The relationship between endogenous testosterone and gonadotrophin secretion

Abstract

The secretion of luteinizing hormone (LH), follicle stimulating hormone (FSH) and testosterone in the adult male was studied by means of 48-hour profiles. The aim of the study was to examine the effects of the endogenous circadian variation in serum testosterone concentration on LH pulsatility, and to determine the temporal relationships between FSH and LH, and between LH and testosterone by cross-correlation, and the pulse frequency of these hormones by spectral analysis, as revealed by extended sampling periods. Hormone profiles were obtained by 20-minute sampling over 48 hours. Six healthy adult males, aged between 21 and 23 years. LH and FSH were measured using an immunoradiometric assay and testosterone with a solid-phase radioimmunoassay. The profiles showed pulsatile secretion of all three hormones, and a circadian rhythm with levels highest between 0200 and 0600 h. Cross-correlation analysis: There was a significant relationship between LH and FSH (r = 0.5; P = 0.001) at 0 minutes (i.e. no time lag). A pulse of testosterone followed on average 60 minutes after a pulse of LH (r = 0.26; P = 0.001). Fourier transformation: Spectral analysis showed the dominant period for LH and FSH pulsatility to be 200 minutes. The dominant period for testosterone pulsatility was 400 minutes. The effect of endogenous variations in serum testosterone concentrations: The LH profiles were divided into periods when the serum testosterone concentration was high or low, and Fourier transformation carried out on these shorter periods. These transforms showed that the dominant period during high serum testosterone concentration was 180 minutes, and during low serum testosterone was 120 minutes (P < or = 0.025). FSH and LH are co-secreted, and a pulse of testosterone follows a pulse of LH by 60 minutes. The physiological changes in serum testosterone concentration that occur during the day result in changes in the pulse frequency of LH. Testosterone concentrations thus have rapidly acting feedback activity at hypothalamic level.