Chronic Fatigue Has Root Causes. Here's What to Ask Your Doctor to Test.
Fatigue is the symptom I hear most often in my practice. Not the normal exhaustion of a hard week that resolves with sleep. The other kind.
The fatigue that's there when you wake up. That makes you cancel things. That you've started explaining as just how you are now. The kind that every test so far has called normal.
Fatigue that doesn't resolve with rest has root causes. Most of them are findable in labs if you're running the right ones.
1. Iron and Ferritin
Iron deficiency is one of the most common nutritional deficiencies in women of reproductive age and one of the most consistently missed in standard labs. The CBC measures hemoglobin and hematocrit, which reflect frank anemia. By the time those are low, iron depletion has been going on for a while.
Ferritin is your iron storage protein. It starts declining before anemia appears. Symptoms including fatigue, hair loss, brain fog, cold intolerance, and restless legs are commonly present when ferritin is below 50 ng/mL. Standard labs consider anything above 12 "normal." That gap between 12 and 50 is where a lot of people are living.
Always ask for ferritin specifically. Your doctor may need to order it separately from your CBC.
2. Thyroid Function
Suboptimal thyroid function is one of the most consistently underdiagnosed drivers of chronic fatigue, particularly in women. TSH within the standard range but above the functional ceiling of 2.0, Free T3 at the bottom of the reference range, Reverse T3 elevated, early Hashimoto's with normal TSH: these patterns explain profound, persistent fatigue and get missed entirely by a TSH-only panel.
3. Vitamin D
Vitamin D deficiency is independently associated with fatigue, muscle weakness, and low mood. It's extraordinarily common, particularly in people with higher melanin concentrations. A Vitamin D level in the 20s clears a standard reference range. Functionally, it can represent a meaningful deficiency.
4. B12
B12 deficiency presents as fatigue, brain fog, tingling or numbness, mood changes, and memory issues. Particularly relevant for people who eat minimal animal products, have a history of proton pump inhibitor use, or carry MTHFR variants that affect B12 utilization.
5. Blood Sugar Dysregulation
Insulin resistance and reactive hypoglycemia are 2 of the most consistently missed drivers of fatigue. Blood sugar swings, spiking after a high-glycemic meal then crashing hard, produce energy crashes that show up as midday fatigue, the 3pm drop, and fatigue that improves after eating. C-Peptide and fasting insulin can catch this long before A1C gives anything away.
6. Cortisol Patterns
Chronic stress dysregulates the HPA axis, producing cortisol patterns that no longer follow a healthy circadian rhythm. Fatigue despite adequate sleep, waking between 2 and 4am, afternoon crashes, difficulty sustaining energy across the day.
Standard blood cortisol is a single snapshot and misses this. A 4-point salivary cortisol test captures the pattern across the day. That's what we use at Magnolia when indicated.
What to Request at Your Next Appointment
• Ferritin (not just iron or CBC)
• Full thyroid panel: TSH, Free T3, Free T4, Reverse T3, TPO and Thyroglobulin antibodies
• 25-hydroxyvitamin D
• B12 and folate
• Fasting glucose and fasting insulin (or C-Peptide as proxy)
• hsCRP (inflammation is a major and consistently underappreciated driver of fatigue)
• Comprehensive metabolic panel
If your provider won't order what you need, or you already have labs and want a second look through a functional lens, you have options.
The Magnolia Lab Deep-Dive Bundle includes a complete fatigue workup, interpreted by a clinician looking for root causes. Already have results? Decode My Labs. -> stan.store/drsarahellis
This post is for educational purposes only and does not constitute medical advice, diagnosis, or treatment.