Selenium and Iodine: The Two Nutrients Most Women Are Missing

You're exhausted. Your hair is thinning. Your periods are heavier than they used to be. You're gaining weight despite eating well. You go to your GP, they run a basic thyroid test, and the result comes back "normal." End of story.

Except it's not normal. Because two nutrients that your thyroid absolutely depends on to function properly are chronically low across the UK population, and almost nobody is testing for them. Those nutrients are selenium and iodine.

The UK selenium crisis nobody is talking about

Here's a fact that surprises most people: UK soil has some of the lowest selenium content in Europe. Selenium levels in British wheat have dropped by roughly 50% since the 1970s, when the UK stopped importing selenium-rich North American wheat and began sourcing more from Europe.

A 2017 study published in Environmental International estimated that up to 1 billion people worldwide may have inadequate selenium intake, with the UK identified as one of the affected regions. Research from the University of Surrey published in The Lancet found that the average UK selenium intake was about 30-40 micrograms per day, well below the recommended 55-75 micrograms.

Why does this matter for women specifically? Selenium is essential for producing a group of enzymes called selenoproteins. One of the most important is deiodinase, which converts your inactive thyroid hormone (T4) into the active form (T3) that your cells actually use for energy. Without enough selenium, your thyroid can be producing T4 normally, your blood tests can look fine, and you can still be functionally hypothyroid because the conversion isn't happening efficiently.

A 2002 meta-analysis published in Thyroid reviewed 14 studies and confirmed that selenium supplementation significantly reduced thyroid antibodies (anti-TPO) in people with Hashimoto's thyroiditis, the most common cause of hypothyroidism. One French study gave patients just 200 micrograms of selenium daily for 6 months and saw a 40% reduction in thyroid antibodies compared to placebo. That's a substantial change from a single nutrient.

Iodine deficiency: the problem the UK thought it had solved

For decades, the assumption was that iodine deficiency was a developing-world problem. That assumption is wrong.

A landmark 2011 study published in The Lancet by researchers at the University of Surrey tested over 700 UK schoolgirls and found that nearly 70% had iodine levels below the WHO adequacy threshold. A follow-up study in 2018 in the European Journal of Nutrition confirmed that UK women of childbearing age were particularly affected, with many falling into the mild-to-moderate deficiency category.

Unlike many countries (the US, Australia, and most of continental Europe), the UK has never had a mandatory iodine fortification programme. No iodised salt. No iodine added to bread. The main dietary source of iodine in the UK is dairy, because iodine is used in cattle feed supplements and as a disinfectant in dairy farming. If you've reduced your dairy intake for health reasons (or because you read one of our other articles about dairy and hormones), your iodine intake may have dropped significantly without you realising.

Iodine is the raw material your thyroid needs to produce thyroid hormones. Without enough of it, your thyroid simply cannot make sufficient T3 and T4. Your body compensates by enlarging the thyroid gland (goitre), increasing TSH, and eventually slowing your entire metabolism.

How selenium and iodine affect your periods, fertility, and weight

Your thyroid is the master regulator of your metabolism. When it's underperforming, the effects ripple through every system in your body. But the effects on women's reproductive health are particularly significant and particularly underappreciated.

Periods. Thyroid hormones directly influence the length and quality of your menstrual cycle. Subclinical hypothyroidism (where TSH is elevated but T4 is technically in range) is associated with heavier, longer, and more painful periods. A 2015 study in Obstetrics and Gynecology found that women with subclinical hypothyroidism were significantly more likely to have menstrual irregularities, and that treatment with thyroid hormone normalised their cycles. But if the root cause is selenium or iodine deficiency, treating with synthetic thyroid hormone alone is addressing the downstream effect rather than the upstream cause.

Fertility. Thyroid function is one of the most important factors in female fertility, and it's one of the first things fertility clinics check. But the testing is often too basic. A 2019 review in Human Reproduction Update found that even "high-normal" TSH (above 2.5 mIU/L) was associated with reduced fertility and higher miscarriage rates. The review recommended that TSH should ideally be below 2.5 before conception. Both selenium and iodine directly affect whether your thyroid can maintain the hormone output required for implantation, early pregnancy, and fetal brain development.

Weight. One of the most frustrating symptoms of suboptimal thyroid function is unexplained weight gain or inability to lose weight despite genuine effort. Thyroid hormones set your basal metabolic rate. When T3 levels drop, your body burns fewer calories at rest, increases water retention, and becomes more resistant to fat mobilisation. You can eat well and exercise consistently and still not see results, because the metabolic thermostat is set too low. Correcting selenium and iodine deficiencies can help restore that thermostat to where it should be.

