"I'm just getting older." It's the most common explanation men give for persistent fatigue — and often the least accurate. While energy does naturally shift with age, profound or persistent low energy is frequently a signal of an identifiable, often treatable underlying cause. Dismissing it as inevitable aging can mean missing something important.
Here are six of the most common, evidence-based causes of low energy in men — many of which are straightforward to evaluate with a healthcare provider.
Key Takeaways
- Persistent fatigue is rarely "just aging" — it usually has identifiable, frequently treatable causes.
- Sleep apnea, low testosterone, iron deficiency, thyroid dysfunction, depression, and poor metabolic health are among the most common culprits.
- Many causes overlap and reinforce each other — for example, sleep apnea lowers testosterone, which worsens fatigue.
- A basic medical workup (bloodwork plus history) can identify most common causes — fatigue warrants evaluation, not resignation.
1Obstructive Sleep Apnea
One of the most common and underdiagnosed causes of male fatigue. OSA fragments sleep hundreds of times per night without the person being aware, preventing restorative deep sleep. It affects an estimated 1 in 4 middle-aged men, yet most cases go undiagnosed. Classic signs include loud snoring, witnessed pauses in breathing, waking unrefreshed, and daytime sleepiness. A sleep study provides definitive diagnosis, and CPAP treatment can be transformative for energy.[1]
2Low Testosterone (Confirmed)
Clinically low testosterone — confirmed by morning blood tests, not symptoms alone — can cause fatigue, low motivation, and reduced energy. Importantly, low energy attributed to "low T" often has other causes, so proper testing matters. Interestingly, sleep apnea and obesity both suppress testosterone, so fatigue causes frequently cluster together.[2] See our article on testosterone after 40 for the full picture.
3Iron Deficiency (Even Without Anemia)
Often thought of as a women's issue, iron deficiency affects men too — particularly endurance athletes, those with GI bleeding, or dietary restrictions. Notably, low iron stores (ferritin) can cause fatigue even before anemia develops. In men, new iron deficiency always warrants investigation of the cause, as GI blood loss must be ruled out.[3]
4Thyroid Dysfunction
Hypothyroidism (underactive thyroid) slows metabolism and is a classic cause of fatigue, often accompanied by weight gain, cold intolerance, dry skin, and low mood. While more common in women, it affects men too and is easily screened with a simple TSH blood test. It's frequently missed because symptoms develop gradually.[4]
5Depression and Chronic Stress
Fatigue is one of the most common physical symptoms of depression — and men often present with fatigue, irritability, and loss of motivation rather than classic sadness, leading to underdiagnosis. Chronic stress similarly depletes energy through sustained cortisol elevation and sleep disruption. Mental health is physical health, and these causes are both common and treatable.[5]
6Poor Metabolic Health and Blood Sugar Dysregulation
Insulin resistance, prediabetes, and blood sugar swings produce energy crashes, particularly after high-carbohydrate meals. Pre-diabetes affects roughly 1 in 3 American adults — most undiagnosed. Stabilizing blood sugar through diet and exercise often produces noticeable energy improvements, and a simple HbA1c blood test screens for it.[6]
The pattern worth noticing: These causes frequently overlap. A man might have mild sleep apnea, borderline-low testosterone (caused partly by the apnea and excess weight), and prediabetes simultaneously — each contributing to fatigue. This is why a comprehensive evaluation, rather than chasing a single cause, is often most effective.
When to See a Doctor
Fatigue warrants medical evaluation when it:
- Persists for more than 2–4 weeks despite adequate sleep
- Is severe enough to interfere with work, relationships, or daily activities
- Is accompanied by other symptoms (weight change, mood changes, snoring, shortness of breath)
- Comes on suddenly or is unexplained
A basic workup typically includes a thorough history, physical exam, and bloodwork (complete blood count, ferritin, TSH, testosterone, HbA1c, vitamin D, metabolic panel). This relatively simple evaluation identifies the cause in the majority of cases.
References
- Peppard, P. E., et al. (2013). Increased prevalence of sleep-disordered breathing in adults. American Journal of Epidemiology, 177(9), 1006–1014. doi.org/10.1093/aje/kws342
- Bhasin, S., et al. (2018). Testosterone therapy in men with hypogonadism: an Endocrine Society clinical practice guideline. JCEM, 103(5), 1715–1744. doi.org/10.1210/jc.2018-00229
- Camaschella, C. (2015). Iron-deficiency anemia. NEJM, 372(19), 1832–1843. doi.org/10.1056/NEJMra1401038
- Chaker, L., et al. (2017). Hypothyroidism. The Lancet, 390(10101), 1550–1562. doi.org/10.1016/S0140-6736(17)30703-1
- Targum, S. D., & Fava, M. (2011). Fatigue as a residual symptom of depression. Innovations in Clinical Neuroscience, 8(10), 40–43.
- Centers for Disease Control and Prevention. (2023). Prediabetes — Your Chance to Prevent Type 2 Diabetes. cdc.gov/diabetes