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Previous studies have suggested that insulin-like growth factor-I (IGF-I) and its binding proteins (IGFBPs) have a pathogenetic role in idiopathic osteoporosis. To investigate this question further we compared 20 men with idiopathic osteoporosis with 12 healthy, age-matched men regarding growth hormone (GH) secretion and sensitivity. GH samples were drawn every 30 minutes for 24 hours from 12 of the patients and all controls, and cumulated GH secretion (24 hGH) was derived. Peak GH secretion (peakGH) was provoked by an insulin tolerance test. There were no differences between the groups in serum IGF-I (162 +/- 30 vs 163 +/- 47 micrograms/liter, mean +/- SD), IGFBP-3 (2474 +/- 263 vs 2568 +/- 197 micrograms/liter), 24 hGH (1.34 +/- 1.26 vs 0.79 +/- 0.43 U), or peakGH (53.0 +/- 21.5 vs 44.1 +/- 19.8 mU/liter). Patients and controls were given GH (2.4 U/day) for 1 week. Serum levels of markers for bone turnover increased significantly in both groups, with no difference in response to GH between the groups. The increase in urinary bone resorption markers was only significant in the controls. In the patients, but not in the controls, there were significant positive correlations between indices for GH secretion and markers for bone turnover at baseline and significant negative correlations with relative changes in bone markers during GH treatment. In this study no difference in GH secretion was found between men with idiopathic osteoporosis and controls, but the findings suggest that the GH/IGF-I axis could play a regulatory role in bone metabolism in men with this condition.

Estrogen deficiency is an important pathogenetic factor in female osteoporosis, and androgens are known to have anabolic effects on bone. In this study we have compared 12 men with idiopathic osteoporosis, age 27-55 years, with 12 age-matched men, with respect to serum levels of sex steroids, biochemical markers of bone turnover, bone density, and body composition. All subjects showed values within the normal range for all hormonal parameters. The patient group compared with the controls had significantly lower serum levels of estradiol (71 +/- 13 versus 85 +/- 14 pmol/liter, P < 0.03); estradiol/sex hormone-binding globulin (SHBG) ratio (22.4 +/- 12.1 versus 39.5 +/- 18.6 pmol/mg, P < 0.02); free androgen index (51.0 +/- 19.4 versus 74.1 +/- 33.1%, P < 0.05); and higher SHBG (3.7 +/- 1.6 versus 2.5 +/- 1.0 mg/liter; P < 0.04). The men with idiopathic osteoporosis had significantly lower body mass index (23.2 +/- 2.8 versus 25.9 +/- 3.3 kg/m2, P < 0.05); and a tendency to lower percentage of total body fat (14.2 +/- 5.5 versus 18.6 +/- 6.0%; P < 0.10) than the controls. Regression analyses revealed that bone mineral density in femoral neck correlated significantly and positively with the ratio estradiol/SHBG (r = 0.67; P < 0.04) and negatively with SHBG concentrations (r = -0.63; P < 0.04) in the group of patients. These findings could represent a pathogenetic mechanism in male idiopathic osteoporosis.

Idiopathic osteoporosis in younger individuals could be related to reduced bone formation rather than increased bone resorption, and disturbances in GH or insulin-like growth factor (IGF)-I production could be involved in its pathogenesis. In the present study, men with idiopathic osteoporosis were compared with healthy men, with respect to bone histomorphometry and to serum levels of IGF-I, IGF-II, IGF binding protein (IGFBP)-2 and IGFBP-3, and 24-h urinary excretion of GH. Mean wall thickness was reduced in the patients (48.3 +/- 7.2 vs. 61.7 +/- 5.4 microns, P < 0.001). Also, resorption depth was decreased, albeit to a lesser degree (54.4 +/- 3.8 vs. 60.7 +/- 5.3 microns, P < 0.01), thus creating a pronounced negative balance (-6.04 +/- 9.8 vs. 0.96 +/- 3.2 microns, P < 0.05). In the patients, serum concentrations of IGFBP-3 were reduced, compared with controls, with a 46% lower mean value; whereas levels of IGF-I, IGF-II, IGFBP-2, and GH were similar in the two groups. Thus, there was a significant negative balance caused by a pronounced decrease in wall thickness in men with idiopathic osteoporosis. The finding of low IGFBP-3 levels in these patients is interesting, in view of previous clinical and experimental findings, but its pathophysiological significance remains to be determined.

