Failure to thrive is a medical term which is used in both pediatric and adult human medicine, as well as veterinary medicine (where it is also referred to as ill thrift).

When not more precisely defined, the term refers to pediatric patients. In MeSH

Mesh

Mesh consists of semi-permeable barrier made of connected strands of metal, fiber, or other flexible/ductile material. Mesh is similar to web or net in that it has many attached or woven strands.-Types of mesh:...

, the term is assumed to refer to an infant or child. In children, it is usually defined in terms of weight

Weight

In science and engineering, the weight of an object is the force on the object due to gravity. Its magnitude , often denoted by an italic letter W, is the product of the mass m of the object and the magnitude of the local gravitational acceleration g; thus:...

, and can be evaluated either by a low weight for the child's age, or by a low rate of increase in the weight.

Infantile/pediatric failure to thrive

As used by pediatricians, it covers poor physical growth of any cause and does not imply abnormal intellectual, social, or emotional development. The term has been used in different ways, and different objective standards have been defined. Many definitions use the 5th percentile as a cutoff.

Traditionally, causes of FTT have been divided into endogenous and exogenous causes. Initial investigation should consider physical causes, calorie intake and psychosocial assessment.

• endogenous
  Endogenous
  Endogenous substances are those that originate from within an organism, tissue, or cell. Endogenous retroviruses are caused by ancient infections of germ cells in humans, mammals and other vertebrates...
  
   (or "organic"): Causes can include inborn error of metabolism
  Inborn error of metabolism
  Inborn errors of metabolism comprise a large class of genetic diseases involving disorders of metabolism. The majority are due to defects of single genes that code for enzymes that facilitate conversion of various substances into others...
  
  . Underlying physical causes may not be immediately obvious such as the mother's mastitis
  Mastitis
  Mastitis is the inflammation of breast tissue. S. aureus is the most common etiological organism responsible, but S. epidermidis and streptococci are occasionally isolated as well.-Terminology:...
  
  , urinary tract infections (UTIs), undiagnosed Cystic Fibrosis
  Cystic fibrosis
  Cystic fibrosis is a recessive genetic disease affecting most critically the lungs, and also the pancreas, liver, and intestine...
  
   (CF) or asthma
  Asthma
  Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...
  
  .

• exogenous
  Exogenous
  Exogenous refers to an action or object coming from outside a system. It is the opposite of endogenous, something generated from within the system....
  
   (or "nonorganic"): Causes can include having a mother with postpartum depression
  Postpartum depression
  Postpartum depression , also called postnatal depression, is a form of clinical depression which can affect women, and less frequently men, typically after childbirth. Studies report prevalence rates among women from 5% to 25%, but methodological differences among the studies make the actual...
  
  . Both neglect and hypervigilance
  Hypervigilance
  Hypervigilance is an enhanced state of sensory sensitivity accompanied by an exaggerated intensity of behaviors whose purpose is to detect threats. Hypervigilance is also accompanied by a state of increased anxiety which can cause exhaustion. Other symptoms include: abnormally increased arousal, a...
  
   can cause failure to thrive. The program outlined in the parenting book On Becoming Babywise is associated with infantile failure to thrive and has been criticized by pediatricians and child abuse advocates.

• Mixed: However, to think of the terms as dichotomous
  Dichotomy
  A dichotomy is any splitting of a whole into exactly two non-overlapping parts, meaning it is a procedure in which a whole is divided into two parts...
  
   can be misleading, since both endogenous and exogenous factors may co-exist. A child with a disease or disability may be more vulnerable to poor care by a caregiver with marginal competence or resources. These infants typically look cachectic
  Cachexia
  Cachexia or wasting syndrome is loss of weight, muscle atrophy, fatigue, weakness, and significant loss of appetite in someone who is not actively trying to lose weight...
  
  , are prone to infections with difficulty recovering, are often developmentally delayed, have unusual postures, and look sad, withdrawn, apathetic OR hypervigilant, irritable, or angry.

Recently the term 'faltering growth' has become a popular replacement for 'failure to thrive', which in the minds of some represents a more euphemistic

Euphemism

A euphemism is the substitution of a mild, inoffensive, relatively uncontroversial phrase for another more frank expression that might offend or otherwise suggest something unpleasant to the audience...

 term.

Adult/geriatric failure to thrive

The term "failure to thrive" is also applied to geriatrics, or more generally in adult medicine.

In adult medicine, failure to thrive is a descriptive, non-specific term that encompasses "not doing well": e.g. malaise

Malaise

Malaise is a feeling of general discomfort or uneasiness, of being "out of sorts", often the first indication of an infection or other disease. Malaise is often defined in medicinal research as a "general feeling of being unwell"...

, weight loss, poor self-care that can be seen in elderly individuals.