Anorexia Nervosa: Given the significant underweight status (80 lbs) and failure to thrive, anorexia nervosa is a highly likely diagnosis. The normal labs, imaging, and hormone levels do not rule out this condition, as individuals with anorexia can have normal laboratory results despite severe malnutrition. The use of escitalopram, an SSRI, also supports this diagnosis, as it is commonly used to treat comorbid depression and anxiety in patients with eating disorders.

Depression: The patient is already on escitalopram, which suggests a diagnosis of depression. Depression can lead to significant weight loss and failure to thrive, especially if the patient has a poor appetite or is experiencing significant psychosocial stressors.
Anxiety Disorder: Similar to depression, anxiety disorders can lead to significant weight loss and failure to thrive, especially if the patient is experiencing significant stress or anxiety that interferes with their ability to eat or maintain a healthy weight.
Avoidant/Restrictive Food Intake Disorder (ARFID): This eating disorder is characterized by a lack of interest in eating or a fear of eating due to concerns about the taste, texture, or nutritional content of food. It can lead to significant weight loss and failure to thrive.

Celiac Disease: Although the labs and imaging are normal, celiac disease can cause malabsorption and failure to thrive, even in the absence of gastrointestinal symptoms. It is essential to consider this diagnosis to avoid missing a potentially treatable condition.
Inflammatory Bowel Disease (IBD): Like celiac disease, IBD can cause malabsorption and failure to thrive, even if the initial labs and imaging are normal. A high index of suspicion is necessary to diagnose and treat this condition.
Hypothyroidism: Although the hormone levels are reported as normal, it is essential to consider hypothyroidism, as it can cause significant weight loss and failure to thrive. A repeat thyroid function test or a free T4 and T3 level may be necessary to rule out this condition.

Gastric Dysmotility: This condition can cause significant weight loss and failure to thrive due to impaired gastric emptying and malabsorption.
Chronic Infectious Disease (e.g., tuberculosis, HIV): Although rare, chronic infectious diseases can cause significant weight loss and failure to thrive, especially if left untreated.
Mitochondrial Disorders: These rare genetic disorders can cause significant weight loss and failure to thrive due to impaired energy production and malabsorption.