(comment: the above 1990 study published in the New England Journal of Medicine was the first to show growth hormone's impact on average, healthy men with no diagnosis of growth hormone deficiency)

According to the United States Hypopituitary Control and Complications Study Advisory Board, patients with a serum IGF-1 level of less than 84 do not require GH stimulation testing for the diagnosis of Adult Growth Hormone Deficiency.

This study investigated what IGF-1 level ranges constitute functional growth hormone deficiency (GHD) commensurate with actual GHD. For men, the median IGF-1 level was 94 (range of 64-141), where for women it was 73 (range 46-103)

The Division of Pediatric Endocrinology and Diabetes, Stanford University Medical Center, examined the pitfalls associated with GH stimulation tests, specifically, the lack of reliability and accuracy of these tests, and their inability to predict who will benefit from GH therapy. They recommend that GH stimulation tests no longer routinely be used for the diagnosis of GHD in children.

Growth hormone treatment for 10 years in growth hormone deficient adults resulted in increased lean body and muscle mass, a less atherogenic lipid profile, reduced carotid intima media thickness, and improved psychological well-being.

GH treatment of hypopituitary GHD men reverses early morphological and functional atherosclerotic changes in major arteries and, if maintained, may reduce vascular morbidity and mortality. GH seems to act via IGF-I, which is known to have important effects on endothelial cell function.

GH and IGF-1 have an important role in the decline in vascular density with age and suggest that decreases in vascular density may have important implications for the age-related decline in cerebral blood flow and brain function. These data indicate that: 1) vascular density on the surface of the cortex decreases with age; 2) vascular density is correlated with plasma levels of IGF-1; and 3) injection of GH increases cortical vascular density in older animals.

Decrease in Carotid Intima-Media Thickness after One Year Growth Hormone (GH) Treatment in Adults with GH Deficiency

Here's a nice overview of the role of IGF-1 in the aging brain.

IGF-I may promote the structural integrity of cerebral arteries, thereby offering protection from hemorrhagic stroke. These considerations may help to explain why tallness is associated with low stroke risk, whereas growth hormone deficiency increases stroke risk.

Neuronal population is reduced in the hypocampus of old rats as compared to young ones and GH treatment is able to significantly enhance the number. Neurotransmitters were measured in several cerebral areas to establish differences between young and old GH-treated or untreated animals. Glutamine, Arginine and Aspartate were reduced in old animals whereas Citruline was increased. GH treatment restored in all cases the levels corresponding to young rats.

Insulin-like growth factor I is required for vessel remodeling in the adult brain.

IGF-1 and brain-derived neurotrophic factor (BDNF) are trophic factors required for the viability and normal functions of various neuronal cells.

Higher IGF-1 levels are associated with higher IQ levels in age-matched children. IGF-1 is associated with brain development in childhood.

Harvard Medical School study documenting higher IGF-1 levels associated with better general cognition.

Growth hormone supplementation increases concentration of beta-endorphin in brain.

Higher mid-life IGF-1 may be associated with better late-life cognition.

Decreased serum IGF-1 level and the progression of carotid atherosclerosis could play a role as independent risk factors for dementia.

Low-dose growth hormone replacement (GHR) improves body composition and quality of life (QoL) as early as 1 month after commencement and the beneficial effects continue at 3 months. Most importantly, these changes occur in the absence of side-effects. We therefore suggest the use of low-dose GH therapy, maintaining IGF-I between the median and upper end of the age-related reference range, for the treatment of AGHD.

After 6 months, patients receiving human growth hormone (hGH) treatment experienced less perceived illness than the placebo group. Significant psychological improvement was noted in the hGH-treated patients' perception of their energy level and mood compared to the placebo group.

Reduced immunocompetence may be one of the consequences of reduced IGF-I levels in human aging. Among the three types of immune cells tested, the T-cells were most sensitive to fluctuations in IGF-I levels. Reduced IGF-I availability may be one of the determinants of the decline in T-cell-mediated immune function in the elderly.

This study confirmed in a large group of patients the beneficial effects of recombinant hGH therapy on body composition, metabolic parameters and general well-being and found a consistent drop in number of sick days and hospitalization rate. These effects were maintained during two years of therapy.

Increased bone density after recombinant human growth hormone (GH) therapy in adults with isolated GH deficiency. These results demonstrate that GH plays an important role in maintaining the integrity of the adult skeleton.

Growth hormone replacement significantly decreases fracture rate.

Growth hormone accelerates osteochondral defect healing by stimulating the formation of osseous and chondral tissue.

The age-related decline in insulin-like growth factor-I dependent proteoglycan synthesis in nucleus pulposus is caused, at least in part, by the increase in insulin-like growth factor binding proteins at the early stages of aging, and further suggest that a loss of proteoglycan synthesis during the late stages of aging is caused by the downregulation of insulin-like growth factor-I receptor in addition to an increase in insulin-like growth factor binding proteins.

The actions of IGF-I in the inner ear suggest that this factor may have therapeutic potential for the treatment of hearing loss.

Local application of IGF-1 to the cochlea significantly protects hearing to loud noise exposure.

Older patients with adult -onset growth hormone deficiency who receive GH replacement therapy achieve beneficial effects equivalent to those seen in younger patients in all age groups and require lower doses of GH.

Physically trained elderly males have increased testosterone, growth hormone, VO2max, and decreased reaction time compared to sedentary elderly males. Elevated serum testosterone and GH levels may be advantageous for brain functions.

Coadministration of low dose growth hormone with low dose testosterone results in beneficial changes in healthy, elderly men with no formal diagnosis of growth hormone deficiency. These beneficial changes include increased lean body mass, decrease in total body fat, increased aerobic capacity and mid-thigh muscle mass, and quality of life questionaire scores. All of these results were superior to placebo, and no major adverse effects were observed.

No evidence of tumor growth stimulation in human tumors in vitro following treatment with recombinant human growth hormone.

Chronic excess of GH and IGF-I cause prostate overgrowth and further phenomena of rearrangement, but not prostate cancer.

Because the simultaneous treatment with GH and testosterone induces an increase of prostate size by 50% of baseline on average, care is suggested in elderly patients with prostate hyperplasia to avoid any risk of prostate symptoms.

We have no evidence as to whether growth hormone in modern dosage regimens is associated with an increased risk of colorectal cancer.

Growth hormone therapy does not appear to increase the risk of disease recurrence or death in survivors of childhood cancer.

Growth hormone therapy does not increase the risk of recurrence of childhood brain tumors.

Women with uterine cancer have significantly lower IGF-1 levels than age-matched controls.

High levels of IGF-1 were associated with good prognosis in patients with advanced lung cancer.