This article is now considered “somewhat obsolete”. Basically, this article studied only the seaweeds that are commercially available, and did not explore the realm of locally (NZ) foraged seaweeds. I have since turned to carefully-selected, locally-foraged seaweeds, which seem to allow safe iodine intake: Iodine in Common Edible Wild NZ Seaweeds – An Alternative to Iodized Salt!
Are you vegan? or on a low-salt diet? or heading there? Then this will be relevant to you. The question of satisfying iodine needs through seaweed is not new as you will read in the history but below. I ask here a specific and very practical question:
Based on published science, can we or not safely rely entirely on seaweeds for iodine intake? If so, which ones? why so? and what to do in practice to implement that?
That is usually my approach, very practical, and I do heavily rely on the work of people that are very focused on non-practical aspects. To figure out the answer, we will have to define what is safe? what is reliable? Since the history of iodine deficiency is frankly quite fascinating, I will start with that, and then get more specifically into the seaweeds talk.
Part One – A quick (recent and epic) history of iodine deficiency
Already in 1813, following the discovery that seaweeds have iodine, a Swiss physician postulated that seaweeds consumption could reverse goiter just like another sea product which the Greeks used for goiter: marine sponges in topical use. 
From 1813, it took no less than a century, for iodine to start reaching our diets, through salt iodization. And there are still problems. The Salt Institute declares “In 1990, only about 20% of the world’s households had access to iodized salt and were protected against Iodine Deficiency Disorders. After a major push, access now exceeds 70%”. It will always fascinate me how slowly are accepted and spread undeniably effective solutions to major medical problems.
21st century – The ironic re-emergence of iodine deficiencies, “for health reasons”
While many countries are still working on making iodized salt the norm, other countries such as New Zealand (and probably others) are already experiencing the next iodine problem, a re-emergence of iodine deficiency . Why? Several reasons, mainly health-driven:
1. People cutting on (iodized) salt to reduce cardiovascular “accidents” 
As salt awareness is growing, people do try to cut on visible salt. They do when using the shaker and in the kitchen. If that salt was iodized, cutting on salt also means cutting on iodine.
Automatically then, your only source of iodine becomes uncontrolled and random, with the execption of New Zealand* which I will develop later. When there is no more iodized salt in your diet, you rely almost entirely on the salt chosen by restaurants and food makers you eat, from the sea products you eat, on the (usually low) iodine content of the soil that grows your food, and even more rarely on the voluntary use of iodized salt in the supermarket food, the last industry I’d expect to care unless forced by law* or consumer trends.
*In New Zealand, bread manufacturers have been forced by law to iodize the salt in bread, since people still eat a lot of bread. Think of bread as a last refuge for salt. Once you got rid of table and cooking salt, the next thing to get rid of is either bread or the salt in it. Inevitably, when that happens, iodine deficiency will become a problem again. Salt being something unnecessary and unhealthy, should not be the carrier for iodine, or anything else that is important for health.
2. People switching to rock salts (i.e. Himalayan salts)
A pink rock salt recently gone quite mainstream, Himalayan salts mined and therefore usually very low in iodine by default. Although any amount of salt is not necessary and therefore contributing to a more unlhealthy status, Himalayan salt has gained popularity as a “healthier salt” due to claims of it containing a wide spectrum of trace minerals. Besides safety issues with the uncontrolled variation of the trace minerals, and a possible content of heavy metals, Hymalayan salts in the “natural” state do not contain a relevant amount of iodine and pose an (ironic) issue of iodine deficiency. In passing, the concept of “healthy salt” reminds me of the $30 million of US taxpayers money spent in 1967 by the National Cancer Institute to create a “safer cigarette”.
3. A rise of vegetarian and vegan diets
In studies done on iodine deficiency, vegans and vegetarians are mentioned as being at particular risk of iodine deficiency. The reason is that most seafood in the Western diet is animal-based, and that most animal produce (meat, milk, cheese) comes from animals that were given a feed artificially supplemented, either for the animal’s own health, as a means of supplementing the final product, or accidentally like in the case of milk-based iodine which comes (or used to come) from iodine containing disinfectants like Betadine applied on cow tits.  Other disinfectants have gradually replaced the iodine-based ones, resulting in lower iodine in cow’s milk.
