Tiredness that does not lift is rarely one single thing. It is usually several ordinary causes layered on each other, sometimes with a lab finding underneath that is easy to check and easy to miss. This page explains both halves. It is educational content and not medical advice.
Most persistent tiredness traces back to everyday causes rather than to a disease, and sleep is the first place to look. Research on fatigue as a presenting complaint describes it as one of the most common reasons adults consult a primary care clinician, with causes spanning sleep, mood, medication effects, lifestyle and a smaller number of organic findings[1].
Sleep quantity, quality and timing. Seven hours in bed is not seven hours of sleep. Fragmented sleep from alcohol, late caffeine, a warm room, a snoring partner, a young child or untreated sleep-disordered breathing can leave total sleep time looking adequate while restorative sleep is cut short. Irregular bed and wake times do the same thing by pushing your body clock out of step with your schedule. Shift work and frequent travel across time zones do this on a larger scale, and consensus work on travel fatigue and jet lag describes how circadian misalignment degrades alertness and recovery for days[2].
Stress and mental load. Sustained psychological stress raises arousal at night and lowers energy during the day. Low mood and anxiety also present as tiredness far more often than people expect, and fatigue is frequently the symptom a person notices first.
Training load and recovery. Hard training produces real fatigue that is both muscular and central, and research on translating fatigue to human performance describes how perception of effort rises alongside physical limits[3]. Heavy weeks without easy weeks, or a sudden jump in volume, will flatten you. Recovery nutrition is an active research area with developing evidence, including integrative reviews of supplements studied for muscle damage recovery and fatigue responses[4].
Eating patterns. Chronically eating less energy than you expend produces tiredness, poor training response, cold hands and low motivation. Very low carbohydrate intake alongside high training volume does something similar in many people. Low intake also makes micronutrient gaps more likely.
Alcohol and medications. Alcohol shortens deep sleep even when it speeds the onset of sleep. Immune signalling itself can produce profound fatigue, which is why interferon-based therapies have fatigue as a well-documented effect[5].
Life stage and recent illness. Pregnancy, the postpartum period and perimenopause all change sleep and energy. Tiredness after an infection is common, and a systematic review of post-COVID-19 syndrome found fatigue and cognitive difficulty among the most frequently reported persistent symptoms[6].
Six routine markers explain most of the lab-findable reasons for ongoing tiredness, and they are inexpensive and widely available. None of them diagnoses anything on its own.
| Marker | Why it is linked to this symptom | What a result can and cannot tell you |
|---|---|---|
| Ferritin | Reflects stored iron, which is needed for oxygen transport and mitochondrial energy production | Low ferritin shows depleted stores but not the reason for them. It can read falsely normal during inflammation |
| Hemoglobin | Carries oxygen to muscle and brain, so a drop reduces oxygen delivery | Shows whether anemia is present, not which deficiency, blood loss or condition sits behind it |
| TSH | The pituitary signal that rises when thyroid hormone output falls, and thyroid hormone sets metabolic rate | Flags a thyroid pattern worth investigating. One value is a snapshot and is usually repeated with free T4 |
| Vitamin B12 | Needed for red blood cell production and for nerve myelin | A low result supports further testing. Mid-range results can still sit alongside functional deficiency |
| Hemoglobin A1C | Estimates average blood glucose over roughly the past three months | Shows glucose control over time. It does not show daily swings, and red cell turnover can distort it |
Typical lab reference ranges for these markers look roughly like the table below.
| Marker | Typical lab reference range | Units |
|---|---|---|
| Ferritin | approximately 10 to 200 (female) and 20 to 400 (male) | ng/mL |
| Hemoglobin | approximately 11.5 to 16.0 (female) and 13.0 to 17.5 (male) | g/dL |
| TSH | approximately 0.45 to 4.5 | mIU/L |
| Vitamin B12 | approximately 211 to 946 | pg/mL |
| Vitamin D (25-OH) | approximately 30 to 100, with many labs regarding under 20 as deficient | ng/mL |
| Hemoglobin A1C | under 5.7 normal, 5.7 to 6.4 the prediabetes range, 6.5 and above the diabetes range | % |
These are typical lab reference ranges and they vary by laboratory, assay, age and sex. Always read your result against the range printed on your own report, and go through it with a clinician.
Low ferritin is one of the most common checkable reasons for ongoing tiredness, particularly in people who menstruate, endurance athletes and people eating little red meat.
Ferritin is the protein that stores iron, so the result is the closest routine estimate of how much iron you have banked. Iron sits at the centre of oxygen transport and of energy production inside mitochondria, and a narrative review of micronutrients and fatigue describes iron as one of the nutrients most consistently associated with tiredness and reduced capacity for work[7]. Stores can fall well before hemoglobin drops, which is why someone can feel flat while a standard blood count still reads normal.
Other things move this marker. Ferritin rises during inflammation, infection, liver stress and heavy alcohol use, so a result inside the normal range can still sit on top of depleted stores when inflammatory markers are elevated. It falls with blood loss of any kind, including heavy periods, gastrointestinal bleeding and frequent blood donation.
