Why selenium matters for your thyroid
Your body converts T4 into the active T3 using enzymes that depend on selenium. Both too little and too much cause problems, which is why the total matters more than the dose on any single label.
Free tool
How much selenium are you actually getting?
Selenium adds up across supplements, multivitamins and food. The safe ceiling for adults is 400 mcg a day. Most people have never counted.
Selenium intake estimate, coreaxis.us/selenium-calculator
Your estimated daily intake
--mcg a day
Add what you take and eat. Your total builds here as you go.
Range used in thyroid studies, 100 to 200 mcg
If you take levothyroxine
Minerals interfere with absorption. Most people take levothyroxine first thing on an empty stomach and keep magnesium, calcium and iron about four hours away from it.
When a blood test is worth it
This is an estimate built on published averages. If your intake sits near either end of the range, or you have symptoms like a metallic taste, brittle nails or hair shedding, your doctor can measure your actual level.
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We make an iodine-free thyroid formula with 100 mcg of selenium as L-selenomethionine, the form used in the research. We built this calculator because most people we talk to are stacking more than they realise. See the formulation
How this is calculated
Every source you enter contributes a range rather than a single number, and the three totals are the sums of those ranges. The headline figure is the middle of the range, not a measurement.
- Supplements and multivitamins are taken at the figure on the label, with no range, because that figure is a declaration.
- One Brazil nut counts as 68 to 200 mcg, typically 90. The spread is real: selenium comes from the soil, and one survey measured 0.03 to 512 mcg per gram across samples.
- A 3 oz tuna serving counts as 60 to 92 mcg, one egg as 13 to 20, one ounce of sunflower seeds as 18 to 27. Weekly entries are divided by seven.
- The rest of your diet is added as a band, 25 to 70 mcg for an average one, because US intake runs near 100 mcg a day before any supplement and starting from zero would tell most people they are low when they are not.
- The verdict compares the middle figure with the recommended intake for your life stage (55 mcg for adults, 60 in pregnancy, 70 when breastfeeding) and with the 400 mcg upper limit, which does not change.
Nothing is stored and nothing is sent anywhere while you use it. The arithmetic runs in your browser.
Questions people ask
How many Brazil nuts a day is too many?
It depends on where they grew. One nut averages about 90 mcg of selenium but can carry anywhere from 68 to 200 mcg, because the tree takes up whatever is in the soil. Two nuts a day sits comfortably under the 400 mcg adult ceiling on average, and can approach it on a bag from a selenium-rich region, before counting anything else you take.
Is 200 mcg of selenium too much?
200 mcg on its own is below the 400 mcg upper limit for adults, and it is the dose most thyroid trials used. The problem is rarely the supplement alone. Add a multivitamin, a couple of Brazil nuts and an ordinary diet and the total can pass the ceiling without any single label looking unusual.
Can I take selenium with a multivitamin?
You can, but check whether the multivitamin already contains selenium. Many carry 20 to 55 mcg and it is easy to miss, which is how people end up taking two selenium doses while believing they take one.
Does selenium help with Hashimoto's?
Selenium is a cofactor for the enzymes that convert T4 into active T3, and it has been studied in autoimmune thyroiditis for more than two decades, usually at 100 to 200 mcg a day. Whether it helps you is a question for your doctor and your blood work, not for a calculator.
What are the signs of too much selenium?
Reported signs of chronic excess include a metallic taste, garlic-smelling breath, brittle nails, hair shedding, nausea and skin rashes. They are not specific, which is why an intake near or above the upper limit is worth raising with a doctor who can measure your actual level.