Failure to thrive is a term which is now a little outdated and the new term which is used is faltering growth or poor growth. Faltering growth is defined as a child whose weight or height is falling across centiles in a downward direction.
Most cases are due to not enough calories and protein being consumed. It can also be due to not enough of these nutrients being absorbed due to problems with the small intestine absorption.
Rarely, other problems can exist such as pancreas and liver conditions, or other problems such as cystic fibrosis, which is uncommon. The fourth component of growth is of course the hormonal mechanisms controlling growth and it is much less common to have a problem involving growth hormone or any of the other hormones such as thyroid hormone or the pituitary gland hormones which control energy and growth.
Investigations usually involve blood and stool tests. Then input from dietitian, endocrinologist sometimes. Sometimes small bowel biopsy via gastroscopy is also recommended.