Under-Recognised Causes of Fatigue: What the Evidence Shows

Why Under-Recognised Causes of Fatigue Matter

Fatigue ranks among the ten most frequent reasons for a primary care consultation, with reported prevalence ranging from 5% to 38% depending on the population studied. Despite how commonly patients present with this complaint, obtaining a definitive diagnosis proves elusive in the majority of cases. One primary care study found that only 27% of young adults with new-onset fatigue received a condition-based diagnosis, while another reported that fewer than 9% had a clear explanation at one year of follow-up.

The clinical and economic consequences are substantial. Patients whose fatigue remains unexplained often undergo repeated consultations, extensive laboratory panels, and specialist referrals — frequently without resolution. Meanwhile, conditions such as non-anaemic iron deficiency, mild obstructive sleep apnea, and medication side effects may be contributing to symptoms but are easily overlooked because standard screening algorithms do not consistently target them. This evidence review synthesises findings from 11 key studies to identify five specific diagnoses that warrant closer attention.

Key Findings: Under-Recognised Causes of Fatigue

1. Non-Anaemic Iron Deficiency: A Treatable and Overlooked Contributor

Two well-designed RCTs provide convergent evidence that iron deficiency can cause clinically meaningful fatigue even when haemoglobin levels are within the normal range. Vaucher et al. (2012) randomised 198 menstruating women with ferritin below 50 µg/L and normal haemoglobin to 12 weeks of oral iron or placebo; the iron group experienced a fatigue reduction roughly 19 percentage points greater than placebo (95% CI −34.5 to −3.2; p = 0.02). These results closely replicate an earlier trial by Verdon et al. (2003), which found a similar magnitude of benefit over a shorter treatment period.

A 2018 systematic review of RCTs by Houston et al. confirmed these findings across a pooled sample of 634 non-anaemic, iron-deficient adults, reporting consistent improvements in both self-reported fatigue and functional physical capacity. The biological plausibility is well-supported: iron serves as a cofactor for enzymes involved in neurotransmitter metabolism and mitochondrial energy production, pathways that can be impaired before anaemia develops. Approximately 20% of European women of childbearing age have ferritin concentrations below 15 µg/L, yet many remain undiagnosed because standard complete blood count screening does not flag tissue iron depletion in the absence of frank anaemia.

2. Occult Obstructive Sleep Apnea: Fatigue Without Classic Symptoms

Obstructive sleep apnea affects an estimated 3–7% of the general adult population, and evidence from large population cohorts consistently demonstrates that a majority of those affected remain undiagnosed. The Wisconsin Sleep Cohort Study and subsequent epidemiological analyses by Young et al. (2002) and Punjabi (2008) found that even mild, undiagnosed OSA was independently associated with daytime sleepiness, fatigue, and diminished quality of life — outcomes that persisted after adjustment for body mass index, age, and other confounders.

The diagnostic gap is particularly pronounced in women, who frequently present with fatigue and low energy rather than the classically recognised triad of loud snoring, witnessed apneas, and excessive somnolence. Clinic-based referral ratios show men outnumbering women 5–8:1, yet community studies suggest the true sex ratio is closer to 2–3:1, indicating substantial under-recognition in female patients. Given the associated cardiovascular, metabolic, and neurocognitive risks of untreated OSA, screening with validated tools such as the STOP-BANG questionnaire warrants consideration in any patient with persistent unexplained fatigue.

3. Subclinical Hypothyroidism: Treatment May Not Relieve Fatigue

Subclinical hypothyroidism — defined by elevated TSH with normal free T4 — affects between 8% and 18% of adults aged 65 and over and is frequently cited as a cause of fatigue. However, the largest trial to date challenges this assumption. The TRUST trial (Stott et al. 2017) randomised 737 older adults with persistent subclinical hypothyroidism to levothyroxine or placebo and found no clinically meaningful difference in tiredness or hypothyroid symptom scores at one year, despite effective TSH normalisation in the treatment arm.

