Key Benefits

• Confirm your selenium status to prevent deficiency and avoid toxicity.
• Spot deficiency or excess early to address hair loss, brittle nails, or GI upset.
• Clarify thyroid symptoms by assessing selenium needed for thyroid hormone activation.
• Guide safe supplementation, especially if using multivitamins, high-dose products, or IV nutrition.
• Protect fertility and pregnancy by correcting low levels affecting thyroid and antioxidant defenses.
• Support immune defenses by ensuring adequate antioxidant enzyme activity during illness or recovery.
• Track trends over time using the same lab and sample type for consistency.
• Best interpreted with your diet, symptoms, and, when relevant, thyroid labs.

What is Selenium?

Selenium is an essential trace mineral the body cannot make; it comes from the diet, especially plants grown in selenium‑rich soil and the animals that eat them. After intestinal absorption, it circulates in blood mainly on selenoprotein P and albumin. Cells use selenium by building it into a select set of proteins as the amino acid selenocysteine. The liver directs distribution, while muscle, thyroid, and immune tissues are major users.

Selenium’s importance is that it enables selenoproteins that protect and fine‑tune cell chemistry. Key roles include clearing peroxides to limit oxidative stress (glutathione peroxidases), maintaining redox signaling and DNA integrity (thioredoxin reductases), and activating or inactivating thyroid hormone (iodothyronine deiodinases). Adequate selenium also supports immune competence and reproductive health. As a biomarker, circulating selenium—and its carrier, selenoprotein P—indicates the body’s selenium supply and capacity to sustain antioxidant defenses and thyroid hormone metabolism.

Why is Selenium important?

Selenium is a trace mineral that powers selenoproteins—enzymes that guard cells from oxidative stress, activate thyroid hormone, fine‑tune immunity, and support fertility and nerve health. The blood selenium biomarker reflects the body’s capacity to build these enzymes; most labs report a narrow normal range, and full selenoprotein activity typically occurs in the middle to upper part of that range rather than at the low end.

When values sit low, the body struggles to quench reactive oxygen species and to convert thyroid hormone to its active form. People may notice fatigue, exercise intolerance, muscle aches, brain fog, brittle hair, and hypothyroid‑like symptoms (cold, slowed metabolism). Immunity can be blunted with more frequent infections. Severe deficiency can injure heart muscle (classically, cardiomyopathy). Men may have poorer sperm motility and integrity; in pregnancy, low selenium is linked to thyroid strain and adverse immune shifts; in children, deficiency may impair growth and increase infection risk.

At the high end, excess selenium becomes pro‑oxidant and toxic (selenosis). Nausea, diarrhea, a garlic‑like breath odor, hair loss, and nail brittleness are common clues; irritability, peripheral neuropathy, skin rash, and elevated liver enzymes can appear. Very high levels may disrupt thyroid signaling and have been associated in some studies with altered glucose regulation.

Big picture: selenium sits at the crossroads of redox balance, mitochondrial function, thyroid axis, and immune surveillance. Adequate—not excessive—status supports metabolic rate, cardiovascular and reproductive health, and resilience to stressors, while both deficiency and excess carry long‑term risks in a U‑shaped pattern.

What Insights Will I Get?

Selenium measures the body’s pool of an essential trace element used to build selenoproteins—enzymes that control antioxidant defenses (glutathione peroxidases), redox signaling (thioredoxin reductases), and thyroid hormone activation (deiodinases). Adequate status supports mitochondrial energy metabolism, vascular and immune resilience, cognitive performance, and reproductive function.

Low values usually reflect insufficient intake, malabsorption, or redistribution during chronic inflammation. Functionally, this limits selenoprotein activity, weakening antioxidant capacity and thyroid hormone conversion (less T3), which can feel like low energy, cold intolerance, and slower metabolism. It is linked with higher infection susceptibility and, in men, reduced sperm quality. In pregnancy, low selenium relates to greater thyroid dysfunction and hypertensive complications.

Being in range suggests that core selenoproteins are saturated, keeping oxidative stress in check and thyroid activation efficient, which supports steady energy, vascular health, and immune balance. Evidence points to a U‑shaped relationship; optimal health signals typically sit around the mid‑range of most reference intervals.

High values usually reflect excess exposure (supplements, high‑selenium foods, or environmental sources) or reduced renal clearance. At a systems level this can flip redox balance toward pro‑oxidant effects and disturb thyroid enzyme activity, with features such as brittle hair/nails, skin changes, gastrointestinal upset, irritability, and peripheral neuropathy. Higher levels have been associated with dyslipidemia and impaired glucose regulation; excess offers no benefit in pregnancy.

Notes: Serum/plasma selenium varies with recent intake and falls with acute illness (negative acute‑phase response) and during pregnancy (hemodilution). Reference ranges differ by region and assay. Selenoprotein P or glutathione peroxidase activity can better reflect functional status, and kidney disease meaningfully alters levels.