A low result shows depleted iron stores. It may reflect intake, absorption or ongoing loss, and the reason matters more than the number itself. It is worth discussing with a clinician, who will usually look at the full blood count, inflammatory markers and your history together. You can read what the marker measures on the Ferritin page.
Hemoglobin is the molecule that carries oxygen in red blood cells, so when it falls, every tissue gets less oxygen per heartbeat and ordinary effort starts to feel hard.
That is the mechanism behind anemia-related tiredness. Breathlessness on stairs, a racing heart during easy exercise, pale skin and poor exercise tolerance often arrive alongside it. The link is clearest in conditions where anemia is common. Reviews of fatigue in chronic kidney disease describe anemia as one contributor among several, including inflammation and disturbed sleep[8], and reviews of fatigue in people receiving maintenance hemodialysis describe the same multi-factor picture[9].
Plenty of other things change hemoglobin. Iron, B12 and folate deficiency, blood loss, pregnancy, altitude, hydration status and plasma volume expansion in endurance athletes all shift the number. Dehydration can make a result look better than it is, and a large training block can make it look worse.
A low result shows reduced oxygen-carrying capacity. It does not say why, and the why is what guides care. A result below your lab range is worth discussing with a clinician promptly rather than monitoring alone. The Hemoglobin page covers what the marker measures.
An underactive thyroid slows metabolic rate across the body, and tiredness is one of its most common features, which is why TSH is on almost every fatigue panel.
TSH is the signal the pituitary sends to the thyroid. When thyroid hormone output drops, TSH typically rises, so a high TSH is an early indicator that the system is working harder to keep output up. Reviews of endocrine causes of fatigue describe thyroid dysfunction among the endocrine conditions in which fatigue is a leading symptom, alongside adrenal and pituitary disorders[10]. Cold intolerance, dry skin, constipation, weight change and slowed thinking often appear with it.
Other things move TSH. It varies across the day and is usually higher in the early morning. Acute illness, pregnancy, recent steroid use, high-dose biotin supplements that interfere with some assays and several medications all shift results. One value is a snapshot, so clinicians commonly repeat it and add free T4 and sometimes thyroid antibodies before drawing conclusions.
An out-of-range TSH is a pattern to investigate, not a diagnosis. Bring it to a clinician, who decides what further testing is appropriate. The TSH page explains the marker in more detail.
Low vitamin B12 is one plausible reason for tiredness because B12 is required for red blood cell production and for healthy nerve function, so a shortfall can show up as both low energy and neurological symptoms.
Pins and needles, numbness, balance problems, a sore tongue and brain fog can accompany it. People following vegan or near-vegan diets, older adults, people with reduced stomach acid and people who have had gut surgery are more likely to run low. Evidence on supplementation is more mixed than many articles suggest.[11] A systematic review and meta-analysis examining B12 supplementation and outcomes including fatigue, cognition and depressive symptoms found the overall picture far from clear-cut[11], which is one reason clinicians check status rather than assume a deficiency.
Other things move the number. Some medication classes used long term lower absorption, pregnancy alters levels, and supplementation taken before the draw can push a serum result up while tissue status lags. Serum B12 is also an imperfect measure, so clinicians sometimes add methylmalonic acid or homocysteine when results sit in a grey zone.
A low or borderline result is worth discussing with a clinician alongside your diet, medications and symptoms. See the Vitamin B12 page for what the marker measures.
Low vitamin D is associated with fatigue, although association is not the same as cause, and raising a level does not reliably change how a person feels.
Vitamin D acts on receptors in muscle, immune cells and many other tissues, and the narrative review of vitamins and minerals for energy and fatigue discusses it among the micronutrients with a plausible link to tiredness and low mood[7]. Low status is common in people with limited sun exposure, darker skin, higher body fat, malabsorption conditions and indoor work through winter.
Other things move the result. Season, latitude, recent sun exposure, supplement use, body composition and some medications all shift 25-hydroxyvitamin D. A single measurement in late summer and the same person in late winter can look like two different people.
A low result shows current status rather than a cause of symptoms. It is worth raising with a clinician, who considers your levels alongside bone health, calcium, medication list and lifestyle before recommending anything. The Vitamin D (25-OH) page covers the marker itself.
Glucose control is one possible reason for daily energy swings, and hemoglobin A1C is the standard way to see the longer trend behind them.
A1C estimates average blood glucose over roughly the lifespan of your red blood cells, which is approximately three months. Sustained high glucose is associated with tiredness, thirst, frequent urination and blurred vision, and insulin resistance is often present for years before glucose rises enough to be noticed. Large meals heavy in refined carbohydrate can also produce a post-meal slump in people whose A1C sits inside the normal range, which is a pattern about meals rather than a laboratory finding.
Other things move A1C. Anything that changes red cell lifespan distorts it, including iron deficiency, recent blood loss, hemolysis, pregnancy, chronic kidney disease and some hemoglobin variants. That is one reason A1C is read next to hemoglobin and ferritin rather than alone.
A result above the standard range is suboptimal rather than a diagnosis, and it is worth discussing with a clinician promptly so the next step can be decided properly. The Hemoglobin A1C page explains what the marker measures.
