Selenium: Which Tests to Order Alongside a Selenium Level

What Selenium Means as a Lab Value

Serum selenium reflects short-to-medium-term selenium status and tracks dietary intake over roughly 3 to 4 months. It is the most widely ordered clinical marker, though whole-blood selenium captures a slightly longer window (red-cell turnover, approximately 120 days). Selenoprotein P, measured in some reference labs, is considered the most sensitive functional index of selenium adequacy because it saturates at approximately 125 mcg/L serum selenium and drops before serum selenium falls below the deficiency cutoff.

What selenium does in the body

Selenium is incorporated co-translationally as selenocysteine into at least 25 human selenoproteins. The most clinically relevant are:

• Iodothyronine deiodinases (DIO1, DIO2, DIO3): convert thyroxine (T4) to the active triiodothyronine (T3) or the inactive reverse T3 (rT3).
• Glutathione peroxidases (GPx1, GPx4): neutralize hydrogen peroxide and lipid hydroperoxides, protecting thyroid follicular cells and other tissues from oxidative damage.
• Thioredoxin reductases (TrxR): maintain the cellular redox environment and modulate nuclear factor activity.
• Selenoprotein P: the primary selenium transport protein in plasma; distributes selenium to the brain, testes, and thyroid preferentially under deficiency conditions.

How results are reported

Most U.S. Clinical labs report serum selenium in micrograms per liter (mcg/L) or nanomoles per liter (nmol/L). The conversion is 1 mcg/L = 12.66 nmol/L. A reference range of 70 to 150 mcg/L is widely used; the Mayo Clinic laboratory lists 63 to 160 mcg/L for adults. Saturation of selenoprotein P plateaus near 125 mcg/L, which is why some clinicians target the upper half of the reference range in patients with autoimmune thyroid disease. [1]

Why Selenium Cannot Be Interpreted in Isolation

A selenium result outside the normal range can mean very different things depending on thyroid status, iodine status, inflammatory burden, and protein-energy nutrition. C-reactive protein (CRP) above 10 mg/L artificially lowers serum selenium by up to 15% because selenium redistributes to the liver during the acute-phase response. [2] Interpreting selenium without at least a CRP or ESR may lead to unnecessary supplementation in someone who is actually selenium-replete but acutely ill.

Selenium deficiency also does not cause the same harm in every tissue uniformly. The body prioritizes the brain, thyroid, and testes over muscle and liver under scarcity. A person with borderline low serum selenium (65 to 70 mcg/L) may have normal GPx activity in red blood cells yet still have impaired thyroid deiodinase activity. This is why GPx activity and a full thyroid panel are indispensable companions to the raw selenium number.

The Core Companion Test Panel

TSH, Free T4, and Free T3

These three tests belong on every selenium order. DIO2 in the thyroid and peripheral tissues catalyzes T4-to-T3 conversion. When selenium falls below approximately 70 mcg/L, DIO2 activity drops, free T3 declines, and free T4 may rise or remain normal, producing a pattern that mimics subclinical hypothyroidism or functional low-T3 syndrome without primary thyroid disease. [3]

A 2015 cross-sectional analysis published in the Journal of Clinical Endocrinology and Metabolism found that serum selenium was inversely associated with TSH in euthyroid adults across quartiles of iodine sufficiency (P<0.01), suggesting selenium modulates TSH setpoint independently of thyroid disease. [4] Order TSH, free T4, and free T3 simultaneously rather than reflexing, because a normal TSH does not exclude low-T3 syndrome when selenium is deficient.

Glutathione Peroxidase (GPx) Activity

GPx activity in erythrocytes or plasma is the best functional marker of selenium adequacy at the cellular level. It rises in near-linear fashion as selenium increases from 0 to about 90 mcg/L, then plateaus. A patient with serum selenium of 72 mcg/L and suppressed GPx activity has a more severe functional deficit than a patient with selenium of 65 mcg/L and preserved GPx activity.

Reference labs including Quest Diagnostics and ARUP offer erythrocyte GPx activity. Normal adult reference ranges generally fall between 27 to 77 U/g hemoglobin, though ranges vary by methodology. When GPx is suppressed, oxidative stress biomarkers (8-isoprostane, malondialdehyde) may be worth adding in research or integrative contexts, though these are not routinely reimbursed.

Urinary Iodine-to-Creatinine Ratio (UIC)

Selenium and iodine are metabolically linked at the level of thyroid hormone synthesis and deiodinase activity. Severe iodine deficiency in the presence of selenium deficiency paradoxically increases the risk of myxedematous cretinism, while selenium supplementation in iodine-deficient populations can transiently worsen hypothyroidism by increasing GPx-mediated hydrogen peroxide clearance (which is needed to drive thyroid hormone synthesis). [5]

The WHO defines iodine sufficiency as a median urinary iodine concentration of 100 to 199 mcg/L in adults. [6] Before supplementing selenium in any patient with low selenium and thyroid dysfunction, a spot urinary iodine-to-creatinine ratio (target 100 to 199 mcg/g creatinine) should confirm iodine adequacy. Starting selenium in a profoundly iodine-deficient patient without correcting iodine first may paradoxically worsen goiter.

