Iodine deficiency shows up as the symptoms of an underactive thyroid: fatigue, cold intolerance, weight gain, and in advanced cases a visible swelling at the base of the neck. Iodine has one job in the body. It builds thyroid hormone. Run short and hormone production falls, and everything thyroid hormone controls slows with it.
That is the textbook version. The complication is that a mild shortfall usually produces no symptoms at all, because the thyroid compensates by working harder. Which makes this harder to spot than most deficiency checklists admit.
The short version
- Iodine has one job: building the thyroid hormones T4 and T3.
- Adults need 150 micrograms a day. The safe ceiling is 1,100.
- Mild shortfall often causes no symptoms because the thyroid compensates.
- The symptoms are non-specific, so they point at a blood test, not a diagnosis.
- In the UK, milk is the main dietary source. Plant milks carry almost none.
What iodine actually does
Iodine is a raw material. The body cannot assemble thyroid hormone without it. Iodine makes up 65 percent of the weight of thyroxine (T4) and 59 percent of triiodothyronine (T3), according to the Institute of Medicine dietary reference intake review. Those two hormones set your metabolic rate, your core temperature, and the pace at which most tissue in your body does its work.
The severity gradient matters more than any symptom list. A 2015 review in Lancet Diabetes and Endocrinology found that severe iodine deficiency causes goitre and hypothyroidism, while mild to moderate deficiency is usually compensated, so most people stay euthyroid. The compensation is not free. Chronic thyroid stimulation raises the population rate of nodular goitre over time.
In English: badly short and you feel it, slightly short and you probably do not, but the gland is working overtime to keep it that way.
1. Fatigue that sleep does not fix
The 2024 Lancet seminar on hypothyroidism lists fatigue, lethargy, weight gain and cold intolerance as the classic presentation, and says plainly in the same sentence that these symptoms are non-specific and the diagnosis is made on blood tests.
Worth holding onto for the whole of this list. Tiredness is the single least specific symptom in medicine.
2. Feeling cold when nobody else is
Thyroid hormone sets your resting metabolic rate, and heat is what a metabolic rate produces. Cold intolerance is one of the four classic signs in that same Lancet review.
What matters is a change against your own baseline. A lifelong tendency to feel the cold tells you very little on its own.
3. Weight creeping up with no change in diet
Lower thyroid output means fewer calories burned at rest for the same body doing the same things. Weight gain sits in the classic four.
The size of the effect is usually modest, and modest weight gain has about forty more likely explanations than a thyroid problem. Treat it as a reason to check.
4. Dry skin and thinning hair
Skin and hair follicles turn over quickly, so they show a slowed metabolic rate earlier than slower tissue does. Both appear in the wider clinical picture described in the Lancet seminar.
They are also what happens when you are short on several other nutrients entirely, which is exactly the problem with reading symptoms backwards.
5. Slowed thinking
The strongest evidence that low iodine affects the brain comes from pregnancy studies. A 2013 ALSPAC analysis in the Lancet found children of UK women with a first-trimester iodine-to-creatinine ratio below 150 micrograms per gram were more likely to score in the lowest quartile for verbal IQ at age 8 (odds ratio 1.58) and for reading accuracy at age 9 (1.69).
That is a real finding about a developing brain, and it is the reason iodine is treated as a public health issue at all. It is not evidence that a mildly low adult gets measurably slower. Stated honestly because the distinction gets flattened constantly.
6. A swelling at the base of the neck
Goitre is the one sign on this list that actually points at iodine specifically. When iodine runs short, the thyroid enlarges to capture more of what is available. The 1998 NHANES analysis opens by noting that iodine deficiency in a population increases the prevalence of goitre.
It sits low and central at the front of the neck, and it moves when you swallow. If you can see or feel one, book a doctor. No supplement decision belongs at that point.
