"I'm just tired all the time, but I figure it's just my age." — every man over 40, ever. Here's the problem with that explanation: persistent fatigue almost never has a single cause called "age." It has identifiable, treatable causes that show up more often as you age but aren't caused by aging itself.
The seven causes below explain the vast majority of fatigue we see in men in their 40s, 50s, and 60s. Most are correctable. Several are easily missed on routine physicals. None of them require you to "just push through."
1. Sleep apnea (the most under-diagnosed cause)
Obstructive sleep apnea (OSA) affects roughly 1 in 4 men over 40, and the majority don't know it. The hallmarks: loud snoring, gasping or choking at night, morning headaches, dry mouth, and the feeling that 8 hours of sleep felt like 4.
OSA wrecks energy on multiple levels — fragmented sleep architecture, drops in oxygen, surges in cortisol, and downstream suppression of testosterone. It's also tightly associated with weight gain, hypertension, and cardiovascular disease. Treating it (typically with CPAP or, in some cases, a dental appliance or weight loss) is one of the highest-leverage interventions in adult medicine.
Quick fact
Treating sleep apnea raises testosterone levels meaningfully in many men — sometimes enough to avoid TRT entirely.
2. Low testosterone
Testosterone declines roughly 1% per year starting in your 30s. By 50, many men have levels low enough to cause symptoms: fatigue, low motivation, reduced muscle, more body fat, lower libido, mood flatness.
Low T isn't always the answer to fatigue, but it's often part of the picture. A proper workup — total testosterone, free testosterone, SHBG, LH/FSH, and related markers — distinguishes whether the testes are underproducing (primary) or the brain is undersignaling (secondary). The treatment is different in each case.
3. Thyroid dysfunction
Hypothyroidism (an underactive thyroid) is more common in women but absolutely happens in men, and it produces fatigue that's hard to distinguish from "just being tired." Other clues: cold intolerance, dry skin, constipation, weight gain, slowed thinking, lower mood.
The basic screen is TSH plus free T4. If TSH is high, the thyroid isn't keeping up. Some labs only check TSH; if your symptoms suggest thyroid involvement, push for the free T4 and free T3 too.
4. Iron deficiency or anemia
Iron deficiency in men is less common than in women but worth checking — and it doesn't always show up as obvious anemia. Low ferritin (the storage form of iron) can cause fatigue, exercise intolerance, and brain fog even when hemoglobin looks normal.
Causes in men include occult GI bleeding (which always warrants follow-up), poor absorption, very heavy training loads, or vegetarian diets without proper iron sources. Ferritin should be on every fatigue panel.
5. Insulin resistance and blood sugar dysregulation
Mid-afternoon energy crashes, hangry-ness, brain fog after meals, and persistent visceral fat are all signs your body is struggling to manage blood sugar. Insulin resistance precedes type 2 diabetes by years or decades and is one of the most common drivers of mid-life fatigue.
Useful markers: fasting glucose, HbA1c, fasting insulin, and triglycerides relative to HDL. Treatment overlaps heavily with weight loss and lifestyle change but can also include medications (metformin, GLP-1 agonists) when warranted.
6. Vitamin D and B12 deficiency
These two vitamin deficiencies are routinely missed and disproportionately common in men with desk jobs, indoor lifestyles, or restrictive diets.
- Vitamin D: low levels are linked to fatigue, low mood, immune dysfunction, and reduced testosterone. Most adults need supplementation; the right dose depends on your starting level.
- Vitamin B12: deficiency causes fatigue, brain fog, neuropathy, and (eventually) anemia. More common in vegetarians and in men taking long-term metformin or PPIs.
7. Chronic stress and burnout
This is the most common cause we see in successful, ambitious men — and the hardest to recognize because most of these patients like being busy. The pattern: years of insufficient recovery, elevated cortisol, sleep disruption, sympathetic overdrive, and eventually a flattening of energy that no amount of caffeine touches.
The fix isn't a pill. It's structural changes to sleep, training load, work hours, and recovery practices. Sometimes a short course of supportive treatment helps, but the foundation has to change.
The right workup
If fatigue has been going on for more than three months and isn't responding to better sleep, a thorough workup is warranted. A reasonable starting panel includes:
- CBC and ferritin
- Comprehensive metabolic panel
- Fasting glucose and HbA1c, ideally with fasting insulin
- Lipid panel
- TSH and free T4
- Total and free testosterone, SHBG, LH, FSH, estradiol
- Vitamin D (25-OH), vitamin B12, folate
- Urinalysis
- Sleep apnea screening if symptoms present
From there, results guide the next steps. Most men find at least one — often several — correctable contributors when they actually look.
Frequently asked questions
Is feeling tired all the time normal as men age?
Some decline in energy is biologically normal — recovery slows, sleep architecture changes, hormone levels drift. But persistent, day-disrupting fatigue is not normal aging. It almost always has an identifiable cause, and most causes are treatable.
What labs check fatigue causes in men?
A reasonable initial workup includes a CBC, comprehensive metabolic panel, fasting glucose and HbA1c, lipid panel, TSH plus free T4, ferritin, vitamin D, vitamin B12, total and free testosterone, SHBG, and a urinalysis. From there, additional testing depends on findings.
How do I know if I have sleep apnea?
Common signs include loud snoring, witnessed pauses in breathing, gasping or choking during sleep, morning headaches, dry mouth on waking, and unrefreshing sleep despite adequate hours in bed. A home sleep study is non-invasive and can be ordered through your primary care doctor or sleep clinic.
What's next?
Fatigue is a signal, not a verdict. The productive move is to get a complete picture — labs, sleep, lifestyle factors — and then act on what you find. Most men leave a thorough evaluation with two or three concrete things to change, and feel meaningfully better within a few months.