Idiopathic or Non-Pathological Fatigue: This is often related to lifestyle factors such as lack of sleep, excessive physical activity, stress, or poor diet. Given the absence of any abnormal findings on exam or labs, this diagnosis is plausible as it doesn't necessarily require an underlying pathological condition.

Depression or Anxiety: These conditions are common in young adults and can present with fatigue as a primary symptom. Even without overt psychiatric symptoms, subtle mood disorders can manifest primarily as fatigue.
Sleep Disorders: Conditions like insomnia, sleep apnea, or restless legs syndrome can cause significant fatigue. These might not be immediately apparent on a routine exam or lab tests.
Chronic Fatigue Syndrome (Myalgic Encephalomyelitis): Although more common in females, it can affect males and is characterized by profound fatigue that is not improved by rest and lasts for more than 6 months.
Vitamin D Deficiency: Common, especially in individuals with limited sun exposure or poor dietary intake, and can cause fatigue among other symptoms.

Hypothyroidism: Although labs are reported as normal, it's crucial to ensure thyroid function tests (TFTs) were included, as hypothyroidism can cause fatigue and may have subtle or non-specific symptoms.
Diabetes: Both type 1 and type 2 diabetes can present with fatigue, among other symptoms. Even if initial labs are normal, the diagnosis should be considered, especially if there's a family history or other risk factors.
Sleep Apnea: A condition that can lead to significant fatigue and may not be detected on routine exam or labs unless specific screening questions are asked.
Substance Abuse: Fatigue can be a symptom of substance abuse or withdrawal, which might not be immediately apparent on physical exam or standard lab tests.

Addison's Disease: A rare endocrine disorder where the adrenal glands do not produce enough cortisol and aldosterone, leading to fatigue among other symptoms.
Hemochromatosis: A genetic disorder causing the body to absorb too much iron, leading to fatigue and other systemic symptoms.
Mitochondrial Myopathies: A group of disorders affecting the mitochondria, leading to muscle weakness and fatigue.
Lymphoma or Other Malignancies: Although rare, certain cancers can present with non-specific symptoms like fatigue, and it's essential to maintain a high index of suspicion, especially with persistent or worsening symptoms.