Anemia: This is a common condition in adolescent females, especially those with heavy menstrual periods. Anemia can lead to weakness and fatigue due to a decrease in red blood cells or hemoglobin, reducing the body's ability to transport oxygen to tissues.

Hypothyroidism: This condition, where the thyroid gland doesn't produce enough thyroid hormone, can cause fatigue, weakness, and other symptoms. It's relatively common in females and can be diagnosed with blood tests.
Depression: Mental health issues, including depression, can manifest as physical symptoms like weakness and fatigue. Given the age and gender, this is a plausible diagnosis that requires consideration.
Vitamin Deficiencies (e.g., Vitamin B12 or Iron Deficiency): Deficiencies in certain vitamins and minerals can lead to symptoms of weakness and fatigue. These are relatively common, especially in individuals with poor diets or certain dietary restrictions.
Chronic Fatigue Syndrome: Also known as myalgic encephalomyelitis, this condition is characterized by extreme fatigue that cannot be explained by any underlying medical condition. It's more common in females and can start at any age.

Diabetes Mellitus: Although less likely, undiagnosed diabetes can present with fatigue and weakness among other symptoms. Missing this diagnosis could lead to serious complications.
Adrenal Insufficiency: A condition where the adrenal glands do not produce adequate amounts of hormones, which can lead to fatigue, weakness, and potentially life-threatening complications if not treated.
Sleep Apnea: Though more common in older adults, sleep apnea can occur in adolescents and cause significant daytime fatigue and weakness.
Cardiac Conditions (e.g., Cardiomyopathy): Certain heart conditions can present with nonspecific symptoms like fatigue and weakness. These conditions are critical to diagnose early to prevent severe outcomes.

Mitochondrial Myopathies: A group of disorders that affect the mitochondria, leading to muscle weakness and fatigue. These are rare genetic conditions.
Muscular Dystrophy: A group of inherited disorders characterized by muscle weakness and loss of muscle mass. While more commonly diagnosed in early childhood, some forms may present later.
Neuromuscular Junction Disorders (e.g., Myasthenia Gravis): Autoimmune conditions that can cause muscle weakness and fatigue. These are rare but important to consider due to their treatable nature.
Cancer: Though extremely rare in this demographic without other symptoms, certain types of cancer (like leukemia) can present with fatigue and weakness.