A nested fatigability study within the TRUST trial (Collet et al. 2020) extended these findings using a more sensitive outcome measure — the Pittsburgh Fatigability Scale — and similarly found no benefit of levothyroxine on physical or mental fatigability. These RCT data suggest that mildly elevated TSH may be an incidental finding rather than the cause of fatigue in many older patients, and that initiating thyroid replacement therapy on the basis of fatigue alone is unlikely to produce benefit in this population.

4. Vitamin D Deficiency: Biologically Plausible but Inconsistently Supported

Low serum 25-hydroxyvitamin D has been cross-sectionally associated with fatigue across multiple populations, and the biological rationale involving mitochondrial function, muscle physiology, and neuroinflammatory pathways is well-described. However, interventional evidence remains mixed. Nowak et al. (2016) conducted a double-blind RCT in 120 otherwise healthy adults with vitamin D deficiency (<20 µg/L) and found that a single high dose of vitamin D₃ produced significantly greater fatigue reduction than placebo at four weeks (mean FAS decrease −3.3 vs −0.8; p = 0.01).

In contrast, a large Mendelian randomisation study using UK Biobank data (n = 327,478) found little genetic evidence supporting a causal relationship between lifelong vitamin D status and self-reported fatigue (OR 1.05; 95% CI 0.87–1.27). The EViDiF study (Roy et al. 2014) observed significant fatigue improvement after vitamin D correction, but its pre-post design without a placebo arm limits causal inference. Taken together, these data suggest that vitamin D supplementation may provide short-term symptomatic benefit in individuals with documented deficiency, but the evidence does not support routine screening or supplementation for fatigue in vitamin D–replete patients.

5. Medication-Induced Fatigue: The Overlooked Iatrogenic Cause

Perhaps the most readily modifiable yet frequently neglected cause of fatigue is medication side effects. The UCSD Statin Study (Golomb et al. 2012) — a randomised, placebo-controlled trial involving 1,016 healthy adults — found that both simvastatin and pravastatin were associated with statistically significant increases in fatigue and reduced energy with exertion compared to placebo. The effect was more pronounced in women taking simvastatin. These findings are consistent with proposed mechanisms involving impaired mitochondrial energy production.

Beyond statins, beta-blockers, antihistamines, antidepressants (particularly sedating SSRIs and TCAs), antiepileptics, and proton pump inhibitors are all commonly prescribed medications with fatigue listed as a recognised adverse effect. In elderly patients receiving polypharmacy — a population where fatigue prevalence is highest — the cumulative sedative and energy-depleting burden of multiple concurrent medications may be substantial. A comprehensive medication review, with specific attention to temporal correlation between drug initiation and fatigue onset, represents a low-cost, high-yield diagnostic step.

What the Evidence Does Not Show

Nearly all iron supplementation trials for non-anaemic fatigue enrolled premenopausal women exclusively. Whether these findings generalise to men, postmenopausal women, or older adults with low ferritin remains unknown; dedicated RCTs in these populations have not been conducted. The optimal ferritin threshold for initiating empiric iron therapy in fatigued patients — whether 30, 50, or 75 µg/L — also lacks consensus-level evidence.

For obstructive sleep apnea, while the epidemiological link to fatigue is well-established, RCTs of CPAP therapy that use fatigue-specific primary endpoints (as opposed to sleepiness scales like the Epworth) are lacking. Whether treating mild OSA specifically improves fatigue, independent of daytime somnolence, is an unanswered question.

The evidence on vitamin D and fatigue lacks adequately powered, placebo-controlled trials in otherwise healthy, non-deficient adults. Current data cannot distinguish whether symptomatic improvement after vitamin D supplementation reflects a true pharmacological effect or regression to the mean. The role of polypharmacy-related fatigue, while clinically intuitive, lacks controlled deprescribing studies that measure fatigue as a primary outcome.

Evidence Grade + Bottom Line