A targeted thyroid and fertility support where intake is low — with a strict upper limit worth respecting.
Selenium is an antioxidant mineral the thyroid depends on to work. Since the thyroid sets the pace for a woman's metabolism, energy, mood and cycles, selenium has an outsized, behind-the-scenes role in hormone health. It is also concentrated in the ovaries and supports fertility and a healthy pregnancy.
For thyroid health, the evidence is reasonably good — selenium is essential for converting thyroid hormones into their active form, and there is some evidence it lowers the antibodies in autoimmune (Hashimoto's) thyroid disease. For fertility it plays a supporting role. The honest framing: it is genuinely useful where intake is low or thyroid antibodies are raised, but it is not a general tonic, and more is definitely not better.
A diagnosed thyroid condition (especially Hashimoto's), trying to conceive, or a diet low in selenium-rich foods (seafood, eggs, Brazil nuts).
Brazil nuts are in a class of their own — just one or two can supply, or exceed, a whole day's selenium, because they concentrate it so dramatically (which is also why a daily handful can tip you over the safe limit). After Brazil nuts, the richest sources are seafood (tuna, sardines, prawns, oysters), then eggs, poultry, organ and muscle meats, and wholegrains, sunflower seeds and mushrooms. One nuance many women don't realise: the selenium in plant foods depends heavily on the soil they were grown in, so the same food can be selenium-rich or selenium-poor by region — which is why deficiency clusters in certain parts of the world. For most women, one or two Brazil nuts a day is a complete, natural selenium strategy with no supplement needed.
A sensible dose is 100–200 mcg/day of selenomethionine, or simply one to two Brazil nuts a day (a natural, concentrated source). Do not exceed 200 mcg from all sources combined.
Take it with food. If you eat Brazil nuts regularly, you may not need a supplement at all — and shouldn't double up.
Selenium has a narrow safe window. Going above 200 mcg/day long term causes hair loss, brittle nails, a garlicky breath odour and, at high doses, toxicity. This is one to respect the ceiling on.
Iodine (the thyroid needs both, in balance), zinc, and vitamin D — a common thyroid-support combination.
Don't combine a selenium supplement with daily Brazil nuts (easy to overshoot). If you take thyroid medication, check with your doctor, as selenium can influence thyroid hormone levels.
Anyone already eating Brazil nuts daily, and those on thyroid medication (coordinate with your doctor).
Selenomethionine (a well-absorbed form) at 100–200 mcg, third-party tested.
Selenium is a targeted thyroid and fertility support, most useful where intake is low or thyroid antibodies are raised. Take 100–200 mcg of selenomethionine (or a couple of Brazil nuts) — and respect the 200 mcg ceiling.
NIH Office of Dietary Supplements — Selenium; trials of selenium in Hashimoto's thyroiditis; research on selenium and fertility.
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Based on guidance from the NHS, NICE, Cleveland Clinic and peer-reviewed research.
General information, not a substitute for personal medical advice — always consult your doctor or a qualified health professional before making health decisions. If you are pregnant, breastfeeding, trying to conceive, under 18, or taking medication, speak to your doctor before starting any supplement.