Understanding Fatigue: Which Biomarkers Can Provide Insight Into Your Energy Levels

You slept seven hours, you've had your coffee, and by mid-afternoon in November you're running on empty again. If you're always tired, a blood test can offer clues about iron, vitamins, thyroid, blood sugar and inflammation. It can't tell you why you feel flat on its own, and no single value will. This article shows what each marker can add, what else moves it, and where a result stops being useful by itself.

Always tired? What a blood test can and can't show

Tiredness rarely has a single cause. In a Dutch study of 571 adults who went to their GP with a new episode of fatigue, about half received no diagnosis within a year that could explain it. Clear physical disease turned up in around 8 percent (Nijrolder et al., 2009).

A German study points the same way. Among 1,700 adults from the Augsburg region, ongoing tiredness was linked with physical inactivity, psychological strain and bodily distress, and inactivity predicted it over seven years. Medical measures as a group, which included haemoglobin, CRP and TSH, showed limited value, although a lower TSH did predict tiredness in women (Atasoy et al., 2026).

So why test at all? Because some causes are measurable and easy to act on, and a result gives you something concrete to bring to your doctor. Think of a blood test as one lens among several. It can show iron stores, vitamin status, thyroid signalling, blood sugar and inflammation. It can't show how well you sleep, how stressed you are or how much you move. If you're not sure where to begin, Feeling off? Where to start with a blood test is a good first read.

Iron and B vitamins: the supply chain behind your energy

Each of these nutrients contributes to the reduction of tiredness and fatigue: iron, vitamin B12, folate, vitamin B6 and vitamin B2 (riboflavin). That's why they come up so often in conversations about tiredness. Whether one of them plays a part in your tiredness is a question for your result and your doctor.

Ferritin, iron and transferrin

Ferritin shows your iron stores, serum iron shows what's travelling in the blood right now, and transferrin saturation shows how full the carrier protein is. In a Swiss trial of 144 women without anaemia but with unexplained fatigue, four weeks of iron lowered fatigue scores more than placebo, but only in women whose ferritin was 50 µg/l or lower (Verdon et al., 2003). A review of 18 trials in iron-deficient adults without anaemia found that iron reduced self-reported fatigue but didn't change measured physical capacity (Houston et al., 2018).

Ferritin has a catch. It rises with inflammation, so in people with inflammatory conditions a normal or even high value can sit alongside iron that isn't available to the body. Transferrin saturation helps you read it (Dignass et al., 2018). What arrives matters as much as what's on the plate: phytate, polyphenols (for example in tea and coffee), calcium, vitamin C and meat all influence how much iron you absorb, and the share absorbed from vegetarian diets is lower (Hurrell and Egli, 2010). Regular blood loss, such as periods, also draws on your stores. More on the topic: Why iron matters and 10 foods to boost it.

Vitamin B12 and folate

B12 and folate work together to build red blood cells. B12 deficiency arises mainly when intake of animal foods is low or absorption is poor. People who eat vegetarian or vegan diets are at risk, malabsorption is most common in older adults, and early shortfalls are subtle and often go unrecognised (O’Leary and Samman, 2010). Metformin taken for more than four months and acid blockers taken for more than twelve months are also listed as risk factors. A US review for family doctors doesn't recommend routine screening for adults at average risk (Langan and Goodbred, 2017).

Thyroid and blood sugar: your pace and your fuel

TSH

TSH is the signal your pituitary gland sends to the thyroid. The thyroid helps set how fast your body uses energy, so TSH is a standard first look when tiredness lingers. A result above or below the lab range is a reason for a conversation with your doctor. In a large trial of 737 adults aged 65 and older with persistently raised TSH and normal free thyroxine, levothyroxine didn't reduce tiredness after a year more than placebo did (Stott et al., 2017). TSH doesn't show the free hormones fT3 and fT4, which belong to a wider panel. More on this: The thyroid-metabolism connection.

Glucose and HbA1c

Fasting glucose is a snapshot of your blood sugar at the moment of the draw. HbA1c reflects your average over roughly the previous three months (Nathan et al., 2008). In people with diabetes, tiredness is a common complaint, and researchers describe physical, psychological and lifestyle routes to it (Fritschi and Quinn, 2010). Without diabetes, a result in range simply takes one possibility off the table.

Vitamin D and hs-CRP: the winter marker and the background signal

Vitamin D

According to the DGE, spending 5 to 25 minutes a day in the midday sun with a quarter of your body surface uncovered is enough for good vitamin D supply in Germany for only about half of the year (DGE, BfR and MRI, 2012). The national health survey shows what that means: among 6,995 adults, one in four people examined in winter had a level below 30 nmol/l, and across the year more than 60 percent were below 50 nmol/l (Rabenberg et al., 2015). The DGE, BfR and MRI call below 30 nmol/l (12 ng/ml) a deficiency and at least 50 nmol/l (20 ng/ml) good supply for the bones. They advise measuring only when there's a justified suspicion of a deficiency or in people at risk.

Cut-offs from the DGE, BfR and MRI. They describe bone health, not tiredness.

In a trial of 120 adults with an average age of 29, who had fatigue and a vitamin D level below 20 ng/ml, a single oral dose of vitamin D lowered fatigue scores more than placebo after four weeks (Nowak et al., 2016). It's one trial in fairly young adults, so it doesn't settle the question. More on the dark months: Winter wellness: vitamin D.

hs-CRP

hs-CRP rises when your body mounts an inflammatory response. A recent infection can lift it, so a high value calls for a recheck once you're well. Its main job here is context: it helps you read ferritin, and it shows whether your body has recently been under inflammatory strain.

Which Aware package covers which marker

No single package covers all six groups. This overview shows what's in each, so you can pick the one that fits your question.