Iodine
The essential trace element that forms the backbone of thyroid hormone — T4 and T3 are literally named for how many iodine atoms they carry.
Both deficiency and excess cause thyroid dysfunction, and the window is narrower than people assume. Deficiency is re-emerging with reduced iodized salt use and plant-based diets.
The guidance says this outright: estimates based on the methodology of casual spot urine sample collection cannot be used to classify individual status. Day-to-day iodine excretion varies so much that one spot sample tells you about a population and almost nothing about a person. Serum or plasma iodine has no established optimum either. If you want to know whether your thyroid has enough iodine, TSH, free T4 and thyroid antibodies answer that question better than any iodine assay does — and the reason to care about over-supplementation is that excess iodine precipitates both hypothyroidism and thyrotoxicosis in susceptible people.
Check a Iodine result against this range →
What Iodine actually measures — the analyte, and the assay
Two different tests are sold under this name and they answer different questions. Urinary iodine concentration (UIC) is the one the world actually uses, because the great majority of ingested iodine is excreted in urine within a day or two, which makes a urine sample a readout of recent intake — and a median UIC is a good indicator of iodine status in populations König 2011. Serum or plasma iodine is the one your portal is more likely to offer, and it has no outcome anchor and no internationally transferable interval. Measurement in either fluid is by the Sandell–Kolthoff catalytic reaction read on a spectrophotometer, or by ICP-MS.
Neither method is the problem. The denominator is. A UIC is micrograms per liter of urine, so it moves with how much you drank. Correcting for creatinine helps and does not fix it: in a seven-day metabolic study in pregnancy the coefficient of variation was 56.16% for the iodine/creatinine ratio against 73.15% for raw UIC within an individual Meng 2025. Both are enormous.
That variation is the whole subject, and it has been quantified twice. In 22 Swiss women who gave 341 twenty-four-hour collections and 177 morning spot samples over 15 months, the mean intra-individual CV was 32% for measured 24-hour excretion and 38% for the spot UIC — and ten repeat collections were needed to place one person's iodine status within 20% precision König 2011. The pregnancy study is worse: at ±20% precision it calculated 33 spot samples for an individual and 39 for a population Meng 2025. Nobody orders 33 urines.
Timing matters too, in a way no report mentions. Iodine excretion per hour is lowest in the morning and highest in the afternoon, and an estimate built from a first-void sample runs about 15% below the measured 24-hour excretion Eriksson 2023. The pregnancy data point the same way, finding the 12:00–18:00 window best correlated with 24-hour excretion Meng 2025. A fasting morning draw — the default for every other test on your panel — is the worst time to collect this one.
Iodine: what changes the blood, and what only changes the reading
What changes the iodine in your body, ranked:
- Iodized salt, or its absence. This is the dominant term worldwide and the reason median UIC is a public-health instrument. Most sea salt and specialty salt is not iodized.
- Dairy, eggs and seafood, then seaweed — where content varies by orders of magnitude between species and harvests, kelp especially.
- Pregnancy and lactation, which raise requirement and renal clearance at the same time. Women in the USA and EU have again been found mildly to moderately iodine deficient in pregnancy and around assisted reproduction Feldt-Rasmussen 2025.
- A plant-based diet without dairy or supplement. In 205 Norwegian vegans, vegetarians and pescatarians the overall median UIC was 57 µg/L — every group below the WHO threshold — and vegans not taking an iodine supplement had thyroglobulin of 27 µg/L against 16 µg/L in supplement users Henjum 2023.
- An iodine load. Iodinated contrast, amiodarone and povidone-iodine deliver milligrams to grams against a daily requirement of 150 micrograms, and distort the number for weeks.
What changes only the reading:
- Hydration. The largest single term in a spot UIC and it is not physiology at all Meng 2025.
- Time of day and which void — up to a 15% systematic shift for a first-void sample Eriksson 2023.
- Sex. Men excrete more: median hourly iodine excretion 7.4 against 5.3 µg/h in the same study Eriksson 2023.
- Season and day-to-day intake. Across 947 spot urines collected over four seasons, log-transformed intra-individual variation was 15.5% against 18.1% between individuals Fuse 2023 — within-person noise almost as large as the between-person signal, which is precisely why one sample cannot rank you against anyone.
- Creatinine itself, if the ratio is used: it varies with muscle mass, age and meat intake, so the correction imports a second person-specific error.
Reference interval or decision threshold — which kind of number Iodine is
Neither a reference interval nor a decision threshold — a population parameter. WHO's numbers (a median UIC of 100–199 µg/L in school-age children, 150–249 µg/L in pregnancy) are epidemiological criteria describing the iodine nutrition of a group. They were never built to classify a person, and the guidance says so.
