Selenium: How to Interpret Your Result

What Selenium Actually Measures in a Blood Test

Serum or plasma selenium quantifies the total concentration of the trace mineral in your blood, reported in nanograms per milliliter (ng/mL) or micrograms per liter (mcg/L). These units are equivalent: 100 ng/mL equals 100 mcg/L. The test captures selenium bound to selenoprotein P, glutathione peroxidase, albumin, and a smaller free fraction.

Most clinical laboratories set the reference interval at 70 to 150 ng/mL, though some widen this to 23 to 190 ng/mL depending on the assay platform and the population sampled [1]. The number on your report reflects circulating selenium at the moment of the draw. It does not directly measure tissue stores in the thyroid, liver, or kidneys. A single value within range does not guarantee functional adequacy, because selenoprotein P (the best biomarker of selenium status) plateaus at a serum selenium of roughly 90 to 125 ng/mL according to data from the SELENOS and BIOSYN trials [2]. Below that plateau, some selenoprotein activities may already be suboptimal even if your result technically falls inside the reference range.

Whole-blood selenium can also be ordered. It includes red blood cell selenium, which reflects longer-term intake over the preceding 120 days. If your provider ordered a serum test, the result mirrors intake over the most recent one to three weeks.

What Does a Normal Selenium Level Mean?

A result between 70 and 150 ng/mL tells your provider that gross deficiency and toxicity are unlikely. It does not confirm that selenium-dependent enzymes are functioning at peak capacity.

The distinction matters for thyroid patients. Type 2 iodothyronine deiodinase (DIO2), the enzyme that converts thyroxine (T4) to the active hormone triiodothyronine (T3), requires selenium as a cofactor [3]. A 2017 systematic review in Thyroid (Winther et al.) found that selenium supplementation in autoimmune thyroiditis reduced thyroid peroxidase antibody (TPO-Ab) titers by a mean of 40% over 12 months compared with placebo, though free T4 and free T3 changes were inconsistent across trials [4]. The European Thyroid Association (ETA) issued a conditional recommendation in 2023 stating that selenomethionine 200 mcg daily may be considered in Hashimoto patients with mild thyroid eye disease, based on the EUGOGO trial [5].

If your selenium lands between 70 and 90 ng/mL and you have elevated TPO antibodies or a suboptimal free T3/free T4 ratio, your clinician might still discuss targeted supplementation despite a "normal" lab value.

What Does a Low Selenium Level Mean?

A serum selenium below 70 ng/mL signals deficiency. Severe deficiency (below 30 ng/mL) is rare in North America but common in regions with selenium-depleted soil, particularly in the Keshan province of China, where endemic cardiomyopathy was first linked to selenium status in the 1970s [6].

Moderate deficiency (40 to 70 ng/mL) is more common and carries subtler consequences. Glutathione peroxidase activity declines, reducing the body's capacity to neutralize hydrogen peroxide and lipid hydroperoxides [7]. Thyroid hormone metabolism slows. Immune surveillance weakens: a 2015 Cochrane review found that selenium supplementation in critically ill adults was associated with a trend toward lower mortality (RR 0.89 to 95% CI 0.78 to 1.02), though the authors graded the evidence as low certainty [8].

Populations at higher risk for low selenium include:

• People undergoing hemodialysis, due to losses through the dialysate membrane
• Individuals with Crohn's disease or short bowel syndrome, where absorption is impaired
• Those following strict vegan diets without deliberate inclusion of selenium-rich plant foods
• Residents of geographic regions with selenium-poor soil
• Patients receiving long-term parenteral nutrition without adequate selenium supplementation

A low result should prompt your provider to check thyroid function (TSH, free T4, free T3), inflammatory markers (CRP, ferritin), and dietary history before initiating supplementation.

What Does a High Selenium Level Mean?