4. The still-existing lack of a holistic agricultural practice
Some posters on the internet, widely shared, claim certain plant foods as reliable sources of iodine. It is not true because it depends heavily on the iodine content of the soil where the plant came from. A potato may be able to accumulate iodine, but since most soils are iodine-deficient, how much iodine should we expect in potatoes? Deficient soils are still far from being thoroughly and routinely balanced with life-sustaining elements, iodine being one of many. In fact, there is more thought being put in supplementing cows and their feed than in conditioning soils for the healthiest feed of humans: plants, fungi, and other non-animal foods. Most fertilization is still entirely focused at productivity and profitability, what is not? Yet we still buy produce based on weight, price, and appearance, not enough based on nutritional content or taste, let alone environmental and social factors. There is however a growing culture of a holistic approach, aiming at going back to basics: taking pride in actually feeding people . Iodine deficiency could likely be resolved by means of fertilization and cultivation methods, among other deficiencies.
Image credits: Alena Kumpta Watercolor Art
Part Two – The search for the perfect seaweed
Criteria to match in order for a reliable, safe, and practical intake of iodine from seaweed only:
- Reliably narrow range of iodine concentration, so that we know accurately enough how much iodine is contained in let’s say a spoon or 10g.
- The range starts away from zero iodine (so that there is no risk of getting close to none)
- Practical concentration: the daily requirement ideally fits in more than a pinch, but less than a lot of tablespoons, so ideally something like a gram or a teaspoon, not one kilogram of seaweed because that would not be realistic nor practical.
- The seaweed has long history of usage and is generally safe
- The seaweed is easy to source
- High absorbability by human digestion
- Low heavy metal content
- The range of suitable iodine for human consumption is based on official recommendations : ~160 µg/day (upper limit UL or maximum: 1,100 µg/day)
- That official recommendation are correct is subject to debate, and here too. Official recommendations have changed a lot historically 
- OBJECTIVE: So rounding up, we will consider safe the range 150 ~ 1,000 µg/day, remember this.
Seaweeds – Variation in content of iodine
Given the large number of seaweeds in existence, the focus was intentionally restricted to a popular few:
- Wakame – Undaria pinnatifida (the fronds are called wakame, the base part of the same plant is called mekabu)
- Kombu – Laminaria digita japonica, also known as kelp
- Nori – which is tricky because it’s a whole genus (Porphyra genus) contaning many species including Laver (Porphyra umbilicalis) or Karengo (Porphyra columbina) and many others. Many seaweeds fall under nori.
These are the ranges in parts per million (ppm) or equivalent units : mg/kg or µg/g (same as mcg/g or micrgrams per gram), all units reflecting the content as packed, usually dry [6,7] For iodine in seaweeds, the microgram per gram unit because the dietary recommendations for iodine are given in micrograms, and one gram of dry seaweed is human sized; if dried it would fit in a hand or a spoon.
- Wakame: 39 ~ 1,571 µg/g
- Kombu: 25 ~ 12,000 µg/g with one measured at 21,000  by EU food safety authorities
- Nori: 0,7 ~ 550 µg/g
How absorbable is iodine from seaweed?
While iodine is an atom, its presence in food can exist in different forms, different molecules or ions referred to as “chemical species”. It’s important to know the chemical species of in foods because that affects bio-availability (how much we can absorb and use) or things like toxicity (less of a concern here).
A study published in 2005 compared kombu to wakame concluded that kombu’s higher content of iodide made it a more bio-available choice than wakame’s various iodine species (monoiodotyrosine and diiodotyrosine) 
Heavy Metals Warning
Seaweeds are notorious for accumulating heavy metals. Interesting work was done by the Health Ranger, a food activist, on a very narrow but US-popular range of seaweeds. One brand of seaweeds harvested in New Zealand showed the cleanest profile in terms of heavy metals. Be careful though when chosing “New Zealand” as a reliable criteria for clean seaweed. The mention “from New Zealand”. All inhabited land in our day dumps pollutants in the sea, New Zealand is no exception, far from the common eco-fantasy. That is why, the cleanest New Zealand seaweeds are most likely harvested in open sea far from the land, ideally South towards Antarctica, in regions far from volcanic/human activity. In my survey ofseaweeds from shops, also saw “NZ-nori”, NZ-seaweed” and other “NZ” prefixed labels in large print. That could just be marketing to write NZ somewhere capitalizing on the good perceived image of New Zealand seaweed. It could also mean the product sold is the NZ species of a certain seaweed, but that by no means guarantees it was grown or harvested in New Zealand, or in clean waters.
