as a result, but I did end 48 years of mercury disability.

    That TPO enzyme has a challenging task, needing careful control to prevent it causing too much damaging oxidation. Se is vitally needed there to prevent an autoimmune reaction to that damage.

    I think is it highly likely that high I2 doses as found useful by Brownstein, Abrahams, et al, are indeed harmlessly therapeutic if Se is indeed adequate, that being adequate in the sense of not just the “proper” 200-400mcg, but instead as much as is needed to counter any mercury intake or existing burden.

    Those studies listed at the top of this article all date from ten or more years ago and don’t mention Se. The one study that does mention Se, as you say – supports the crucial role of Se in preventing the I toxicity! Would be good if more such studies were done or maybe already are.

    • I am curious about iodine’s detox benefits as well. I have especially heard a lot about its unique ability to detox fluoride and bromide, stuff we all have too much of in our bodies these days. But the general thinking (from what I’ve read) is that iodine intake needs to be ramped up to fairly high doses to really detox thoroughly.