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Biology of Reproduction, Vol 11, 624-630, Copyright © 1974 by Society for the Study of Reproduction
1 Department of Physiology, University of Pittsburgh School of Medicine, Pittsburgh,
Pennsylvania 15261 A radioimmunoassay for serum testosterone that does not require chromatographic purification of samples was developed and used to describe the temporal pattern of this steroid
in adult male rhesus monkeys. Serum gonadotropins were measured by radioimmunoassays
previously described. Testosterone levels in male rhesus monkeys show a threefold diurnal
variation with peak levels of 17.0 ± 1.5 (SEM) ng/ml appearing at 2100 h. No concomitant
variations in circulating LH are observed, however. Adrenal androgen secretion does not
contribute significantly to this nocturnal testosterone elevation since castrated males show
no circadian variation in testosterone levels. In orchidectomized animals implanted with
Silastic capsules containing testosterone, the resulting serum levels of the steroid are constant, indicating that changes in metabolic clearance rate cannot account for the testosterone
fluctuations seen in the intact male. These observations suggest that the diurnal shifts in
testosterone concentration are the consequence of changes in testicular secretion rate. In
contrast to intact animals, orchidectomized monkeys show a significant nocturnal elevation
in serum LH concentrations suggesting that, in normal males, increases in circulating LH
too small to be measured with our radioimmunoassay may account for the nocturnal rise
in serum testosterone levels.
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