Why You're Always Tired: The Biomarkers Behind Chronic Fatigue

"I'm just tired all the time." It's one of the most common complaints in general practice — and one of the most frequently dismissed. Fatigue is a symptom, not a diagnosis, and its causes range from simple nutritional deficiencies to complex hormonal imbalances. The good news? A targeted blood panel can often identify the root cause.

The Systematic Approach to Fatigue

Rather than testing everything at once, experienced clinicians follow a logical investigation pathway. Starting with the most common and treatable causes, then working through to less obvious culprits, ensures efficient diagnosis without unnecessary testing.

1. Iron and Ferritin: The Most Common Culprit

Iron deficiency is the world's most prevalent nutritional deficiency, and fatigue is its hallmark symptom. Critically, you can be iron-depleted without being anaemic — your haemoglobin may be "normal" while your ferritin (iron stores) is dangerously low. A ferritin below 30 μg/L is associated with fatigue even without anaemia. Optimal levels are 70–100 μg/L for energy and cognitive function.

Women of reproductive age are particularly vulnerable due to menstrual blood loss. If your ferritin is below 50 μg/L and you're experiencing fatigue, iron supplementation is warranted — but always test first, as excess iron is harmful.

2. Vitamin B12 and Folate

B12 deficiency affects up to 20% of older Australians and is increasingly common in younger adults, particularly those following plant-based diets. Symptoms include fatigue, brain fog, pins and needles, and mood disturbances. The standard lower limit of 150 pmol/L is considered too low by many experts — optimal is above 300 pmol/L. Folate (vitamin B9) works in concert with B12, and deficiency in either impairs red blood cell production and methylation.

3. Thyroid Function: TSH, Free T3, and Free T4

The thyroid gland is the master regulator of metabolism, and even subtle dysfunction can cause profound fatigue. Standard TSH testing has a wide reference range (0.5–4.0 mIU/L), but many patients with "normal" TSH between 3.0–4.0 experience hypothyroid symptoms. Request a full thyroid panel including free T3 and free T4 to get the complete picture.

Subclinical hypothyroidism — elevated TSH with normal T3/T4 — is particularly common in women and is often missed on standard testing. Symptoms include fatigue, weight gain, cold intolerance, dry skin, and brain fog.

4. Cortisol: The Stress Hormone

Cortisol follows a natural diurnal rhythm — peaking in the early morning and declining throughout the day. Chronic stress, poor sleep, and overtraining can dysregulate this pattern, leading to either elevated cortisol (anxious fatigue, wired-but-tired) or flattened cortisol curves (burnout, adrenal insufficiency). Morning cortisol below 200 nmol/L warrants further investigation.

5. Vitamin D

Despite Australia's reputation as the sunburnt country, vitamin D deficiency is remarkably common — particularly in southern states during winter, in office workers, and in people with darker skin. Low vitamin D (below 75 nmol/L) is associated with fatigue, muscle weakness, and depressed mood. Aim for 100–150 nmol/L for optimal energy and immune function.

6. Inflammation Markers

Chronic low-grade inflammation (elevated hs-CRP, ESR) can drive persistent fatigue independently of other causes. If your inflammatory markers are elevated, investigate further — common drivers include gut dysfunction, autoimmune conditions, chronic infections, and metabolic syndrome.

When to See Your GP

If fatigue persists for more than 2–4 weeks despite adequate sleep and rest, it's time for blood work. Request a comprehensive panel including full blood count, iron studies (including ferritin), B12, folate, thyroid function (TSH, fT3, fT4), vitamin D, hs-CRP, and fasting glucose. Armed with these results, you and your GP can identify the cause rather than treating the symptom.

References

1. World Health Organization. Iron Deficiency Anaemia: Assessment, Prevention, and Control. WHO/NHD/01.3. Geneva: WHO; 2001.
2. Vaucher P, et al. Effect of iron supplementation on fatigue in nonanemic menstruating women with low ferritin. CMAJ. 2012;184(11):1247-1254. doi:10.1503/cmaj.110950
3. Stabler SP. Vitamin B12 Deficiency. N Engl J Med. 2013;368(2):149-160. doi:10.1056/NEJMcp1113996
4. Canaris GJ, et al. The Colorado Thyroid Disease Prevalence Study. Arch Intern Med. 2000;160(4):526-534. doi:10.1001/archinte.160.4.526