Anemia or Iron Deficiency: This is a common condition in young adults, especially in women due to menstrual losses. It can lead to fatigue, weakness, and shortness of breath.
Vitamin Deficiencies (B12, D): Deficiencies in vitamins B12 and D are prevalent and can cause fatigue, among other symptoms like neurological issues and bone health problems.

Depression or Anxiety: Mental health disorders are common in young adults and can manifest with fatigue, along with mood changes, loss of interest in activities, and sleep disturbances.
Sleep Disorders (Insomnia, Sleep Apnea): Poor sleep quality or quantity can lead to significant fatigue. Sleep disorders are often underdiagnosed and can have a profound impact on daily functioning.
Chronic Fatigue Syndrome (Myalgic Encephalomyelitis): A condition characterized by extreme fatigue that cannot be explained by any underlying medical condition. It's more common in young adults and can be debilitating.
Hypothyroidism: Although less common in young adults without a family history, hypothyroidism can cause fatigue, weight gain, and cold intolerance.

Diabetes Mellitus: Both type 1 and type 2 diabetes can present with fatigue, among other symptoms like polyuria, polydipsia, and weight loss. Early diagnosis is crucial to prevent long-term complications.
HIV Infection: Fatigue can be an early symptom of HIV infection. Given the potential for severe consequences if left untreated, it's critical to consider HIV in the differential diagnosis, especially in sexually active individuals or those with high-risk behaviors.
Lymphoma or Leukemia: Although rare, these cancers can present with fatigue, night sweats, and weight loss. Early detection significantly improves prognosis.
Adrenal Insufficiency (Addison's Disease): A rare condition where the adrenal glands do not produce enough cortisol and aldosterone, leading to fatigue, weight loss, and changes in skin pigmentation.

Mitochondrial Myopathies: A group of disorders that affect the mitochondria, leading to muscle weakness and fatigue. These conditions are genetic and can present at any age.
Porphyrias: A group of rare genetic disorders that can cause a variety of symptoms, including fatigue, neurological issues, and abdominal pain.
Wilson's Disease: A genetic disorder that leads to copper accumulation in the body, causing liver disease, neurological symptoms, and fatigue. It's essential to diagnose early to prevent irreversible damage.