Thyroid
Iodine deficiency and what your thyroid test can show
Reviewed by a qualified clinician · analysed at UKAS-accredited UK labs (ISO 15189)
Last reviewed October 20266 min read
Every Helvy guide is written by our health editors, then checked by a qualified clinician before it goes live and re-checked as the science moves. We name clinical roles, not individuals, until each reviewer has agreed to be credited publicly. This is wellness guidance to help you understand your own data, not a diagnosis.
QUICK ANSWER
Mild iodine deficiency is common in UK women. No blood test measures your iodine directly. What a thyroid test can show is the effect: TSH may drift up if iodine runs short. Too much iodine, often from kelp, can also upset the thyroid. Checking TSH and free T4 before and after a change shows how yours is coping.
Not sure which markers you need? Build your test →You swapped to oat milk, or you rarely eat fish. Now you keep reading that you might be short of iodine. And somewhere online, a kelp tablet is being sold as the answer.
As of October 2026, the question has fresh weight. A review in Clinical Endocrinology, accepted in May 2026 and written by UK thyroid and nutrition experts, says iodine deficiency has come back in the UK. It also warns about the opposite problem: people taking far too much.
1. Is iodine deficiency common in the UK?
Mild deficiency is, especially in women. Iodine is a mineral your thyroid uses to make its hormones. Adults need 140 micrograms (µg) a day, according to the NHS.
Population iodine is measured in urine. The WHO treats a group as having enough when its middle value sits at 100 µg/L or above. The review gives these UK figures.
| Group | Median | Adequate if at least |
|---|---|---|
| Women 16 to 49 (NDNS 2019 to 2023) | 82 µg/L | 100 µg/L |
| Pregnant women in Belfast | 73 µg/L | 150 µg/L |
The trend is down, too. Since 2013, levels have fallen 25% in adults aged 19 to 64 and 29% in girls aged 11 to 18. Unlike most rich countries, the UK does not add iodine to salt.
2. Who is most at risk?
Anyone who eats little dairy or fish. In the UK, cows’ milk is the main source. Dairy gives adults about a third of their iodine.
- People on plant milks: an unfortified glass has about 2 µg, against about 60 µg in cows’ milk.
- Vegans and many vegetarians, as the British Dietetic Association notes.
- Women planning a pregnancy, who are pregnant or who are breastfeeding.
- Anyone with a very narrow diet.
Only 28% of plant milks in the review were fortified, so check the label. Sea salt and Himalayan salt are not iodine sources. Our vegan blood test guide covers the other nutrients that run low on a plant diet.
3. What can iodine deficiency feel like?
Often nothing at all. Mild deficiency rarely has clear symptoms of its own. When it does show, it shows through the thyroid.
Short of iodine, the thyroid struggles to make enough hormone. Over time that can bring the signs of a slow thyroid: tiredness, feeling cold, weight gain and low mood. Our underactive thyroid symptoms guide lists them in full.
Long-term shortage can also swell the thyroid. That swelling at the front of the neck is called a goitre. The NHS says goitre from low iodine is rare in the UK. See a GP if you notice one.
4. Can a blood test show iodine deficiency?
Not directly. No standard blood test measures your iodine. Urine tests exist, but iodine in urine swings day to day with what you ate. The review says a single urine test is unhelpful for one person, and that your diet history tells a clinician more.
What a blood test can show is how your thyroid is coping. Two markers do most of the work.
- TSH, the signal your brain sends to push the thyroid harder.
- Free T4, the main hormone the thyroid makes.
When iodine runs short, TSH tends to creep up. The review notes that in the UK, average TSH drifts upwards without a full underactive thyroid. It asks specialists to weigh diet when they see a small TSH rise.
So a raised TSH is a clue, not proof. The common cause of a slow thyroid in the UK is autoimmune, and a qualified clinician tells the two apart. Our guides to high TSH and subclinical hypothyroidism explain what a mildly raised number means.
5. Why is kelp the wrong fix?
Because the dose is a guess. Brown seaweeds like kelp hold huge and very uneven amounts of iodine. The British Dietetic Association says kelp tablets can carry far more than the label claims.
“Taking high doses of iodine for long periods of time could change the way your thyroid gland works.”
NHS, iodine
Too much iodine can push the thyroid either way. It can slow it down, or tip it into overdrive. Which one depends on your own gland.
Some online “thyroid reset” plans push 12.5 to 50 mg a day. That is 100 to 300 times the daily need. The NHS says 0.5 mg or less a day from supplements is unlikely to cause harm. Our thyroid supplements guide covers other products that can shift a result.
6. What should you do if you think you are low?
Start with food. A glass of cows’ milk, a pot of yoghurt, an egg or a portion of white fish all help. A portion of cod or haddock can cover your whole day.
If you avoid dairy and fish, pick an iodine-fortified plant milk. If you want a supplement, choose one that states its dose and uses potassium iodide or iodate, not kelp. The NHS advises asking a healthcare professional first. Ask your GP or midwife if you are pregnant or planning to be.
Testing helps most around the change. A thyroid result before you start gives a baseline. A second one a few months later shows whether TSH and free T4 have moved. That matters most if you already have a thyroid condition, since a normal intake can affect you differently.
7. Which test reads your thyroid?
None of our panels measure iodine itself. They measure how your thyroid is working. The Thyroid & Vital Organs panel (£159) reads TSH, free T4 and free T3, the active hormone your cells use.
If tiredness is the main worry, the General Energy & Wellness panel (£149) reads TSH and free T4 beside vitamin D and B12. Our guide to normal thyroid levels shows the ranges, and the full blood count guide covers the wider markers often checked with tiredness.
A blood test measures biomarkers. It offers wellness insight, not a diagnosis. The thyroid hub has the rest of our thyroid guides.
READY TO TEST?
Ask your blood
Helvy’s home blood tests report your thyroid markers with plain-English context, so you can see how your thyroid is coping. Samples are processed by UKAS-accredited UK labs.
Frequently asked questions
Is iodine deficiency common in the UK?
Mild deficiency is common in women. A 2026 review in Clinical Endocrinology reported a median urinary iodine of 82 µg/L in UK women aged 16 to 49, below the WHO adequacy line of 100 µg/L. The UK does not add iodine to salt.
Can a blood test check my iodine levels?
Not directly. No standard blood test measures iodine, and urine iodine swings day to day. A thyroid blood test shows the effect instead: TSH tends to rise when the thyroid is short of iodine.
What are the symptoms of iodine deficiency?
Mild deficiency often has no clear symptoms. When it affects the thyroid, it can bring tiredness, feeling cold, weight gain and low mood. Long-term shortage can cause a goitre, a swelling at the front of the neck.
Should I take kelp for iodine?
No. The British Dietetic Association says kelp and seaweed supplements should not be used as an iodine source, because the iodine they contain varies widely and can be far higher than the label claims.
How much iodine do I need a day?
The NHS says adults need 140 micrograms a day. Milk, yoghurt, eggs and white fish are the main UK sources. The NHS says 0.5 mg or less a day from supplements is unlikely to cause harm.
Should I check my thyroid before taking iodine?
It gives you a baseline. Too little and too much iodine can both upset the thyroid, so a TSH and free T4 result before you start, and again a few months later, shows whether anything has moved.