Short stature is defined as a height significantly below the average for a child’s age and sex, typically falling below the 3rd percentile, or more than two standard deviations below the mean. When a child presents with this finding, a full medical evaluation is initiated to find the underlying cause. The term “idiopathic” means the cause is unknown or unexplained. Idiopathic Short Stature (ISS) is diagnosed when extensive testing fails to identify an organic reason for the child’s small size. This article explains what ISS means, the evaluation process required, and the medical interventions available for this growth condition.
Understanding Idiopathic Short Stature
Idiopathic Short Stature (ISS) is a diagnosis of exclusion, assigned only after all known causes of short stature have been thoroughly ruled out. A child is considered to have ISS when their height is significantly low (usually defined as a height standard deviation score (SDS) below -2.0), yet they show no evidence of systemic, endocrine, nutritional, or chromosomal abnormalities. This diagnosis accounts for an estimated 80% of children referred for short stature after a complete work-up.
The child must be otherwise healthy, with normal body proportions and no history of being small for gestational age at birth. ISS is distinct from familial short stature (FSS), where height is consistent with short parents and bone age is normal. It is also differentiated from constitutional delay in growth and puberty (CDGP), where the child has a delayed bone age but will eventually reach a normal adult height. Children with ISS often exhibit a normal growth velocity but remain consistently below the typical growth curve because they are growing from a low starting point.
The Evaluation Process
A diagnosis of ISS requires a comprehensive medical workup, typically managed by a pediatric endocrinologist, to confirm the absence of any other medical condition. The process begins with a detailed review of the child’s medical history, including birth size, parental heights, and serial measurements of the child’s growth velocity over time. A consistent, slow growth rate is often seen with ISS, while a rate falling further off the curve may suggest an underlying issue.
A standard diagnostic tool is the bone age assessment, which involves an X-ray of the non-dominant hand and wrist to determine skeletal maturity. In ISS, the bone age is often close to the child’s chronological age, distinguishing it from constitutional delay where the bone age is significantly delayed. Extensive laboratory testing is conducted to rule out systemic diseases, including blood tests for thyroid function, markers of celiac disease, kidney and liver function, and genetic screenings like testing for Turner syndrome in girls.
Ruling out Growth Hormone Deficiency (GHD) is a particularly important step in the evaluation. This requires specialized pharmacological stimulation tests where a medication is given to provoke the pituitary gland to release growth hormone. For an ISS diagnosis, the child’s stimulated growth hormone level must be above a certain threshold (usually 10 nanograms per milliliter), indicating they are growth hormone sufficient. This rigorous process ensures that the “idiopathic” label is applied only after exhausting all diagnostic possibilities.
Medical Interventions
For children diagnosed with ISS who meet specific height criteria, the primary medical intervention is treatment with recombinant human Growth Hormone (GH) therapy. Although GH is traditionally used to treat GHD, the U.S. Food and Drug Administration (FDA) has approved its use for ISS in children whose height is significantly low, typically defined as an SDS of -2.25 or less. The goal of this intervention is to increase the child’s final adult height, which is often projected to be below the expected range for their family.
Growth hormone is administered through daily subcutaneous injections and continues for several years until the child reaches their final adult height or their growth plates fuse. Studies suggest that GH therapy for ISS can lead to an increase in final adult height, with typical gains ranging from four to seven centimeters. The mechanism of action involves GH stimulating the liver to produce Insulin-like Growth Factor-1 (IGF-1), which promotes the growth of bone and tissue.
Before initiating treatment, families and physicians must weigh the potential benefits against the risks. These risks can include temporary side effects like joint pain, fluid retention, and a theoretical risk of increased intracranial pressure. Observation without intervention is also a valid and common pathway, especially if the child is otherwise healthy and well-adjusted. The decision to pursue GH therapy is a highly individualized one, considering the child’s current height, predicted adult height, growth velocity, and the potential psychosocial impact of their short stature.
Long-Term Outlook
Individuals with a diagnosis of Idiopathic Short Stature are generally healthy; the diagnosis itself does not imply any underlying chronic disease or intellectual impairment. The long-term physical prognosis is excellent, as these individuals typically function normally in all respects other than their final adult height. Predicting the final adult height is complex, but it often falls within the expected parental range, remaining below the average for the general population.
The focus shifts to the psychological and social well-being of the individual. Short stature can present social challenges, and children with ISS may face issues related to self-esteem and body image. Psychological support and counseling are important tools to help children and adolescents develop coping mechanisms and a strong sense of self-worth independent of their height. Individuals with ISS can lead full and productive lives, and the goal is to foster resilience and a positive outlook regardless of their final height.