Best seaweed verdict
Kombu: too much iodine, extremely wide range, and starts close to zero. It’s out.
Nori: wide range, start close to zero. It’s out.
Wakame: relatively narrow range (good) but starts close to zero. It’s out.
Among these three popular seaweeds I cannot see any candidate for a best seaweed taken daily on its own as the one and only source of dietary iodine. The fantasy of getting all iodine in precise amounts from seaweed alone is over for me. So what to do now?
Well, there is good news. There are other more viable avenues.
The solution: Get your iodine from more reliable sources than seaweeds
The natural way, and its limitations
- Do not rely only on daily seaweeds unless it passes the above-discussed criteria/checklist
- If relying partly on seaweeds, only consume a safe amount based on the method here: trusting observed ranges more than nutritional facts (generally one-shot, or copied off the net rather than measured) and always calculate to avoid the worst case scenario of toxic doses
- If you forage, that’s wonderful. Do it intelligently though:
- Select a clean foraging spot: Learn about the history of activity and pollution of the area you forage in.
- Identify: Always identify the species you collect.
- Quantify: Once you know the name of what you forage, look into the research to quantify the amount of iodine in different parts of plant.
- Research iodine ranges of other seaweed species (dulse, arame, …) <== and of course write me an email to show me what you found 😉
Relying on iodized salts/products
- People with a very low-salt diet, and no other iodine source may choose a highly-iodized salt, that provides enough iodine in a very small daily intake of salt. That means a higher concentration than in normal salt. Note that iodized salts in general have by law wide ranges of allowed iodine (45±20 µg/g for NZ/Aus.). I read some research showing that the legal (already wide) ranges are not generally not respected and that an even wider range of iodine is actualy found in iodized salt products. This makes iodized salt an unreliable source of iodine if you are aiming at a rather reliable range of iodine intake.
Or even better, iodine supplements
If you consume a very low-salt diet, get a good iodine supplement, preferably pharmaceutical-grade.
Best of the best: Pharmaceutical-grade prescription iodine
Did you know the iodine in your iodized last is extracted from seaweeds? So the same concerns you would have for seaweeds (heavy metals, sea-borne contaminants etc…) are legitmate to have when it comes to iodized supplements. Supplements also being very poorly regulated worldwide, my current advice is to always try to get the pharmaceutical grade, prescription-only iodine. The dosage will be very precise, contaminants likely checked for, and manufacturing process perhaps not perfect but far more regulated and accountable than free-market supplements.
Dealing with probable relunctance from your G.P.: If your diet has such incredibly low salt that you are realistically at risk of a predictable iodine deficiency, there is no reason why a G.P. would not take you seriously when you request an iodine prescription. You may just need to be ready that they would suggest seafoods and free-market supplement and you may need to be ready to tell them kindly why that is not an option for you. I would change GPs (and I have been) if he/she does not generally share similar views on nutrition and prevention.
I strongly believe in prevention over therapy, it also costs less in many ways. But unfortunately the practice of things, still reserves a lot of supplements and testing to pathologies rather than preventive actions. You may experience that worldview from your doctor when you ask him/her for supplements or preventive/routine testing in a non-therapeutical context.
If you still want “free-market” supplements
If this is not available to you, or you still prefer free-market supplements, get a clean iodine supplement (i.e. low on heavy metals). I looked at this link, the only (apparently) genuinely independent lab that looked at the heavy metals. Strangely it did not look at iodine content itself! At the moment I write this, I roughly trust this source but have not examined it thoroughly enough. If you have researched the trustworthiness of the Health Ranger, (whom as supplement vendor might or not have a conflict of interest) please comment below.
Selenium variation in Brazil nuts – Can we get all our selenium from Brazil nuts?
 Research on Iodine Deficiency and Goiter in the 19th and Early 20th Centuries
 IODIZED SALT by The Salt Institute – July 13, 2013
 Iodine, New Zealand Ministry of Health
 Are vegetarians an ‘at risk group’ for iodine deficiency? British Journal of Nutrition (1999), 81, 3–4
 Based on the works of Claude & Lydia Bourguignon.
 Analysis of iodine content in seaweed by GC-ECD and estimation of iodine intake
 SURVEY OF IODINE LEVELS IN SEAWEED AND SEAWEED CONTAINING PRODUCTS IN AUSTRALIA
 Notification details – 2005.050 RASFF Portal
 Iodine speciation studies in commercially available seaweed by coupling different chromatographic techniques with UV and ICP-MS detection