The outcome data behind them are U-shaped, which is the unusual part. In 13,487 Chinese adults with thyroid ultrasound and function testing, a UIC at or above 300 µg/L carried an adjusted odds ratio of 1.451 (1.252–1.681) for subclinical hypothyroidism, while a UIC below 100 µg/L carried an OR of 0.831 for the same outcome — and 1.196 (1.099–1.301) for thyroid nodules Li 2025. Too much and too little are both wrong, in opposite directions, and a single number cannot be read as ‘higher is safer’.
The national experiment says the same. When China's median UIC fell from 219.7 to 175.9 µg/L between two nationally representative surveys, the prevalence of overt hyperthyroidism, subclinical hyperthyroidism, Graves' disease and goitre all fell, while thyroid nodules rose Li 2021. That is a whole country moving down the U.
Serum iodine is the weaker claim of the two. The interval printed beside it is a laboratory's own measured distribution; no guideline body anchors a serum iodine to a thyroid outcome, and this site does not invent an optimum for it.
How you would know your Iodine was wrong — and when to redraw
Do not retest the urine. Retest the thyroid. That is the honest answer and it follows from the numbers above: repeating one spot sample draws a second value from a distribution with a 38% intra-individual CV König 2011, so the second number disagrees with the first for reasons that have nothing to do with your diet. If you genuinely want an individual iodine status, the price is ten collections König 2011 — 33 in pregnancy Meng 2025 — under matched conditions.
Conditions that must match if you do repeat it: same time window (afternoon, not first void), same hydration, same laboratory, same fluid, and creatinine measured on the same sample so the ratio can be calculated Eriksson 2023.
How you would know your iodine was wrong. Order TSH, free T4 and thyroid antibodies on the same draw, before you change anything. Iodine is one of the few nutrients where the downstream markers are better than the nutrient marker: thyroid hormone synthesis is the function iodine exists for, and TSH moves when supply is genuinely inadequate Shulhai 2024. Thyroglobulin is the marker that responded to supplement use in vegans while UIC was uniformly low Henjum 2023. And the antibodies are a safety check rather than a status check: if TPO antibodies are positive, pushing iodine upward is the intervention most likely to make the thyroid worse, because the risk curve turns back up above 300 µg/L Li 2025.
What Iodine cannot tell you
It cannot classify you. A spot urinary iodine describes a population median; one sample from one person carries a 38% intra-individual CV and needs nine friends before it means anything König 2011. This is not a limitation that better technique fixes — the analytical precision is fine and the biology is what is noisy.
It cannot separate low intake from dilute urine without a creatinine, and even the ratio carries a 56% within-person CV Meng 2025.
It cannot tell you whether your thyroid has enough. Most body iodine sits in the gland as thyroglobulin-bound hormone precursor, and the gland defends its supply hard. TSH, free T4 and thyroglobulin answer that question and a urine does not Shulhai 2024.
A serum iodine cannot be compared to WHO's criteria at all — those are urinary numbers, in a different fluid, for a different purpose.
The wrong inference is the expensive one on this page: a single low spot UIC read as deficiency, treated with a milligram-dose kelp or ‘iodine protocol’ product, in somebody who never checked their thyroid antibodies — walking straight into the upper arm of a U-shaped risk curve Li 2025.
Sources read for these sections
- König F, et al. Ten repeat collections for urinary iodine from spot samples or 24-hour samples are needed to reliably estimate individual iodine status in women. Journal of Nutrition 2011 · PMID 21918061
- Meng Q, et al. Sample Size and Optimal Timing for Urine Collection Based on Diurnal Variability of Urinary Iodine/Creatinine Ratio in Pregnant Women. Biological Trace Element Research 2025 · PMID 41240297
- Eriksson J, et al. Urinary iodine excretion and optimal time point for sampling when estimating 24-h urinary iodine. British Journal of Nutrition 2023 · PMID 36744548
- Li X, et al. Urinary iodine levels and thyroid disorder prevalence in the adult population of China: a large-scale population-based cross-sectional study. Scientific Reports 2025 · PMID 40274899
- Fuse Y, et al. Variability and Seasonal Change of Urinary Selenium, Molybdenum, and Iodine Excretion in Healthy Young Japanese Adults. Biological Trace Element Research 2023 · PMID 36394795
- Li Y, et al. Effect of the transition from more than adequate iodine to adequate iodine on national changes in the prevalence of thyroid disorders: repeat national cross-sectional surveys in China. European Journal of Endocrinology 2021 · PMID 34762596
- Henjum S, et al. Thyroid function and urinary concentrations of iodine, selenium, and arsenic in vegans, lacto-ovo vegetarians and pescatarians. European Journal of Nutrition 2023 · PMID 37592132
- Feldt-Rasmussen U. Importance of iodide sufficiency and normal thyroid function in fertility and during gestation. Thyroid Research 2025 · PMID 40420166
- Shulhai AM, et al. The Role of Nutrition on Thyroid Function. Nutrients 2024 · PMID 39125376
Where to start with Iodine
In this order. Start at the supplement and you learn nothing, because you never established the number was real.
