Selenium RBC (Red Blood Cell) Biomarker Testing
It measures longer-term selenium status in red blood cells to assess deficiency or excess, with convenient ordering and Quest-based lab access via Vitals Vault.
With Vitals Vault, you have access to a comprehensive range of biomarker tests.
Selenium is a trace mineral your body uses to build “selenoproteins,” which support antioxidant defenses, thyroid hormone activation, and immune signaling. Because you only need small amounts, both too little and too much can matter.
A Selenium RBC test measures selenium inside your red blood cells (RBCs). Compared with a standard serum/plasma selenium test, RBC selenium is often used as a steadier snapshot of longer-term status, since red blood cells circulate for about 3–4 months.
This test is most useful when you are trying to confirm suspected deficiency, evaluate possible excess from supplements, or monitor a plan over time. Your result is one piece of the picture and is best interpreted alongside your symptoms, diet, medications, and related labs.
Do I need a Selenium RBC test?
You might consider a Selenium RBC test if you have a reason to suspect your selenium intake or absorption has been off for a while. This can come up if you follow a very restricted diet, avoid many selenium-containing foods, have a history of gastrointestinal conditions that affect absorption, or have had bariatric or other intestinal surgery.
Testing can also be helpful if you are taking selenium supplements (including “thyroid support” or antioxidant blends) and want to confirm you are not drifting into an unnecessarily high range. Selenium has a relatively narrow window between “enough” and “too much,” so checking levels can be more reliable than guessing from a label.
If you are evaluating thyroid symptoms (fatigue, cold intolerance, hair changes, constipation, palpitations) or immune concerns, selenium is not the first test to order, but it can be a meaningful add-on when the basics do not fully explain what you are feeling.
This test supports clinician-directed care and follow-up planning, but it cannot diagnose a specific disease on its own.
Selenium RBC is performed in a CLIA-certified laboratory; results should be interpreted with your clinician and are not a standalone diagnosis.
Lab testing
Order Selenium RBC through Vitals Vault and complete your draw at a participating lab location.
Schedule online, results typically within about a week
Clear reporting and optional clinician context
HSA/FSA eligible where applicable
Get this test with Vitals Vault
With Vitals Vault, you can order a Selenium RBC test without needing to schedule a separate doctor visit just to obtain a lab requisition. You complete your blood draw at a participating lab location, and your results are delivered in a clear, easy-to-review format.
If your number is low, high, or simply doesn’t match how you feel, PocketMD can help you think through common next steps to discuss with your clinician, such as checking related nutrients, reviewing supplement doses, or choosing a reasonable retest interval.
Selenium is a “context” biomarker. Vitals Vault makes it easy to add companion labs when you want a broader map (for example, thyroid markers or other micronutrients) rather than making decisions from a single value.
Key benefits of Selenium RBC testing
- Gives a longer-term view of selenium status than a single serum/plasma snapshot for many people.
- Helps confirm suspected selenium deficiency when diet, absorption, or medical history raises concern.
- Flags potential over-supplementation when you are using selenium-containing products regularly.
- Adds useful context when you are working up thyroid-related symptoms alongside standard thyroid labs.
- Supports safer dose adjustments by giving you a baseline before you change supplements or diet.
- Helps you track trends over time, which is often more actionable than a one-time result.
- Guides smarter follow-up testing (and avoids guesswork) when symptoms and routine labs don’t align.
What is Selenium RBC?
Selenium RBC is a measurement of the amount of selenium contained inside your red blood cells. Because red blood cells live for roughly 120 days, the selenium inside them can reflect your selenium exposure and status over the past few months rather than only the last few days.
Selenium is incorporated into proteins that do important work in the body. One of the best-known roles is antioxidant defense through enzymes such as glutathione peroxidases, which help limit oxidative damage. Selenium is also involved in thyroid hormone metabolism through deiodinase enzymes that help convert thyroxine (T4) into the more active triiodothyronine (T3).
RBC testing can be particularly helpful when short-term intake fluctuates (for example, intermittent supplement use or variable diet). That said, no selenium test is perfect for every situation. Your clinician may choose serum/plasma selenium, RBC selenium, or both depending on your question and timing.