7. Your diet changed and you did not notice
Here is the one almost nobody checks. In the UK, dairy is the main iodine source, contributing roughly 30 to 40 percent of daily intake, according to a 2020 survey of 98 herds published in the Journal of Trace Elements in Medicine and Biology. That survey found milk iodine ranging from below the limit of detection to 1,558 micrograms per litre, with a summer median of 197 and a winter median of 297.
Then people swap it for oat milk. A 2017 British Journal of Nutrition analysis of 47 milk-alternative drinks found the unfortified ones had a median iodine concentration of 7.3 micrograms per kilogram, about 1.7 percent of the winter cows' milk they measured. Three of the 47 were fortified with iodine.
| Drink (2017 UK analysis) | Median iodine |
|---|---|
| Conventional cows' milk, winter | 438 mcg/kg |
| Organic cows' milk, winter | 324 mcg/kg |
| Fortified oat drink | 287 mcg/kg |
| Fortified soya drink | 280 mcg/kg |
| Unfortified plant drinks (44 products) | 7.3 mcg/kg |
The salt does not save you either. The UK has no programme of food or salt iodisation, which the 2011 Lancet survey of UK schoolgirls states directly before reporting a median urinary iodine of 80.1 micrograms per litre, with 51 percent of participants in the mild deficiency range and 16 percent moderate. So if you are in the UK, the sea salt versus rock salt question is irrelevant to your iodine intake. Whatever else your salt is doing for your electrolytes, it is not doing this. Check the label. If it does not say iodised, it is not.
A 2023 meta-analysis of 4,421 adults found vegan diets had the lowest iodine intake at 17.3 micrograms a day, and that none of the diet groups studied had a median urinary iodine inside the optimal 100 to 200 range.
How much you need, and the ceiling
The RDA for adults 19 and over is 150 micrograms a day, and the tolerable upper intake level is 1,100 micrograms, both set in the Institute of Medicine review. That is a narrow band by micronutrient standards, and the top of it is a real limit rather than a formality. A 2019 review in Annals of the New York Academy of Sciences found excess iodine can precipitate hyperthyroidism, hypothyroidism, goitre or thyroid autoimmunity, with people who have existing thyroid conditions more susceptible.
Which is the argument against fixing this with kelp tablets. A 2004 Thyroid paper measuring 12 seaweed species found iodine ranging from 16 micrograms per gram in nori to over 8,165 micrograms per gram in processed kelp granules, and concluded some seaweed dishes may exceed the 1,100 microgram upper limit. A 500-fold spread between products on the same shelf.
Worth noting for the Americans reading: the US does have iodine in the food supply, and NHANES 2001 to 2002 put the national median urinary iodine at 167.8 micrograms per litre, which is comfortably adequate. The US drop was in the decades before that, from 320 down to 145 micrograms per litre between the early 1970s and the early 1990s. It stabilised. The UK never had the programme in the first place.
The half of this that is not iodine
Iodine builds T4. Converting T4 into the active hormone T3 is done by iodothyronine deiodinases, and those enzymes are selenoproteins. Selenium sits inside the structure of the enzyme that runs the conversion. If you have read our piece on what selenium actually does, you already know where this goes.
Fireblood contains 75 micrograms of iodine from sea kelp and 25 micrograms of selenium as L-selenomethionine. The iodine figure is half the adult RDA, which leaves the rest to your food. That is deliberate on a nutrient where the gap between the requirement and the ceiling is this tight, and where the kelp on the shelf next to it varies 500-fold.
The thing to take from this
Six of the seven signs above are things a bad week produces on its own. The seventh is a question about your fridge. If you want to know where you actually stand, the honest order is: look at what you have replaced in your diet over the last few years, then ask for a thyroid function test, then think about a supplement. Most deficiency articles run that list backwards.
Fireblood contains 75mcg of iodine from sea kelp and 25mcg of selenium alongside 37 other nutrients, at doses printed on the label rather than hidden in a blend. It will not fix a diagnosed thyroid condition and it is not meant to. It covers the everyday gaps in a diet that is otherwise doing the work. The full formula is on the product page if you want to check it against what you are already taking. Or against a bag of kelp granules.