You have the range, where it came from and the first two moves. Inside is the rest of the five-pathway protocol — supplements, hormones, peptides — the order to run them in, and what to change when the number will not move.
Get the full protocol — $10/mo →📚 WHO/UNICEF/ICCIDD iodine status assessment. American Thyroid Association statement on iodine supplementation.
🩸 Test your Iodine
Order directly through Marek Diagnostics — no doctor's visit needed, drawn at any Quest location in the US. Code CAMERON applies 10% off automatically.
Order this test — 10% off → Browse all 103 markers →What Iodine is usually tested alongside
One marker is a data point. These panels add the markers that make Iodine interpretable, name why each is on the list, and load the set into your cart at 10% off.
includes this + 8 more markers — Vegan, vegetarian, or mostly plant-based — whether you've been doing it a decade or three months.
includes this + 10 more markers — Restrictive diet, plant-based, post-bariatric, on a GLP-1, heavy training load, or you just want to stop guessing which supplements you actually need.
What people use Iodine to decide
Nobody orders a test for its own sake. Iodine is on the test list for these pathways — each one links to what the pathway claims, and what its test list is read for before you spend anything on it.
Every nutrient here is harmful in excess as well as in deficiency, which is exactly why guessing is the wrong move. Iron and iodine are the two that most reliably hurt people who supplement them without looking first.
The one pathway you should never guess at. Normal TSH with low free T3 and high reverse T3 is the classic dieting-induced conversion problem — the metabolic rate has dropped and no appetite suppressant addresses that. Equally: if thyroid is genuinely fine, this whole pathway is money spent on a problem you don't have.
TSH alone is not a thyroid panel. Normal TSH with low free T3 is impaired conversion, and it is the single most common thing missed in fatigue — you need the full set to see it.
A multivitamin is a hedge against gaps you haven't measured. This panel tells you which gaps are real — and RBC values beat serum for zinc, magnesium and folate, because serum is defended at the expense of tissue.
What moves your Iodine
1 supplement in the Vault have a documented effect on this marker, or are a reason to have measured it first:
Browse all 278 compounds & 371 supplements →
Would you feel it? Symptoms Iodine helps explain
People search for how they feel, not for a marker. These are the complaints where this one is worth checking, and whether it is first-line or a follow-up.
Why your Iodine might be wrong
Most abnormal results are interference, not disease. Check these before you change anything. Each says whether the number is wrong (repeat it), badly timed (redraw it), or real with a cause.
Spot urinary iodine reflects the last day or so and varies hugely with single foods. Seaweed, dairy or an iodine-containing supplement will dominate the result.
Spot iodine describes populations well and individuals poorly. Repeat over several days if it matters.
CT contrast delivers an enormous iodine load that distorts results and thyroid function for weeks to months.
Note any recent scan with contrast.
What Iodine means in combination
One marker tells you a little; combinations tell you the story. These are the named patterns this one takes part in.
That is a malabsorption pattern, not a diet pattern. Celiac disease, atrophic gastritis, H. pylori, pancreatic insufficiency and inflammatory bowel disease all present this way, and correcting the individual nutrients without finding the cause is a treadmill.
Investigate why absorption is failing rather than supplementing indefinitely. Celiac serology must be done before removing gluten. Any neurological symptom with low B12 or copper needs urgent attention.
The thyroid compensates for a long time before TSH moves, so adequate TSH does not confirm adequate iodine. It matters most in pregnancy, where requirements rise sharply and deficiency affects fetal neurodevelopment.
Correct with a modest dose — a prenatal covers it. Excess iodine is also harmful and precipitates dysfunction in autoimmune thyroid disease, so check thyroid antibodies before supplementing. And a spot urinary iodine reflects the last day, not your status.
What to test next
These put Iodine in context — each with its own full breakdown.
Frequently asked questions
Serum 40–92 µg/L; urinary iodine is the better population measure. Ranges vary by laboratory and assay — always compare to the range printed on your own report.
No individual optimum exists. WHO’s iodine criteria are population medians, and the guidance says outright that spot samples cannot classify one person. WHO/UNICEF/IGN epidemiological criteria for iodine nutrition, as set out in the Nutrition International survey toolkit.
Excess iodine can trigger or worsen autoimmune thyroid disease and cause both hypo- and hyperthyroidism. High-dose iodine protocols circulating online are genuinely risky in people with thyroid autoimmunity.
Hypothyroidism, goiter; in pregnancy, impaired fetal neurodevelopment.
You can order Iodine directly through Marek Diagnostics without a doctor's visit — drawn at any Quest Diagnostics location in the US. Code CAMERON applies 10% off automatically.
Where this goes next
This page is the free framework. The protocol itself — the dosing, the order to correct things in, the week-by-week schedule and what to retest — is a lesson inside Skool.