FIGURE 10-3 Representative endocrine axes, pathways, and interrelations, with particular regard to resource partitioning.

NOTE: Based on Anderson et al. (2001), Arnalich et al. (1999), Baskin et al. (1999), Bruunsgaard et al. (2000), Harris (2000), Imura et al. (1991), Mantzoros and Moschos (1998), Mantzoros et al. (2001), Owen et al. (2001, Rasmussen (2000), Straub et al. (2000), Wauters et al. (2000), and reviews in Worthman (1999a, 2002).

From the upper left to the right in Figure 10-3, the axes are (1) the hypothalamic-pituitary-adrenal (HPA) axis, which regulates arousal and stress via cortisol, a hormone that also extensively affects the other axes; (2) the hypothalamic-pituitary-gonadal (HPG) axis, which regulates gonadal function, including output of gonadal steroids (estradiol, E2; testosterone, T), which are responsible for reproductive functions and behaviors and many sex differences; (3) the somatotropic axis, which regulates growth and resource allocation via a cascade of trophic factors and binding proteins produced in the liver and locally; and (4) the thyrotropic axis, responsible for regulating metabolic rate and required for neurological development and alertness. The relationship of growth to energy availability is built into the organization of this axis.