Hair and skin. Thyroid hormones regulate the growth cycle of hair follicles. Low thyroid function extends the resting phase of hair growth, leading to diffuse thinning (not patchy loss, but overall thinning across the scalp). Dry skin, brittle nails, and loss of the outer third of your eyebrows are all classic signs that practitioners trained in functional medicine look for. These symptoms often appear long before blood tests show a clear thyroid problem.

The selenium-Hashimoto's connection

Hashimoto's thyroiditis is the most common autoimmune condition in the UK, and it overwhelmingly affects women (about 10:1 female to male ratio). In Hashimoto's, the immune system attacks the thyroid gland, gradually destroying it and leading to hypothyroidism.

Selenium has a uniquely important role here. The thyroid gland contains the highest concentration of selenium per gram of any organ in your body. Selenoproteins protect thyroid cells from oxidative damage caused by the hydrogen peroxide produced during thyroid hormone synthesis. When selenium is insufficient, thyroid cells are more vulnerable to immune attack.

A 2010 Cochrane-style systematic review in Clinical Endocrinology analysed multiple randomised controlled trials and concluded that selenium supplementation at 200 micrograms daily for 3-12 months significantly reduced anti-TPO antibody levels in Hashimoto's patients. A 2016 meta-analysis in Thyroid pooled data from 16 trials and confirmed the finding, reporting average reductions in anti-TPO antibodies of 20-40%.

This doesn't mean selenium cures Hashimoto's. But it does mean that ensuring adequate selenium status is one of the most evidence-based nutritional interventions available for managing the autoimmune component of the condition.

Iodine and breast health

This is an area of research that most people have never heard of, but it's worth knowing about. Breast tissue has iodine transporters and concentrates iodine in a similar way to thyroid tissue. Research suggests that iodine plays a role in normal breast cell development and may have protective effects against abnormal cell growth.

A 2005 study in Breast Cancer Research and Treatment found that molecular iodine (I2) had antiproliferative and apoptotic effects on breast cancer cell lines. Epidemiological data from Japan, where iodine intake is significantly higher than Western countries due to seaweed consumption, shows lower rates of breast and thyroid disease. A 2008 study in The Proceedings of the Society for Experimental Biology and Medicine reviewed the evidence and concluded that iodine deficiency may be a contributing factor in breast disease, including fibrocystic breast changes.

This research is still developing, and nobody is suggesting iodine supplementation as a standalone breast cancer prevention strategy. But it adds to the picture of why adequate iodine intake matters for women's health far beyond just thyroid function.

How to test and what to supplement

Selenium testing: A serum selenium level is the standard test. Optimal range is 1.0-1.6 micromol/L. Many UK labs can run this, or you can get it privately for around £30-40. If you're deficient, 100-200 micrograms of selenomethionine daily for 3-6 months typically restores levels. Retest to confirm.

Iodine testing: A spot urinary iodine test gives a reasonable snapshot of your iodine status. The WHO considers a median urinary iodine concentration of 100-199 micrograms/L as adequate. Below 100 indicates deficiency. Iodine loading tests (24-hour urine collection after a set dose) give more precise information but are harder to arrange.

Food sources: For selenium, Brazil nuts are the richest source (1-2 nuts can provide your daily requirement, though content varies by region). Fish, eggs, and meat also contribute. For iodine, seaweed (especially kelp and nori), dairy products, eggs, and white fish are the main sources. If you eat little to no dairy and rarely eat fish or seaweed, your iodine intake is almost certainly inadequate.

Supplementation: For iodine, doses of 150-300 micrograms daily are appropriate for most people and are found in most quality prenatal and multivitamin formulations. Higher doses (above 500 micrograms) should only be used under practitioner supervision, especially if you have Hashimoto's, as excess iodine can temporarily worsen thyroid inflammation in some people.

An important nuance: if you have Hashimoto's or elevated thyroid antibodies, ensure adequate selenium intake before supplementing iodine. Selenium protects the thyroid from the oxidative stress that iodine metabolism produces. Taking high-dose iodine without adequate selenium can be counterproductive.

Why these nutrients aren't tested routinely

The honest answer is that standard NHS guidelines focus on TSH-based thyroid management rather than nutritional optimisation. If your TSH is within the reference range, you're typically told everything is fine. There's no standard protocol for checking selenium or iodine, even in women presenting with classic thyroid symptoms.

This isn't a criticism of individual doctors. It's a systems-level issue. The UK has no national iodine supplementation programme, no routine selenium testing, and limited awareness in mainstream medicine of the degree to which these nutrients are depleted in the British diet.

The research is there. The evidence is strong. The testing is inexpensive. What's missing is the clinical pathway that connects nutritional status to thyroid function in everyday practice. Until that changes, the responsibility falls on individuals to understand the connection and take action.

If you're tired, gaining weight, losing hair, and struggling with your periods, and your basic thyroid test came back "normal," selenium and iodine deficiency should be at the top of your investigation list. The fix might be simpler than you think.