What causes delayed bone age?

Q: My 13 years old son's growth hormone report showed insulin like growth factor – 1 as 178 ng/ml, igf-bp3 - 4.4 ug/ml, human growth hormone is 0.11 ng/ml and growth hormone 1,2,3 is 0.16, 16.60 and 10.40ng/ml. A radiologist told that his bone age is about 6-8 years. Please guide.

A:Your son has short stature due to growth hormone insensitivity syndrome (GHIS). This is evidenced by presence of short stature, delayed bone age compared to actual age, high levels of growth hormone (GH) following stimulation and low serum IGF-1 levels for his age and gender. Most children with this disorder are capable of producing GH, but there may be an abnormality in the receptors of the GH, and/or synthesis of IGF-1. The diagnosis can be confirmed by further tests which may include the estimation of acid labile subunit of GH (ALS) and growth hormone binding protein (GHBP) in some children. An alternative is to do IGF-1 generation test in which growth hormone is administered to see whether the levels of IGF-1 rise. A specific mutation test can be done in some children after a review by experts. Some of these children improve with IGF-1 treatment.