Impact of Testosterone Replacement on the Maturation of the Growth Hormone—IGF-I Axis

The regulated mode of growth hormone (GH) secretion is sexually dimorphic in both animals (1) and humans (2–6). During human pubertal development, there is preferential augmentation of the amplitude of spontaneous GH pulses, with a subsequent return to or a fall below prepubertal values in early adulthood (2). Some clinical data indicate a sex difference in the timing of these physiological changes in activity of the somatotropic axis (3). Moreover, a large body of evidence has been accumulated showing significant sex differences in GH responses to various pharmacological stimuli in young adults (4). However, the majority of data on sex hormone-mediated regulation of spontaneous and stimulated GH secretion in the human has been derived from cross-sectional population studies including healthy subjects of normal stature representing one or more stages of pubertal development (2,5,6). In relation to the specific actions of testosterone on GH secretion, these previous experiments have suggested an augmentation of the calculated maximal rate of GH release attained per secretory episode, resulting in a greater mass of GH released per secretory burst (6). A positive correlation between the magnitude of plasma GH responses to some provocative stimuli and androgen concentrations in human subjects (6) has also been suggested by cross- sectional studies.