Growth hormone secretion and bone mineral density in prepubertal black and white boys.

Racial differences in bone mineral density (BMD) appear to account in part for racial differences in the incidence of osteoporosis and fractures. We previously reported that the greater BMD in adult blacks compared with whites is associated with a higher serum 17 beta-estradiol and greater secretion of growth hormone (GH) in men but not women. To determine whether these racial differences occur in prepubertal boys, we measured spontaneous overnight GH secretion, serum testosterone, 17 beta-estradiol, IGF-I, and IGFBP3, IGF-I/ IGFBP3 ratio, BMD of the total body, forearm, lumbar spine, trochanter, and femoral neck, and lean body mass and body fat in 14 healthy black and 16 white boys ages 6-7 years. Measurements of GH were obtained at 20-minute intervals for 12 hours. Results were analyzed by deconvolution and are expressed as mean +/- SE. Whereas BMD of the hip (0.755 +/- 0.020 vs 0.663 +/- 0.021 g/cm(2), P = 0.0037), trochanter (0.617 +/- 0.014 vs 0.552 +/- 0.018 g/cm(2), P = 0.0102) and femoral neck (0.710+/-0.018 vs 0.6381 +/- 0.021 g/cm(2), P = 0.0157) were significantly greater in black compared with white boys, BMD of the total body (0.768 +/- 0.010 vs 0.741 +/- 0.012 g/cm(2), NS), forearm (0.405 +/- 0.010 vs 0.380 +/- 0.008 g/cm(2), NS), and lumbar spine (0.612 +/- 0.013 vs 0.609 +/- 0.021 g/cm(2), NS) was not different in the two groups. Stepwise regression analysis showed significant correlations between BMD and race at each skeletal site except the lumbar spine and trochanter. Deconvolution analysis revealed no racial difference in any of the GH measurements. Whereas serum testosterone, serum 17 beta-estradiol, and serum IGF-I were not different, serum IGFBP-3 was higher and the molar ratio of serum IGF-l/IGFBP-3 was lower in white than in black males. In summary, prepubertal BMD is higher in black than in white males at the hip, trochanter, and femoral neck, and the racial difference does not result from differences in secretion of GH.

Two sensitive immunometric assays for serum thyroid stimulating hormone evaluated.

This study was conducted to evaluate the analytical performance (functional sensitivity, reproducibility, parallelism, and accuracy) of two recent commercial kits marketed as third generation immunometric assays for measuring serum thyroid stimulating hormone (TSH). One assay is automated; the other is manual. Accuracy by method comparisons was evaluated using 86 patient samples assayed by an established third generation immunometric assay as the comparative method. The new assays met the third generation criterion for functional sensitivity (CV < or = 20 percent at TSH < or = 0.02 mIU/L), were reproducible (CVs < 11 percent), and measured serum TSH in parallel with the calibrator curves. Linear regression analysis of the intermethod comparison data showed highly correlated (R > .095) results; however, the regression slopes were non-unity, indicating patient sample results were not transferable between methods. Clinical laboratories choosing a third generation TSH assay should validate the performance characteristics of the selected method to ensure reliable results for patient care.

Increased serum 1,25-dihydroxyvitamin D after growth hormone administration is not parathyroid hormone-mediated.

To assess the effects of growth hormone (GH) on serum 1,25-dihydroxyvitamin D [1,25(OH)2D], we performed the following prospective crossover study in six healthy, young, adult, white men. During each of two admissions for 2 1/2 days to a general clinical research center, subjects were placed on a daily dietary calcium intake of 400 mg. Serum calcium, phosphorus, 1,25(OH)2D, immunoreactive intact parathyroid hormone (PTH), insulin-like growth factor I (IGF-I), IGF binding protein 3 (IGFBP3), tubular reabsorption of phosphate (TMP/GFR) were measured. Recombinant human GH (rhGH, Humatrope) (25 microg/kg/day subcutaneously for 1 week) was administered prior to and during one of the admissions. Results are expressed as mean +/- SEM. Whereas serum 1,25(OH)2D (58.9 +/- 7.7 versus 51.6 +/- 7.4 pg/ml, P< 0.01), serum phosphorus (4.5 +/- 0.1 versus 3.7 +/- 0.1 mg/dl, P < 0.01), TRP (92.0 +/- 0.5 versus 87.8 +/- 0.7 mg/dl, P < 0.005), TMP/GFR (4.6 +/- 0.1 versus 3.5 +/- 0.2, P < 0.005), and urinary calcium (602 +/- 49 versus 346 +/- 25 mg/day, P < 0.001) increased significantly, serum PTH decreased significantly (19.9 +/- 1.9 versus 26.8 +/- 4.0 pg/ml, P < 0.05) and serum calcium did not change when subjects received rhGH. These findings indicate that in humans, GH affects serum 1,25(oh)2D independently of circulating PTH and that this effect is mediated by IGF-I. We propose, therefore, that one potential mechanism by which GH stimulates increases in bone mass is via modest increases in serum 1,25(OH)2D.

Demonstration of a lack of racial difference in secretion of growth hormone despite a racial difference in bone mineral density in premenopausal women--a Clinical Research Center study.

We previously found GH secretion to be higher in black than white men. Therefore, we performed studies to determine whether this racial difference in GH secretion also occurs in women. Measurements of GH were obtained at 20-min intervals over 24 h and analyzed by deconvolution in 12 healthy black and 12 healthy white premenopausal women. Bone mineral density (BMD) was determined by dual energy x-ray absorptiometry, and GM allotypes were measured as a genetic marker for race. Racial distribution of the groups, as determined by analysis of GM haplotypes, were typical for black and white American populations. Twenty-four-hour integrated GH concentration, GH secretory burst amplitude, burst frequency, half-duration, mass, and half-life were not different in the two groups. Serum testosterone was modestly, but significantly, greater in the black than in the white women (1.1 +/- 0.1 vs. 0.9 +/- 0.1 nmol/L; P < 0.05). Serum 17 beta-estradiol and insulin-like growth factor (IGF)-binding protein-3 were not different in the two groups. However, the IGF-I/IGF-binding protein-3 molar ratio was significantly greater in the black than the white women (2.0 +/- 0.1 vs. 1.6 +/- 0.1; P < 0.02). The BMD of total body (1.12 +/- 0.02 vs. 1.07 +/- 0.02 g/cm2; P < 0.05) and total hip (0.96 +/- 0.04 vs. 0.86 +/- 0.04 g/cm2, P < 0.05) were greater in the black (n = 13) than in the white (n = 12) women. There was a trend toward greater BMD of the forearm in the black women (0.58 +/- 0.01 vs. 0.56 +/- 0.01 g/cm2; P = 0.06) and no racial difference in the BMD of the spine. When examining all subjects together, the BMD of the total body, trochanter, and spine correlated with total integrated GH secretion. Thus, the racial difference in GH secretion that we had previously found in men does not occur in women despite the higher BMD values at several skeletal sites in black women.