A serum selenium above 150 ng/mL warrants attention. Above 200 ng/mL, early signs of selenosis can appear: garlic-odor breath, brittle nails, hair loss, nausea, and peripheral neuropathy. The Institute of Medicine established the tolerable upper intake level (UL) at 400 mcg per day for adults [9]. Acute selenium toxicity (above 800 to 1 to 000 ng/mL) is a medical emergency that can cause acute respiratory distress syndrome, myocardial infarction, and renal failure.

The SELECT trial (N=35,533) raised a separate concern. Men randomized to 200 mcg/day of L-selenomethionine showed a non-significant 9% increase in type 2 diabetes incidence (HR 1.07 to 99% CI 0.94 to 1.22) compared with placebo over a median of 5.5 years [10]. A subsequent analysis of the same cohort found that men with baseline selenium above 122 ng/mL who received supplementation had a statistically significant increase in high-grade prostate cancer risk (HR 1.91 to 95% CI 1.21 to 3.02) [11].

These findings do not mean that selenium above 122 ng/mL is automatically dangerous. They do mean that supplementing on top of an already-replete level introduces risk without documented benefit. If your level exceeds 150 ng/mL, your clinician should ask about Brazil nut consumption (a single nut contains 68 to 91 mcg), selenium-containing multivitamins, and selenium-yeast supplements before considering any intervention beyond dietary adjustment.

How to Raise a Low Selenium Level

Dietary correction is first-line. Two Brazil nuts daily provide approximately 140 to 180 mcg of selenium, enough to bring most mildly deficient adults into the optimal range within four to six weeks [12]. Other reliable sources include yellowfin tuna (92 mcg per 3 oz), halibut (47 mcg per 3 oz), sardines (45 mcg per 3 oz), and eggs (15 mcg per large egg).

When supplementation is warranted, the two common forms are selenomethionine and sodium selenite. Selenomethionine has higher bioavailability and is incorporated into body proteins in place of methionine, providing a more sustained rise in selenium status [13]. Sodium selenite is less well absorbed and generates more oxidative intermediates during metabolism, which is why most clinical trials (including the EUGOGO thyroid eye disease trial) used selenomethionine [5].

Dosing for documented deficiency typically starts at 100 to 200 mcg per day, with a recheck at eight to twelve weeks. Dr. Margaret Rayman, Professor of Nutritional Medicine at the University of Surrey and lead author of the 2012 Lancet selenium review, has stated: "The narrow margin between selenium intakes that are beneficial and those that are harmful makes it essential to target supplementation to people whose status is low, rather than adopting a population-wide approach" [14].

After repleting, maintenance through dietary sources alone is preferred. A selenium level between 90 and 125 ng/mL at follow-up confirms adequate selenoprotein saturation.

How to Lower a High Selenium Level

Reducing elevated selenium is simpler than raising it, because the body excretes excess selenium through the kidneys (as trimethylselenonium ion) and lungs (as dimethyl selenide) within days of removing the source [9].

Stop all selenium-containing supplements first. Audit multivitamins and greens powders, many of which contain 55 to 200 mcg per serving. Eliminate or reduce Brazil nut intake. A person eating five Brazil nuts daily could be consuming over 400 mcg from that single food. Because selenomethionine gets incorporated into muscle protein, washout from tissue stores may take eight to twelve weeks even after dietary sources are eliminated.

Recheck serum selenium at the twelve-week mark. If the level remains above 200 ng/mL despite supplement and dietary elimination, investigate occupational or environmental exposure (glass manufacturing, electronics production, coal combustion) and consider referral to a toxicologist. There is no chelation agent approved specifically for selenium toxicity, though case reports describe supportive care with N-acetylcysteine for acute ingestions [15].

For mild elevations (150 to 200 ng/mL), dietary modification alone is sufficient. No pharmacologic intervention is indicated.

Selenium and Thyroid Function: Why Your Endocrinologist Cares

The thyroid gland contains more selenium per gram of tissue than any other organ in the body. Three families of selenoproteins are active there: the iodothyronine deiodinases (DIO1, DIO2, DIO3), which regulate T4-to-T3 conversion and T3 degradation; the glutathione peroxidases (GPx1, GPx3), which protect thyrocytes from the hydrogen peroxide generated during thyroid hormone synthesis; and thioredoxin reductases, which maintain intracellular redox balance [3].

