In The use of a GH receptor antagonist (GHRA) to explore the relationship between GH and IGF-I in adults with severe growth hormone deficiency (GHD), the authors research the question of why some folks with deficient GH still have "normal" IGF-I levels but others with deficient GH have low IGF-I levels.
[A]pproximately 50% of middle-aged patients diagnosed with severe GHD, defined by peak stimulated GH levels of <3>Using Pegvisonmant, a highly selective GH receptor antagonist used to treat acromegaly by blocking GH action, they determined those who had low IGF-I levels with AGHD could not increase secretion of GH but "[i]n those with a normal IGF-I, baseline and stimulated GH secretion can be increased by reducing IGF-I." This suggested to them that patients with low IGF-I levels were that way because they weren't regulated by GH (GH independent) but that patients with normal IGF-I levels were GH-dependent.
Does this mean there is a difference in severity in the two types of patients? According to the authors it does:
They do admit the need for further study, indicating a possible "genetic contribution to the difference in IGF-I levels between [AGHD] patients with low and normal IGF-I."
These data show that in these patients, who meet the consensus criteria for severe adult GHD, the hypothalamic-pituitary axis is capable of increasing GH secretion in response to a fall in IGF-I thereby implying that the previously unexplained paradox of patients with severe adult GHD having normal IGF-I levels can be accounted for by subtle differences in on-going physiological GH secretion not discernable by the ‘crude’ stimulation tests used to diagnose adult GHD. Hence, this study shows that patients with normal and low IGF-I differ in the severity of GHD.
CA Berg, A Pokrajac, M Bidlingmaier, CJ Strasburger, SM Shalet, PJ Trainer (2008). The use of a GH receptor antagonist (GHRA) to explore the relationship between GH and IGF-I in adults with severe growth hormone deficiency (GHD) Clinical Endocrinology DOI: 10.1111/j.1365-2265.2008.03481.x