A completely normal panel is useful information, but it does not rule out several of the most common reasons for persistent tiredness, because many of them leave no mark in blood.
Obstructive sleep apnea is the clearest example. It needs a sleep study, not bloodwork, and loud snoring with witnessed pauses in breathing or morning headaches should be raised with a clinician regardless of lab results. Chronic insomnia, shift work and an irregular sleep schedule are the same story. So are depression and anxiety, which commonly present as fatigue and are assessed clinically rather than by blood test.
Neurological and post-viral fatigue also sit outside routine panels. Work on the mechanisms of fatigue in multiple sclerosis describes central processes involving brain networks and cognition rather than anything measurable in a standard blood draw[12], and the same distinction applies to fatigue that persists after infection[6]. Medication effects, chronic pain, perimenopausal sleep disruption, under-eating relative to training, celiac disease and food intolerances round out the list, and reviews of fatigue in primary care stress how often the explanation is found in the history rather than the laboratory[1].
The practical reading of a normal panel is that several specific causes are now less likely, so attention moves to sleep, mood, load and medications.
Some symptoms alongside tiredness deserve prompt or urgent attention rather than another month of waiting, and none of them should be self-diagnosed.
Seek urgent care for chest pain, breathlessness at rest, fainting, a fast or irregular heartbeat that does not settle, or confusion. Arrange a prompt appointment for unintentional weight loss, fever that keeps returning, night sweats, blood in your stool or black stools, vomiting blood, very heavy periods, new or severe headache, new weakness, numbness or vision change, or tiredness that has worsened steadily over weeks. Snoring with witnessed pauses in breathing also deserves a prompt conversation, since sleep-disordered breathing is both common and assessable.
Mental health symptoms count here too. If low mood, hopelessness or loss of interest have come with the tiredness, that is a reason to speak with a clinician soon rather than later. If you are having thoughts of self-harm, call or text 988 (US) or your local emergency number now.
Never ignore an abnormal result, and never stop or change a prescribed medication on your own. Take the report to a clinician or licensed provider and discuss what it means for you.
All six markers come from a single standard blood draw, and they are usually ordered together rather than one at a time. The FitScript Essential panel includes ferritin, hemoglobin, TSH, vitamin B12, vitamin D and hemoglobin A1C alongside the rest of a baseline workup, which lets each result be read in the context of the others rather than in isolation.
After testing, Atlas, the AI health advisor, explains what each result means in plain language and how markers relate to each other, and FitScript membership covers 150+ biomarkers tested per year so that changes over time are visible rather than guessed at. Any decision about a prescription therapy is made by a licensed provider who evaluates whether it is appropriate for the individual after reviewing the results and the clinical picture.
Retesting matters as much as testing. A single value is a snapshot, and most of these markers move with season, training, diet and illness, so a second draw months later often says more than the first one did.
Constant tiredness usually has several overlapping causes rather than one. Short or fragmented sleep, chronic stress, heavy training without recovery, under-eating and alcohol are the most common. A smaller group of people have a lab finding such as low ferritin, low B12 or an out-of-range TSH. A clinician interprets bloodwork alongside your history.
Start with the three things that explain most cases. Those are total sleep time, sleep quality and the gap between your energy intake and your training load. If those look reasonable and tiredness has lasted several weeks, a basic blood panel covering iron stores, thyroid signalling, B12, vitamin D and glucose control is worth discussing.
Ongoing fatigue is a symptom with a wide differential, so it is mapped rather than guessed. Clinicians look at sleep, mood, medications, recent illness and training, then order bloodwork to check iron stores, red blood cells, thyroid signalling, B12, vitamin D and average glucose. Most explanations are found in the history.
Sleeping more without feeling better points toward sleep quality rather than quantity. Sleep-disordered breathing, alcohol before bed, irregular timing, low mood and untreated pain all fragment sleep while total hours look fine. Bloodwork can rule several causes in or out, but a sleep assessment is often the more useful next step.
Six markers cover most of what bloodwork can show. Ferritin reflects iron stores, hemoglobin shows oxygen-carrying capacity, TSH screens thyroid signalling, vitamin B12 supports red cells and nerves, vitamin D is associated with fatigue, and hemoglobin A1C shows average glucose. A clinician decides which are appropriate for you.
Yes, iron stores can be depleted before hemoglobin falls, which is why ferritin is measured alongside a full blood count. Iron is one of the nutrients most consistently associated with fatigue and reduced work capacity. Inflammation raises ferritin, so results are read together with inflammatory markers and your history.
Several weeks of unexplained tiredness that has not improved with better sleep and a lighter training load is a reasonable point to test. Sooner is appropriate when other symptoms are present, such as breathlessness, heavy periods, unintentional weight loss or new neurological symptoms. Speak with a clinician about timing and which markers fit.
It means several specific explanations are now less likely, which is genuinely useful. It does not rule out sleep apnea, chronic insomnia, low mood, medication effects, under-eating or post-viral fatigue, since none of those show on a routine panel. The next step is usually a clinical assessment rather than more blood tests.