Anorexia Nervosa: This is a common condition in adolescent girls, characterized by a distorted body image and excessive dieting that leads to significant weight loss. The psychological aspect of the condition, combined with the physical symptoms of weight loss, makes it a prominent consideration.

Thyroid Disorders (Hyperthyroidism): An overactive thyroid gland can lead to increased metabolism, resulting in weight loss despite increased appetite. This condition is relatively common and can be diagnosed with blood tests.
Diabetes Mellitus (Type 1): Although more commonly associated with symptoms like polyuria and polydipsia, unexplained weight loss is a key feature, especially in the absence of other obvious symptoms.
Depression: Can lead to changes in appetite and weight loss. Given the age group, it's essential to consider mental health conditions.
Gastrointestinal Issues (e.g., Crohn's Disease, Ulcerative Colitis): Chronic inflammation and malabsorption can result in weight loss. These conditions often present with other symptoms like diarrhea, abdominal pain, and fatigue.

Cancer (e.g., Leukemia, Lymphoma): Although rare, cancers can present with unexplained weight loss. Early detection is crucial for treatment outcomes.
Tuberculosis: Can cause weight loss, fever, and night sweats. It's less common in some regions but critical to diagnose due to its infectious nature and the availability of treatment.
HIV/AIDS: Weight loss is a common symptom, especially in the later stages. Given the potential for long-term health consequences, diagnosing HIV early is vital.
Pituitary or Adrenal Gland Disorders: Conditions like Addison's disease or a pituitary tumor can lead to weight loss among other symptoms.

Celiac Disease: An autoimmune reaction to gluten, leading to malabsorption and potential weight loss. It's less likely to present solely with weight loss but is worth considering, especially with gastrointestinal symptoms.
Mitochondrial Disorders: Rare genetic conditions affecting the mitochondria, which can lead to a variety of symptoms including weight loss, muscle weakness, and neurological issues.
Chronic Infections (e.g., Parasitic Infections): Certain infections, though rare in many parts of the world, can cause weight loss among other symptoms.
Genetic Disorders: Such as Prader-Willi syndrome, which can lead to significant weight gain initially but may present with failure to thrive or weight loss in infancy, with potential for weight fluctuations later in life.