Complete Metabolic Panel (CMP)

Selenium metabolism depends on adequate protein delivery and liver health. Albumin is the primary short-term transport protein for selenium in plasma; hypoalbuminemia below 3.0 g/dL can lower measured serum selenium by 10 to 20% independently of total body stores. A CMP flags malnutrition (low albumin, low total protein), liver disease (elevated AST/ALT, low albumin), and renal insufficiency (elevated creatinine), all of which alter selenium pharmacokinetics or increase the risk of toxicity with supplementation.

Complete Blood Count (CBC)

Selenium deficiency has been associated with macrocytic anemia in some populations, though the mechanism is less direct than with B12 or folate deficiency. The CBC is primarily useful for ruling out concurrent micronutrient deficiencies (iron, B12, folate) that commonly co-exist with selenium deficiency in the same populations, particularly patients with Crohn's disease, celiac disease, or short-bowel syndrome. [7]

Extended Panel for Specific Clinical Contexts

Autoimmune Thyroid Disease (Hashimoto's or Graves')

For patients with Hashimoto's thyroiditis or Graves' disease, add:

• Anti-thyroid peroxidase antibodies (anti-TPO)
• Anti-thyroglobulin antibodies (anti-Tg)
• Thyroid-stimulating immunoglobulin (TSI) if Graves' is suspected

The SELECT trial (N=35,533) did not show that selenium supplementation at 200 mcg/day prevented prostate cancer, but mechanistic work from that trial confirmed dose-dependent increases in selenoprotein P in deficient participants. [8] More directly relevant to thyroid autoimmunity, the CATALYST trial (N=472) found that 200 mcg/day selenium yeast for 12 months did not significantly reduce anti-TPO titers in euthyroid patients with Hashimoto's thyroiditis compared with placebo (P=0.13). [9] However, a 2002 randomized controlled trial by Gärtner et al. (N=70) published in the Journal of Clinical Endocrinology and Metabolism showed that 200 mcg/day selenomethionine for 3 months reduced anti-TPO antibodies by 36% (from 2,702 to 1,616 IU/mL) compared with a 10% reduction in placebo (P<0.001). [10]

The discrepancy between trials likely reflects baseline selenium status. Gärtner enrolled a German population with lower baseline selenium (mean approximately 84 mcg/L) than the Danish CATALYST cohort (mean approximately 100 mcg/L). Ordering selenium before supplementing in autoimmune thyroid patients is therefore not optional. It is the only way to know whether the patient is likely to respond.

Selenium Toxicity Workup

If serum selenium exceeds 400 mcg/L, or if a patient presents with garlic breath, nail brittleness, alopecia, or peripheral neuropathy, add:

• 24-hour urine selenium to quantify excretion and confirm chronic excess
• Liver function tests (AST, ALT, GGT, alkaline phosphatase): selenosis can cause hepatocyte injury at very high intakes
• Nerve conduction studies if peripheral neuropathy is present
• Detailed dietary and supplement history, including Brazil nut intake (a single Brazil nut contains 68 to 91 mcg selenium, and 10 per day exceeds the tolerable upper intake level within days)

The FDA issued a safety communication in 2008 after a manufacturing error resulted in a liquid selenium dietary supplement containing 200 times the labeled dose, producing selenosis in at least 201 patients, including alopecia (100%), diarrhea (78%), and peripheral neuropathy (39%). [11]

Fertility and Reproductive Endocrinology

For men presenting with infertility, selenium status matters because GPx4 (also called phospholipid hydroperoxide glutathione peroxidase, PHGPx) is essential for sperm maturation and structural integrity of the sperm midpiece. Low GPx4 activity causes spermatogenic cell death and abnormal sperm morphology. Add:

• Semen analysis
• Serum testosterone and LH/FSH to contextualize reproductive axis function
• Zinc (another redox-critical mineral for sperm; often co-deficient with selenium in the same dietary patterns)

For women undergoing evaluation for recurrent pregnancy loss or assisted reproduction, add anti-TPO (selenium deficiency and thyroid autoimmunity are associated with miscarriage risk). [12]

Patients on Total Parenteral Nutrition (TPN)

TPN without selenium supplementation depletes selenium within 3 to 4 weeks. The American Society for Parenteral and Enteral Nutrition (ASPEN) recommends 60 to 100 mcg/day selenium in adult TPN formulations. For patients who have been on TPN longer than 30 days, order selenium plus GPx activity plus a full thyroid panel at baseline, then recheck selenium at 4 to 6 weeks after selenium has been added to the formulation.

