What laboratory tests should be ordered for a patient presenting with low energy?

Laboratory Tests for Patients Presenting with Low Energy

For patients presenting with low energy, a comprehensive laboratory panel should include complete blood count, urinalysis, serum electrolytes (including calcium and magnesium), blood urea nitrogen, serum creatinine, fasting blood glucose (glycohemoglobin), lipid profile, liver function tests, and thyroid-stimulating hormone. 1

First-Line Laboratory Tests

Essential Tests

• Complete Blood Count (CBC) - To evaluate for anemia, infection, or other hematologic abnormalities
• Comprehensive Metabolic Panel:
  • Electrolytes (sodium, potassium, chloride, calcium, magnesium)
  • Kidney function (BUN, creatinine)
  • Liver function tests (AST, ALT, alkaline phosphatase)
  • Glucose
• Thyroid Function Tests:
  • TSH (primary screening test)
  • Free T4 (if TSH is abnormal)
  • Free T3 (particularly important when evaluating energy deficiency) 1, 2
• Iron Studies:
  • Serum ferritin
  • Serum iron
  • Total iron binding capacity (TIBC)
  • Transferrin saturation 2
• Fasting Lipid Profile
• Urinalysis

Second-Line Laboratory Tests

If first-line tests are inconclusive or specific conditions are suspected:

• Vitamin B12 and Folate - To rule out deficiencies causing anemia and fatigue 2
• Vitamin D Level - Deficiency can contribute to fatigue and muscle weakness
• HbA1c - For evaluation of glucose metabolism over time 2
• Inflammatory Markers (ESR, CRP) - To screen for inflammatory conditions
• Cortisol Level - Morning cortisol to evaluate adrenal function

Special Considerations for Athletes

For athletic patients with low energy, consider additional tests to evaluate for Relative Energy Deficiency in Sport (RED-S) or Female Athlete Triad:

• Hormonal Assessment:

  • Sex hormones (estradiol, progesterone, testosterone)
  • Triiodothyronine (T3) - A sensitive marker of energy availability 1
  • Insulin-like growth factor-1 (IGF-1) 1

• Energy Availability Calculation:

  • Consider calculating energy availability using the formula: Energy Intake (kcal) - Exercise Energy Expenditure (kcal) / Fat-Free Mass (kg) 1
  • Values below 30 kcal/kg FFM/day may indicate low energy availability 1

Interpretation Pitfalls

• Thyroid Function Tests: Be cautious with interpretation as non-thyroidal illness, medications, and assay interference can affect results 3, 4, 5
• Normal Laboratory Values: A patient may have clinically significant energy deficiency even with laboratory values within the normal range 1
• Weight Stability: Do not use weight stability as an indicator of adequate energy availability, as physiological adaptations can maintain weight despite low energy availability 1

Clinical Correlation

Laboratory results should always be interpreted in the context of:

• Patient's symptoms
• Medical history
• Medication use
• Activity level
• Dietary patterns

Follow-Up Recommendations

• If thyroid dysfunction is identified, appropriate treatment and monitoring should be initiated 2
• For nutritional deficiencies, supplement accordingly and address underlying dietary issues
• In cases of RED-S or energy deficiency in athletes, consider referral to sports medicine, nutrition, and/or endocrinology specialists

Remember that laboratory tests are just one component of evaluation. A thorough history focusing on diet, exercise patterns, sleep quality, stress levels, and psychological factors is essential for understanding the cause of low energy.