If gigantism was only caused by excess HGH levels than why is gigantism so rare.  It is likely that extremely high levels of HGH or other factors are needed to cause gigantism.  This study will help us determine how much HGH is above the norm in cases of gigantism.

Gigantism caused by growth hormone secreting pituitary adenoma.

“Gigantism indicates excessive secretion of growth hormones (GH) during childhood when open epiphyseal growth plates allow for excessive linear growth. Case one involved a 14.7-year-old boy presented with extreme tall stature. His random serum GH level was 38.4 ng/mL, and failure of GH suppression{Resistance to GH surpression may be another factor needed for gigantism to be caused} was noted during an oral glucose tolerance test (OGTT; nadir serum GH, 22.7 ng/mL). Magnetic resonance imaging (MRI) of the brain revealed a 12-mm-sized pituitary adenoma. Transsphenoidal surgery was performed and a pituitary adenoma displaying positive immunohistochemical staining for GH was reported. Pituitary MRI scan was performed 4 months after surgery and showed recurrence/residual tumor. Medical treatment with a long-acting somatostatin analogue for six months was unsuccessful. As a result, secondary surgery was performed. Three months after reoperation, the GH level was 0.2 ng/mL{So the GH level was higher by a factor of 192 which is exceptionally high} and insulin-like growth factor 1 was 205 ng/mL. Case two involved a 14.9-year-old boy, who was referred to our department for his tall stature. His basal GH level was 9.3 ng/mL, and failure of GH suppression was reported during OGTT (nadir GH, 9.0 ng/mL). Pituitary MRI showed a 6-mm-sized pituitary adenoma. Surgery was done and histopathological examination demonstrated a pituitary adenoma with positive staining for GH. Three months after surgery, the GH level was 0.2 ng/mL{His GH was elevated by a factor of 46.5} and nadir GH during OGTT was less than 0.1 ng/mL. Pituitary MRI scans showed no residual tumor. ”

So key factors involved in Gigantism involve HGH much higher than normal and a resistance to HGH surpression.  According to this steriods forum, some experienced elevated serum HGH levels as high as 18ng/mL which is within the range of the serum levels of the two cases of gigantism.  Which means in terms of serum GH level, HGH injections may be sufficient to cause gigantism.  However, resistance of the HGH to surpression is one thing that is not conferred by the injections as the HGH injections have to be cycled.

“True gigantism is extremely rare”<-Which is why again it is not likely to be caused by elevated HGH alone.

“normal range, 0-5 ng/mL [For GH]”

In the 38.4ng/ml case for Gigantism, his IGFBP-3 was additional elevated.  IGF-1 levels were slightly above normal range.  Prolaction was within normal range.

The other boy with 9.3ng/ml case for Gigantism, had prolaction, IGFBP-3 and IGF-1 all in normal range.  But this boy was actually taller than the other case beside having about a 1/4 less HGH.

“Approximately 100 cases of children with pituitary gigantism have been reported”<-But yet gigantism was caused with levels of only about twice the maximum the normal range.

“Hyperprolactinemia is a common finding in GH excess presenting in childhood, undoubtedly related to the fact that mammosomatotrophs (GH and prolactin-secreting cells) are by far the most common type of GH secreting cells involved in childhood gigantism. However, gigantism caused by a pituitary tumor comprised of somatotropes (GH-secreting cells) show a normal prolactin level”