Chronic Fatigue Syndrome: This condition is characterized by persistent and unexplained fatigue that is not relieved by rest. Given the normal lab results, it's a plausible diagnosis, as chronic fatigue syndrome often doesn't have abnormal lab findings.

Anxiety or Depression: These mental health conditions can cause significant fatigue. Normal labs would not rule out these diagnoses, as they are primarily diagnosed based on clinical presentation and psychological evaluation.
Sleep Disorders: Conditions like insomnia, sleep apnea, or restless leg syndrome can lead to fatigue. Normal labs do not exclude these possibilities, as diagnosis often requires specific sleep studies.
Hypothyroidism (Subclinical): Although labs are normal, subclinical hypothyroidism might not always present with abnormal thyroid function tests, especially if the patient is in the early stages or has a mild form of the disease.
Adrenal Insufficiency (Mild): Similar to hypothyroidism, mild adrenal insufficiency might not be detected by standard lab tests, especially if the deficiency is partial or the patient is seen during a period of compensation.

Addison's Disease: Although rare, this condition can present with fatigue among other symptoms. It's crucial to consider because it can be life-threatening if not treated.
Hematologic Malignancies (e.g., Leukemia, Lymphoma): These conditions might not always present with abnormal labs initially, especially in early stages. They are critical to diagnose early due to their potential for severe outcomes.
Sleep Apnea with Normal Oxygen Saturation: Some cases of sleep apnea might not significantly impact oxygen saturation levels but can still cause significant fatigue.
Chronic Infections (e.g., Lyme Disease, Tuberculosis): Certain infections can cause prolonged fatigue and might not always be detected by routine lab tests, requiring specific diagnostic tests.

Mitochondrial Myopathies: These are a group of disorders that affect the mitochondria, often presenting with muscle weakness and fatigue. They are rare and require specific diagnostic tests.
Porphyrias: A group of disorders that can result in fatigue among a wide range of other symptoms. They are rare and often require specific tests for diagnosis.
Celiac Disease with Minimal Gastrointestinal Symptoms: Some individuals with celiac disease may present with fatigue as a primary symptom without significant gastrointestinal symptoms, making it a rare but possible diagnosis to consider in the context of unexplained fatigue.