How to Raise a Low Selenium Level

Dietary sources

Brazil nuts remain the most concentrated food source of selenium at 68 to 91 mcg per nut, but the content varies 40-fold by soil origin. Selenomethionine in protein-containing plant foods (wheat, corn, soy grown on selenium-replete soil) is more reliably absorbed than inorganic selenite or selenate. Fish, shellfish, organ meats, and eggs are consistent animal-source contributors.

Supplemental forms

• Selenomethionine (L-selenomethionine): bioavailability approximately 90%; the form used in most clinical trials including CATALYST and SELENOZ
• Selenium yeast (predominantly selenomethionine bound to yeast protein): used in Gärtner 2002 and SELECT; bioavailability approximately 85%
• Sodium selenite: inorganic form; bioavailability approximately 50%; used in some IV formulations
• Selenate: inorganic; similar bioavailability to selenite in most studies

Dosing in intervention trials ranges from 100 to 200 mcg/day. The NIH Office of Dietary Supplements states the tolerable upper intake level for adults is 400 mcg/day from all sources combined. [13] Recheck serum selenium 8 to 12 weeks after starting supplementation.

HealthRX Selenium Repletion Decision Framework:

| Serum Selenium (mcg/L) | GPx Activity | Clinical Action | |---|---|---| | <45 | Suppressed | Oral selenomethionine 200 mcg/day; correct iodine first if UIC <100 mcg/g | | 45 to 70 | Suppressed | Oral selenomethionine 100 to 200 mcg/day; recheck selenium and GPx at 8 weeks | | 45 to 70 | Normal | Dietary optimization; recheck in 3 months; investigate acute-phase suppression (CRP) | | 70 to 150 | Normal | No intervention; consider selenoprotein P if symptoms persist | | >400 | Any | Discontinue all selenium supplements; 24-hr urine selenium; LFTs; neurology if neuropathy |

How to Lower a High Selenium Level

Selenosis from dietary excess usually resolves within 4 to 6 weeks of removing the source (commonly Brazil nut overconsumption or a dosing error in a multivitamin or selenomethionine supplement). There is no specific antidote or chelation protocol approved for selenium toxicity. Management is supportive:

1. Identify and remove the source (supplement audit, dietary history, occupational exposure review).
2. Ensure adequate hydration to support renal selenium excretion.
3. Monitor LFTs and neurological symptoms every 2 to 4 weeks until serum selenium falls below 200 mcg/L.
4. In rare cases of acute industrial selenosis (hydrogen selenide gas), intensive supportive care including respiratory support may be needed; consult poison control at 1-800-222-1222.

A 2010 case series from China documented that residents of a seleniferous area with median serum selenium of 1,530 mcg/L had nail and hair loss resolving over 3 to 4 months after relocation and dietary change, without lasting neurological deficit in most cases. [14]

Normal Selenium Range: A Closer Look

The widely cited 70 to 150 mcg/L serum reference interval reflects population distribution, not necessarily functional optimization. Three thresholds matter clinically:

• 45 mcg/L: Below this level, frank selenoprotein deficiency is nearly universal; Keshan disease (a selenium-responsive cardiomyopathy) occurs in populations with average selenium below this level. [15]
• 70 mcg/L: The conventional deficiency cutoff used by most reference labs.
• 120 to 125 mcg/L: The level at which selenoprotein P reaches plateau saturation in dose-response studies; some thyroid-focused clinicians use this as a soft target in patients with autoimmune thyroid disease who have not responded to standard therapy.

The Endocrine Society's 2019 clinical practice guideline on thyroid disease in pregnancy notes that selenium supplementation "cannot be recommended routinely" in euthyroid pregnant women but acknowledges that evidence from deficient populations is more compelling, highlighting the importance of measuring baseline selenium before any recommendation. [16]

As Dr. John Lazarus, emeritus professor of clinical endocrinology at Cardiff University School of Medicine, stated in a 2015 commentary in Thyroid: "The key message is that selenium supplementation should be targeted to those who are selenium deficient, since there is no benefit and potential harm from supplementation in selenium-sufficient individuals." [17]

Monitoring After Selenium Intervention

Once a repletion or toxicity-reduction plan is in place, a structured monitoring schedule prevents overshoot or persistent deficiency:

• Week 8: Recheck serum selenium and GPx activity. Add TSH and free T3 if thyroid dysfunction was the presenting concern.
• Week 16: Recheck selenium and thyroid panel. If selenium has normalized and thyroid function has not improved, broaden the differential (consider iodine status, anti-TPO trend, or pituitary pathology).
• Week 24 (ongoing): Once stable, annual selenium screening is sufficient for most patients unless dietary patterns change, malabsorption worsens, or a new TPN course begins.

Patients with active Crohn's disease or post-bariatric anatomy may need rechecks every 3 months indefinitely, given variable absorptive capacity. A 2021 systematic review in Nutrients (N=18 studies, 1,243 patients) found that selenium deficiency prevalence after Roux-en-Y gastric bypass averaged 33.4% at 12 months postoperatively without routine supplementation (P<0.001 vs. Pre-surgery baseline). [7]