Failure to thrive (FTT) is a common term used to describe lack of adequate weight gain in pediatric-aged patients. Accepted definitions include a weight for age less than the fifth percentile on standardized growth charts, a decrease in weight percentile of more than two major percentile lines on the growth chart, or less than the 80 percentile of median weight for height ratio weight/length ratio.  Failure to thrive is important to recognize and treat because it can result in developmental delays and other long-term effects for the developing child.
Failure to thrive is often multifactorial in etiology. One way to classify potential causes is to think about three big categories of why a patient is failing to gain weight adequately. The categories include decreased intake, increased output, and increased caloric demand. Also, causes can be described as either organic or inorganic. There are multiple underlying pathologic conditions or organic causes that can lead to the three categories outlined above. For example, a patient with a swallowing abnormality may not be able to physically take in enough calories to gain weight. Conversely, a patient with chronic diarrhea may be losing more calories than he consumes. Lastly, a patient with congenital heart disease may have increased caloric demands and be unable to keep up. While there is a myriad of potential organic causes, inorganic causes are more common. Inorganic causes include some outside reasons for not getting enough calories. Examples might include improper mixing of infant formulas, feeding refusal, or parental neglect.
While failure to thrive can affect patients of all ages, it is most common in infants and younger children. There is no gender or race predisposition. Failure to thrive has been associated with lower socioeconomic status, lower parental education level, and with other increased psychosocial stressors in the home environment.
The pathophysiology of failure to thrive depends on the underlying etiology. However, at its base is a lack of necessary calories for adequate growth. This could be from not taking in enough calories, losing too many calories, or increased caloric demand. There are multiple formulas available for calculating caloric needs based on age and gender that can be helpful for catch-up growth once the underlying etiology is identified.
The history and physical exam are vital when assessing a patient with failure to thrive. Often, the patient may have no specific presenting symptoms but is then found to have inadequate growth when they present for a routine well visit. Other patients may have more obvious signs and symptoms associated with their underlying condition which have led to the failure to thrive. In the history, it is important to note a detailed feeding history including types of food, amount, and frequency. Feeding refusal, texture preferences, difficulty swallowing, or frequent emesis should be noted. If the infant is formula fed, identifying the formula mixing technique used can be helpful. Tachypnea, fatigue, or diuresis with feeds may also be important if present. In addition, documenting urine output, any unusual urine smells, and stool frequency and consistency are also helpful. Developmental milestones should be assessed and any delays documented. Past medical history may provide clues to an underlying organic etiology. Specifically, congenital abnormalities such as known congenital heart disease, esophageal abnormalities, intestinal abnormalities, endocrine disorders, and genetic disorders are often associated with failure to thrive. Family medical history including prenatal history can be pertinent, especially when considering potential underlying organic etiologies. The physical exam should note the rate of weight gain or loss from last visit as well as the current weight and height percentiles. Carefully assessing for any hints to an organic etiology such as an oral motor dysfunction, heart murmur, tachypnea, abnormal abdominal or genitourinary (GU) exam, or prominent skin lesions is essential. Often, the exam will be notable only for the poor weight gain and a thin appearing infant without providing any specific clues to the underlying cause.
Evaluation of failure to thrive should include confirmation of poor weight gain over time (depending on the severity of weight loss/clinical status at initial presentation). If possible, an observation of feeding, especially in younger infants can be very informative. If truly failure to thrive, an initial laboratory evaluation can be helpful to rule out common organic causes and guide the provider to more specific tests if needed. Initial labs should include a complete blood count (CBC) to assess for anemia, iron panel because the most common type of anemia in this age group is iron deficiency, and a metabolic panel to assess electrolyte and hydration status, liver, and kidney function. An erythrocyte sedimentation rate (ESR) can be useful to identify nonspecific underlying inflammation. Prealbumin can assess overall nutritional status and if low initially can be used to trend treatment response later on. Send screening thyroid function studies including thyroid-stimulating hormone (TSH) and Free T4. A normal newborn metabolic screen should be documented; if not, sending one can provide clues to less common organic causes of failure to thrive. A urinalysis is also important to assess kidney function. If there is a prominent history of abnormal stooling, stool studies including culture, guiac, and reducing substances may be helpful. Pending results of the first set of basic labs, more specific labs may be indicated such as a chloride sweat test, pancreatic function testing, or other metabolic-specific testing. While imaging is not routinely indicated initially, chest x-ray, ECG/echocardiogram, and endoscopy may be useful to determine certain conditions if there is a concern for an anatomic abnormality. 
Treatment for failure to thrive includes identifying the underlying etiology and addressing the caloric deficit. Often, patients are admitted to an inpatient setting to facilitate the workup, observe feeds and to ensure appropriate weight gain prior to discharge. However, depending on the clinical status of the patient this may not always be warranted. Whenever possible, enteral feeds should be provided. For patients with feeding refusal or inability to consume enough calories, nasogastric tubes (in the short-term) and gastrostomy tubes (long-term) are sometimes needed. Parenteral nutrition is a last resort. For patients with increased caloric demands, working with nutritionists to prescribe a high-calorie diet that is appropriate for the patient is essential. Establishing specific meal times and routines can help with toddlers who have difficulty feeding. Minimizing fruit juices and empty calories is also helpful. For patients with increased caloric losses, there are often disease-specific treatments to address this issue. An example would be pancreatic enzyme replacement in cystic fibrosis patients. Frequent follow-ups to confirm weight gain, once it is established, is essential. If needed, parental education and additional psychosocial support can be very helpful.
The importance of psychosocial factors such as low socioeconomic status, parental drug or alcohol abuse, postpartum depression, and parental education must be considered. Remember that most causes of failure to thrive (FTT) are inorganic. Addressing these psychosocial factors is often the key to restoring adequate weight gain for the patient. While an uncommon cause, parental neglect, and abuse must be a consideration.