This finding contrasts with those of other reports of reduced T in response to spaceflight. The use of urine samples may provide a more comprehensive analysis of effects on the hypothalamic-pituitary-gonadal (HPG) axis. Clearly, more studies are required before definitive statements can be made regarding effects of spaceflight on male reproductive hormones.
Reproductive changes do not always occur at the testes but may occur at the level of the hypothalamus or pituitary. The hypothalamus produces GnRH, which acts on the pituitary gland to stimulate secretion of FSH and LH. Decreased T in various biological fluids of male astronauts occurred in parallel with increased plasma LH, which returned to normal after flight. The in-flight increase in LH was suggested to be a compensatory mechanism in response to decreased T levels. However, Ortiz et al. reported increased urinary LH in rats in response to increased urinary T after flight. In both animals and humans, the pituitary responded to changes in plasma T, indicating that the HPG axis was not impaired by spaceflight. Reproductive changes in males following spaceflight suggest that hypogravity or other factors associated with spaceflight may compromise male reproduction.