Fatigue is one of the most common reasons people seek medical care—yet it often remains unexplained. Many individuals are told their labs look “normal,” their thyroid is “fine,” or that their symptoms are simply part of getting older. But deep down, they know something still isn’t right.
Persistent fatigue is more than just an inconvenience. It affects the ability to focus, move, show up for work and relationships, and enjoy life. And in many cases, it’s not caused by a lack of sleep or a busy schedule alone. It’s a sign that something in the body is out of balance.
From nutrient deficiencies to chronic stress, hormone shifts, and cellular energy breakdown, there are many reasons someone might feel persistently tired—even when standard lab work doesn’t reveal a clear problem. In functional medicine, we look at fatigue as a clue that points to deeper patterns within the body that need attention and support.
Here are some of the most common and overlooked reasons for fatigue:
Mitochondrial Fatigue: When the Cell’s Powerhouse Fizzles Out
You may remember from high school biology that mitochondria are the “powerhouses of the cell.” These tiny structures are where the body produces ATP—the molecule that fuels every cellular function.
But when mitochondria are underperforming, the whole system slows down. Signs of this include:
- Limbs feeling “heavy”
- Mentally foggy and forgetful
- Easily overstimulated by light or noise
- Wiped out after physical activity or socializing
What drains mitochondria? Chronic stress, blood sugar swings, inflammation, toxins, and nutrient deficiencies—especially CoQ10, B2, magnesium, and iron. Mitochondrial fatigue isn’t fixed by more coffee or more sleep. It’s a deep energy deficit that needs root-cause attention.
Blood Sugar Dysregulation
Energy levels are like a campfire. Eating too many refined carbs or sugary snacks can cause blood sugar to spike and crash. It’s like throwing paper on a flame, burning bright at first, then gone in a flash.
When blood sugar crashes, so does energy. Shakiness, irritability, or hitting a wall in the afternoon are all signs of a blood sugar crash. At night, blood sugar crashes can cause a cortisol rise that wakes a person up and keeps them from entering the deep, restorative sleep needed to truly recharge.
Stabilizing blood sugar with protein, healthy fats, and fiber at meals—and cutting back on refined carbohydrates—can make a major difference in daily energy and mood.
Iron and B12 Deficiency
Iron and B12 are both essential for oxygen delivery and cellular energy production. Being low in either can cause weakness, shortness of breath, or difficulty recovering from even light exercise.
Eating foods with enough of these nutrients is the first step. But beyond that, absorption is key. Low stomach acid, chronic gut issues, or certain medications can block the absorption of these critical nutrients. Women with heavy periods, vegans and vegetarians, and those on acid-reducing medications are especially at risk.
If iron or B12 levels look “fine,” but a person still feel exhausted, a functional medicine provider can check functional ranges and additional markers that paint the full picture of these nutrient levels.
Thyroid Function: “Normal” Isn’t Always Optimal
The thyroid is the body’s thermostat. It helps regulate metabolism, temperature, and energy. Even if TSH is within the “normal” range, the thyroid may not be functioning optimally.
Common signs of suboptimal thyroid function include:
- Low energy
- Cold hands and feet
- Hair thinning
- Constipation
- Brain fog
Comprehensive thyroid testing should include Free T3, Free T4, Reverse T3, and thyroid antibodies—not just TSH. Thyroid and mitochondrial health are closely connected, and both matter when it comes to lasting energy.
Hormone Imbalance and Circadian Disruption
The body runs on rhythms. Cortisol is the primary daytime hormone. It should be highest in the morning and gradually taper off at night.
Chronic stress, irregular sleep, shift work, and late-night eating can all disrupt this pattern. The result is waking up tired and feeling wired at night. Some people even develop a flat cortisol curve, where energy levels are low all day long.
Balancing the daily circadian rhythm by anchoring wake/sleep times, limiting screen exposure after dark, and supporting the stress response can help reset the internal clock.
Emotional, Mental, and Physical Stress
We often think of stress as just mental or emotional, but stress comes in many forms: overexercising, skipping meals, poor sleep, illness, and even chronic negative self-talk. All of these keep the nervous system stuck in “fight or flight” mode.
When the body is constantly mobilizing to respond to stress, it doesn’t have the resources to repair, restore, and energize. Fatigue, in this case, is the body asking for a pause. Without intentional recovery, the energy tank stays empty.
Chronic or Lingering Infections
Fatigue is a common symptom of post-viral syndrome—something many have become more familiar with after COVID-19. But it can also show up after Epstein-Barr virus (EBV), mono, or other lingering infections.
Some infections never fully resolve. Others hide in the gut, mouth, or sinuses and quietly keep the immune system on high alert. The result is feeling inflamed, tired, and achy, without an obvious reason. Identifying and addressing these hidden infections can result in increased energy.
Gut Health and Food Sensitivities
Gut health plays a central role in energy. Inflammation in the gut from dysbiosis, leaky gut, or food sensitivities can trigger an immune response that saps energy.
Two of the most common culprits are gluten and dairy. For some people, these foods don’t cause digestive symptoms but show up as fatigue, joint pain, or brain fog. An elimination diet or functional testing can help determine if food sensitivities are contributing to fatigue.
Nutrient-Poor Diet
A diet heavy on processed foods and light on whole foods results in getting calories but not real nourishment. Many of the nutrients involved in energy production like magnesium, B vitamins, and amino acids are depleted in ultra-processed diets. And relying on caffeine or sugar to get through the day only masks the problem temporarily.
We recommend the Mediterranean Diet as an all-around excellent nutrient-dense diet that focuses on whole foods with colorful vegetables and fruits, quality protein and healthy fats at every meal to nourish the body’s energy systems.
Medication Side Effects
Some medications can cause fatigue, either directly or by depleting nutrients. When starting a new medication, it’s wise to look up what nutrients may be affected by taking it.
One newer category getting attention is GLP-1 agonists, which are used for blood sugar control and weight loss. These drugs can reduce appetite significantly, which may result in lower intake of protein, iron, or B12. Nutrition counseling can help individuals who are taking these medications.
Age-Related Energy Changes
After 40, energy metabolism begins to shift. Hormonal changes, reduced muscle mass, and declining mitochondrial density can all affect how energized someone feels.
But this doesn’t mean fatigue is inevitable. With strength training, targeted nutrients, and proper stress recovery, our 40s, 50s, and beyond can still be vibrant and full of energy.
Fatigue Is Real and It’s Fixable
When a person is exhausted but their labs are “fine,” it’s time to look deeper. When we zoom out and address the full picture—from mitochondria to hormones to stress resilience—energy can be restored. Working with a functional medicine practitioner can help identify what’s really going on and create a personalized path to healing.
At Radiance Functional Medicine, we offer functional medicine and nutrition counseling. We hope that you will allow us to help you heal your gut, balance your hormones, or find a way of eating that helps you thrive! Schedule an appointment to get started. Whether you are looking for a Nutritionist or Functional Medicine Doctor in Denver or your local area, we see patients in person and virtually. Call our office at 303.333.1668 to schedule your Initial Consultation.
