This test is a specialized biochemical analysis that measures the concentration of selenoprotein P (SelP) in the serum. Selenoprotein P is the primary transport and storage form of selenium, an essential trace element for human health.
Selenoprotein P is a glycoprotein containing up to 10 selenocysteine residues per molecule - a natural variant of the amino acid cysteine, in which the sulfur atom (S) is replaced by selenium (Se). The protein is produced mainly in the liver and then distributed through the bloodstream to peripheral tissues and organs with high selenium demand, such as the brain, testes, and thyroid gland.
The test uses advanced analytical methods to precisely quantify the bioavailable form of selenium in the body. Unlike total selenium measurements, which also include non-functional forms (e.g., selenomethionine), the selenoprotein P assay is considered a functional indicator of selenium status.
Interpretation
The purpose of this test is to assess the functional selenium status of the body and to provide an accurate picture of selenium sufficiency or deficiency, an element essential for endocrine and immune homeostasis.
Low levels of selenoprotein P indicate functional selenium deficiency, even when total serum selenium appears normal. This condition is linked to increased oxidative stress, impaired T4-to-T3 conversion, weakened immune defenses, and greater susceptibility to inflammatory or autoimmune diseases.
Elevated levels of selenoprotein P may result from high-dose selenium supplementation and do not necessarily correspond to enhanced physiological function. Significantly elevated values may reflect the body's protective effort to eliminate excess selenium and warrant further evaluation for potential selenium toxicity (selenosis).
Optimal levels of this marker indicate adequate selenium intake and efficient biological utilization, allowing the body to maintain strong antioxidant protection, endocrine balance, and immune regulation. The interpretation of results should always be made within the context of the individual’s clinical profile, in conjunction with total selenium measurements, clinical symptoms, or other related indicators such as thyroid hormones or inflammatory markers.
Unlike total selenium, the measurement of selenoprotein P reflects true bioavailability of selenium at the tissue level, not just circulating concentrations. In cases of deficiency or increased physiological demand (e.g., chronic inflammation, malignancy, pregnancy), selenoprotein P production and circulation may drop early, often before total selenium levels decrease measurably.
It is important to note that in certain cases, even when selenium intake is sufficient, the element may not be properly utilized by the body’s cells. This may be due to autoantibodies against selenoprotein P, which interfere with selenium transport and cellular uptake. In such cases, a functional selenium deficiency may occur, despite normal laboratory values of Selenoprotein P or total selenium. As such, SelP functions as a dynamic biomarker, reflecting disturbances in selenium metabolism and revealing deficiencies that standard measurements may overlook.
Diagnostic Value of the Test
The selenoprotein P assay has broad clinical utility in situations where functional selenium deficiency is suspected - cases that may go undetected with a stand-alone measurement of total selenium. Such scenarios may include endocrine, immunological, oncological, and metabolic disorders, as well as chronic inflammatory conditions. More specifically, its role has been extensively studied in the following contexts:
- Thyroid Disorders and Autoimmunity: In cases of hypothyroidism or Hashimoto’s thyroiditis, where the conversion of T4 to T3 is affected by selenium deficiency, this test provides confirmation of potential functional insufficiency. It can help distinguish cases in which thyroid hormone levels appear normal, yet the patient continues to experience low energy, offering further guidance for targeted selenium supplementation.
- Chronic Fatigue and Low Energy: This test is useful in patients with unexplained fatigue, stress intolerance, or symptoms such as “brain fog,” given selenium’s essential role in mitochondrial function. Selenoprotein P provides an objective measure of selenium bioavailability in tissues beyond serum selenium levels alone.
- Pregnancy and Fertility Planning: Selenoprotein P contributes to evaluating the antioxidant profile and metabolic balance during pregnancy. Selenium deficiency has been associated with gestational diabetes and, in certain cases, with prolonged pregnancy. This test aids in the early detection of functional deficiencies and in targeted micronutrient support.
- Cancer and Inflammation: Low levels of selenoprotein P have been observed in patients with breast, liver, and colorectal cancers, while higher levels are associated with a better prognosis in these diseases. Particularly in patients under treatment, the test is used as a complementary tool for antioxidant monitoring and to assess inflammatory burden.
- Recovery and Post-Infectious States: Following severe infections, COVID-19, or septic episodes, selenoprotein P reflects the body's response to increased immune and metabolic demands. Its measurement can support tracking of recovery after illness and guide targeted micronutrient-based interventions.
Therapeutic Approaches
Therapeutic strategies targeting abnormal levels of selenoprotein P aim to enhance the functional availability of selenium in tissues, supporting essential biological pathways such as antioxidant protection, thyroid homeostasis, neuromuscular function, and immune system balance.
The measurement of selenoprotein P serves as a guidance tool for personalized interventions, particularly in the context of functional and preventive medicine, and it may support the evaluation of the need for dietary adjustments, targeted selenium supplementation, or monitoring of therapeutic response in comorbid conditions such as hypothyroidism or chronic fatigue.
Last updated: 20/1/2026
