Pitfall in diagnosing growth hormone deficiency in a hypochondroplastic patient with a delayed puberty

Hypochondroplasia is a clinically and genetically heterogeneous skeletal dysplasia with less obvious disproportion in childhood and a reduced pubertal growth spurt. We report on a young hypochondroplastic man who had been misdiagnosed and treated as being growth hormone (GH) deficient in the early phase of puberty. The delay of his puberty which is unusual in hypochondroplasia might have confused the results of provocative GH testing. At the age of 17 years measurement of body proportions revealed an increased upper to lower body segment ratio. Skeletal radiographs showed a lack of increase in the interpedicular distance from the first to the fifth lumbar vertebra, anteroposterior shortening of the lumbar pedicles, short femoral necks, a fibula longer than the tibia, and short tubular bones. As the clinical and radiographic features suggested the diagnosis of a skeletal dysplasia, a DNA sequence analysis of the fibroblast growth factor receptor 3 gene on chromosome 4 p16.3 was performed, which identified the missense mutation C1620 G in the tyrosine kinase domain resulting in an Asn540Lys substitution. Hypochondroplastic children with this common mutation (N540K) were previously found to respond to GH treatment with an increase in sitting height compared to leg length, which accentuated the existing disproportion. We want to emphasise that in children with normal serum IGF-I and IGFBP-3 levels accurate measurements of body proportions and skeletal radiographs in disproportionate cases are more important than reiterative GH stimulation tests, which prepubertally and in the early phase of puberty often show subnormal responses.