Injections with growth hormone (GH) or insulin-like growth factor I (IGF-I) have been proposed for anabolic therapy in osteoporosis. In a cross-over study, 12 men with idiopathic osteoporosis received daily subcutaneous injections of GH (2 IU/m2) or IGF-I (80 micrograms/kg) for 7 days with 12 weeks of wash-out. Serum levels of procollagen type I increased by 29% following treatment with GH (P < 0.001) and by 43% with IGF-I (P < 0.001 compared with pretreatment levels; P < 0.05 compared with GH injections), whereas both treatments rendered a 20% increase in osteocalcin concentrations (P < 0.001), indicating enhanced bone formation. There was also evidence of stimulated bone resorption, as the urinary levels of deoxypyridinoline increased by 44% following GH injections (P < 0.001) and by 29% following IGF-I (P < 0.001), and there were 28% higher serum concentrations of IGF-I after GH than after IGF-I injections. Although markers of bone metabolism increased under both treatments, comparison of the treatments suggests that IGF-I enhanced formation of collagen type I more than did GH. Furthermore, the stimulation of bone resorption was detected as soon as 4 days after the initiation of GH injections. Some of the differences might be dose-dependent, but could also indicate separate mechanisms at the cellular level.

In response to treatment with growth hormone, serum levels of lipoprotein(a) increase, while those of LDL cholesterol decrease. To establish if increased levels of insulin-like growth factor-I may be of importance for these changes, we analyzed serum lipoprotein concentrations in 11 male patients with idiopathic osteoporosis who were treated with growth hormone (2 IU.m-2.d-1) or insulin-like growth factor-I (80 micrograms.kg-1.d-1) in a randomized, double-blind, cross-over study. LDL cholesterol was reduced by 0.7 mmol/L (P < .01) during growth hormone treatment but was not affected when the same patients received insulin-like growth factor-I. In contrast, mean lipoprotein(a) levels increased from 519 to 571 mg/L (P < .03) in response to growth hormone but were reduced from 538 to 478 mg/L (P < .04) during treatment with insulin-like growth factor-I. These results indicate that growth hormone exerts its effects on lipoprotein metabolism independent of insulin-like growth factor-I. Furthermore, the results suggest that treatment with insulin-like growth factor-I may reduce lipoprotein(a) levels.

Experimental studies in vitro indicate that insulin-like growth factor 1 (IGF-1) could be of importance for normal bone growth and remodelling, but the clinical relevance of these observations is unknown. In 12 consecutive young to middle-aged male patients (mean age (+/- SD) 46 +/- 8 years, range 30-57 years) with symptomatic idiopathic osteoporosis, the plasma concentrations of IGF-1 were significantly lower than in healthy subjects (0.51 +/- 0.25 vs. 0.73 +/- 0.23 U ml-1; P less than 0.01). The bone mineral densities in the spine, the femoral neck, and the forearm were significantly different between patients and control subjects. There were positive correlations between the plasma IGF-1 concentrations and the bone densities of the spine and the forearm. Three of the patients received a 5-d course of human recombinant growth hormone (GH). During this short period significant increases in plasma IGF-1 levels and in biochemical indices of bone turnover (serum bone-specific alkaline phosphatase, urinary calcium excretion) were recorded. These observations indicate that circulating IGF-1 could have an important role in maintaining bone mass, and suggest that impairment of IGF-1 production is involved in the pathogenesis of osteoporosis.