What do my Selenium RBC results mean?
Low Selenium RBC
A low Selenium RBC result generally suggests that your selenium status has been low over the past several weeks to months. This can happen with low dietary intake, malabsorption, long-term restrictive eating patterns, or increased needs during certain health states. If your result is low, it is reasonable to review your diet and any digestive history, and to consider checking related markers that can travel together (such as zinc, copper, or vitamin B12) depending on your situation. Your clinician may also look at thyroid labs if symptoms point in that direction.
In-range (optimal) Selenium RBC
An in-range result usually means your longer-term selenium exposure is adequate for most physiologic needs. If you are taking a supplement, an in-range value can support staying at the same dose or even simplifying your regimen if you are using multiple products that contain selenium. If you still have symptoms, it is worth remembering that selenium is only one contributor to thyroid function, immune health, and energy, so follow-up often focuses on broader labs and lifestyle factors rather than pushing selenium higher.
High Selenium RBC
A high Selenium RBC result often points to higher-than-needed intake over time, most commonly from supplements or multiple fortified products taken together. Persistently high selenium can increase the risk of side effects (sometimes called selenosis), which may include hair or nail changes, gastrointestinal upset, or a metallic/garlic-like breath odor, although symptoms vary and are not specific. If your result is high, do not “balance it out” with other supplements; instead, review all sources of selenium and discuss a safer plan and retesting timeline with your clinician.
Factors that influence Selenium RBC
Supplement dose and consistency are the biggest drivers of higher RBC selenium, especially when you combine a standalone selenium product with multivitamins or thyroid-support blends. Diet matters too, but selenium content in foods can vary widely by soil and sourcing, which is why intake estimates are often imprecise. Gastrointestinal conditions or surgeries that affect absorption can contribute to low values, and inflammation or acute illness can complicate interpretation of micronutrient labs in general. Different labs and methods may use different reference ranges, so interpret your result using the range printed on your report and focus on trends when retesting.
What’s included
- Selenium, Rbc
Frequently Asked Questions
What is the difference between Selenium RBC and serum selenium?
Serum (or plasma) selenium reflects what is circulating in the liquid portion of your blood and can shift with more recent intake. Selenium RBC measures selenium inside red blood cells, which can reflect longer-term status because RBCs live for about 120 days. Your clinician may choose one or the other based on whether you are assessing recent changes or longer-term patterns.
Do I need to fast for a Selenium RBC test?
Fasting is not usually required for Selenium RBC. However, if you are getting other labs at the same time (such as lipids or glucose/insulin testing), your overall order may require fasting. Follow the instructions provided with your lab order.
How long does it take for Selenium RBC to change after I change my supplement or diet?
Because RBC selenium reflects longer-term exposure, it typically changes more slowly than serum selenium. Many people retest in roughly 8–12 weeks after a meaningful change, but timing depends on how high or low your starting value is and what your clinician is monitoring. If your level is high, your clinician may recommend stopping or reducing selenium and retesting after an appropriate washout period.
What can cause a falsely high or low Selenium RBC result?
Lab-to-lab method differences and reference ranges can affect how a number is labeled. Recent supplement use is a common reason for higher results, while malabsorption or long-term low intake can drive lower results. Severe illness and inflammation can complicate micronutrient interpretation, so if you were acutely unwell around the time of the draw, discuss whether retesting when stable makes sense.
What symptoms are associated with low selenium?
Selenium deficiency can be subtle and may overlap with many other issues. Some people report low energy, frequent infections, or hair and nail changes, but these symptoms are not specific to selenium. That is why testing is most helpful when you have a clear risk factor (dietary restriction, malabsorption, or long-term low intake) and you interpret the result alongside other labs.
Can too much selenium be harmful?
Yes. Selenium is essential, but excessive intake over time can cause side effects and may be harmful. High results are often linked to supplements or stacking multiple products that contain selenium. If your Selenium RBC is high, review all sources (including multivitamins and specialty blends) and make changes with clinician guidance.