Selenium

Urinary selenium can help assess recent exposure, especially when a supplement, workplace, or environmental source may be supplying too much. The result also depends on urine concentration and how your kidneys handle selenium. It doesn't directly measure how much your tissues need or store.

Its use for predicting heart disease, diabetes, or longevity remains exploratory. Studies have linked urinary selenium with these outcomes, but they haven't established an optimal urine level or shown that adjusting your intake to change this number improves health.

What Urinary Selenium Reflects

Your body builds selenium into 25 proteins involved in thyroid hormone metabolism, immune function, and protection against cell damage. These are called selenoproteins. The kidneys help regulate selenium balance by removing selenium-containing compounds in urine.

This test measures total selenium in a spot urine sample using ICP-MS. This laboratory technique measures the amount of an element in the sample. It combines selenium from different chemical forms into one result; it doesn't identify each form separately. Selenosugars are among the main compounds in human urine, and the mixture varies with intake, the form consumed, and inherited differences in metabolism.

Urine is sensitive to recent intake, but there isn't a fixed one- or two-day window that applies to every selenium source. Some forms remain in the body longer than others. Serum and plasma selenium also respond to recent intake, while whole-blood and red-cell measurements generally reflect a longer period.

Urine has performed reasonably well for comparing selenium status between communities in Malawi, but it was much less useful for distinguishing individuals within those communities. Findings from Zimbabwe also showed poor agreement with plasma selenium at smaller geographic scales. A population-level relationship doesn't make a spot sample a reliable nutritional assessment for you.

Deficiency and Toxicity

Severe selenium deficiency can damage heart muscle. It contributes to Keshan disease in regions with very low selenium intake, although infection and other factors also matter. Chronic excess can cause hair loss, brittle or damaged nails, garlic-smelling breath, stomach upset, skin changes, and nerve symptoms.

These conditions require interpretation of exposure, symptoms, and appropriate laboratory findings together. A high urine result can support evidence of excess exposure, but it doesn't establish how much harm has occurred. A low urine result alone cannot diagnose deficiency.

Heart Disease

In 2,727 American Indian adults in the Strong Heart Study, higher selenium in spot urine was associated with a higher rate of developing cardiovascular disease. The adjusted difference was about 11% when comparing the 75th with the 25th percentile. This was a relative comparison within a population with relatively high selenium exposure, not a boundary between safe and unsafe results.

Evidence from blood measurements has often pointed in the opposite direction. A review of 20 cohort studies linked higher serum or plasma selenium, or higher levels of the selenium transport protein selenoprotein P, with lower mortality. Those are different measurements from urinary selenium. Neither set of observational findings establishes that changing your urine result will change your cardiovascular risk.

Survival After a Kidney Transplant

In 693 Dutch kidney transplant recipients followed for a median of 8.2 years, those in the lowest third of 24-hour urinary selenium excretion had more than twice the mortality rate of those in the highest third. The association persisted after adjustment for several health and dietary factors. Protein intake was an important dietary determinant of selenium intake in this group.

The researchers measured the total amount excreted during a full day. That differs from the concentration in this spot urine test, so the study's comparisons cannot be applied directly to a spot result. Low excretion may reflect nutrition, kidney handling, or illness. The study did not show that raising urinary selenium improves transplant survival.

Brain Health in Later Life

Among 728 older American Indians, selenium measured in urine was compared with brain scans and thinking tests performed about 21 years later. Higher urinary selenium was associated with poorer verbal fluency. Associations with processing speed and MRI measures were less clear after adjustment for other factors.

There were no thinking tests at the time of the original urine collection, so the study couldn't establish a rate of cognitive decline. These findings do not validate urinary selenium as a screen for dementia or brain shrinkage.

Blood Sugar and Liver Markers

A study of 137 healthy, nonsmoking blood donors in Northern Italy reported small, imprecise associations between higher urinary selenium and higher fasting glucose, triglycerides, and LDL cholesterol, with lower HDL cholesterol. A separate analysis of the same group linked urinary selenium with higher ALT. ALT is an enzyme that can rise with liver injury, but this association did not establish that selenium had damaged the liver.

Another analysis in this group found little association between total urinary selenium and a urine marker of DNA damage across the whole sample. Some patterns differed by sex and by the chemical form of selenium measured. It would overstate the findings to say that higher urinary selenium generally showed more DNA damage.

These were measurements taken at one point in time. They cannot establish whether selenium exposure caused the metabolic differences, or whether other dietary and health factors explained them.

Pregnancy and Gestational Diabetes

A Chinese study compared 226 women with gestational diabetes with 452 women without it. Women in the lowest third of urinary selenium had about 2.35 times the odds of gestational diabetes compared with those in the highest third. Urine was collected within three days before the glucose tolerance test, so this was not evidence of a warning signal months before diagnosis.

The finding does not establish selenium deficiency as the cause or validate this urine test for gestational diabetes screening. Glucose testing remains the direct way to assess gestational diabetes.

When Results Can Be Misleading

• Urine dilution: drinking more fluid can lower the concentration without lowering selenium exposure. Creatinine correction helps account for dilution, but muscle mass and creatinine production can also affect the corrected result.
• Recent food and supplements: a recent dose or selenium-rich meal can change the result. Different selenium compounds are absorbed, retained, and excreted differently, so equal doses need not produce equal urine readings.
• Kidney disease and protein loss: reduced filtration and protein leaking into urine can alter selenium excretion. The direction and size of the change depend on the kidney problem; a low result doesn't automatically mean low intake.
• Severe illness: studies in intensive care found changes in blood selenium and urinary losses, including losses associated with tissue breakdown. These findings may reflect acute illness rather than your usual nutritional status.
• Collection and analytical interference: contamination can falsely raise a trace-element result. Recent gadolinium or iodine contrast used for imaging can interfere with trace-element testing, including this ICP-MS measurement.

What Repeated Measurements Can Show

Urinary selenium can vary substantially between visits. In one study of older adults with relatively high selenium exposure, spot urine measurements had poor repeatability. Other smaller studies found less variation, so the amount of fluctuation depends on the population and collection conditions.

Repeated measurements can help distinguish a persistent exposure pattern from an isolated result. They still don't turn urinary selenium into a validated measure of nutritional adequacy, treatment success, or future disease risk.