The clinical translation of this biochemistry is most clear in autoimmune thyroid disease. A 2021 meta-analysis published in Frontiers in Endocrinology pooling 16 randomized controlled trials (N=1,494) confirmed that selenium supplementation (most commonly selenomethionine 200 mcg daily) significantly reduced TPO-Ab concentrations at 6 and 12 months compared with placebo [16]. The effect on thyroglobulin antibodies (TgAb) was less consistent, and changes in TSH, free T4, and free T3 did not reach significance in most trials.

The American Thyroid Association (ATA) does not currently recommend routine selenium supplementation for Hashimoto thyroiditis outside of clinical trials, citing heterogeneity in trial populations and outcomes [17]. The ETA is more permissive, endorsing a six-month trial of selenomethionine 200 mcg for mild, active Graves ophthalmopathy based on the Marcocci et al. EUGOGO trial, where selenium improved quality of life scores and reduced disease progression compared with placebo (OR 0.36 to 95% CI 0.16 to 0.81) [5].

If you have Hashimoto or Graves disease and your selenium is below 90 ng/mL, your endocrinologist has reasonable grounds to recommend a targeted supplement trial with follow-up labs at three and six months.

Factors That Affect Your Selenium Result

Several variables can shift your number independent of true selenium status.

Recent dietary intake. Serum selenium reflects recent intake (one to three weeks). A large seafood meal the day before your blood draw can transiently raise the result. Fasting status does not meaningfully affect selenium, but consistency in dietary patterns before testing improves interpretability.

Geographic location. Soil selenium content varies dramatically. The Great Plains and western U.S. states tend to have selenium-rich soil, while the Pacific Northwest and parts of the Southeast have lower concentrations [18]. People sourcing most of their food locally may have systematically higher or lower levels depending on regional soil chemistry.

Inflammation. Selenium is a negative acute-phase reactant. During acute infection, surgery, or trauma, serum selenium drops by 10 to 30% due to redistribution and increased selenoprotein consumption in immune cells [19]. If your blood was drawn during an illness or within two weeks of surgery, the result may underestimate your baseline status. Drawing a concurrent CRP can help your clinician distinguish true deficiency from inflammatory redistribution.

Kidney function. Because trimethylselenonium ion is cleared renally, impaired kidney function can raise serum levels modestly. Conversely, hemodialysis removes selenium, creating a net deficit over time.

Medications. Cisplatin and other platinum-based chemotherapies increase renal selenium excretion. Statin therapy has been associated with lower selenium in some observational cohorts, though the mechanism is unclear and the magnitude is small [20].

When to Retest Selenium

Retesting depends on your initial result and the intervention applied.

If your level was low and you started supplementation: recheck at eight to twelve weeks. Selenomethionine at 200 mcg/day raises serum selenium by approximately 20 to 30 ng/mL over this interval in most adults [13].

If your level was high and you stopped supplements: recheck at twelve weeks to allow washout from tissue protein pools.

If your level was normal and no intervention was made: routine retesting is not indicated unless clinical circumstances change (new thyroid diagnosis, change in renal function, initiation of parenteral nutrition, or development of symptoms consistent with deficiency or toxicity).

There is no guideline-endorsed screening interval for selenium in asymptomatic, healthy adults. The U.S. Preventive Services Task Force (USPSTF) has not issued a recommendation on selenium screening [21]. Testing is most valuable when ordered with a specific clinical question: unexplained thyroid dysfunction, refractory hypothyroidism despite adequate levothyroxine dosing, suspected malabsorption, parenteral nutrition monitoring, or evaluation of symptoms like hair loss and nail changes in the context of supplement use.

Target a follow-up value of 90 to 125 ng/mL, the range at which selenoprotein P activity plateaus and the SELECT trial's risk signal has not been observed.