**1. Introduction**

Androgens have well-known trophic actions on the reproductive, nervous, skeletal, and cardiovascular systems throughout the lifespan. A focus in behavioral neuroendocrinology has been on understanding dose-dependency, brain targets, cellular mechanisms of testosterone (T) and its metabolites for reproduction through androgen receptors. Research has challenged the existing paradigm about androgen action,

#### **Figure 1.**

*Androgens target brain, heart, muscle, liver, kidneys, adrenals, testes, penis, bones (left) and (right) changes in androgens across the lifespan with a decade by decade decline naturally or by points indicated by arrows with pharmacological interventions or disease [1].*

as well as taking a novel approach to understand how these trophic actions in the brain occur in relation to growth in the body (reproductive organs). Given androgens' pervasive trophic effects from early development throughout adulthood, unique studies are discussed that impact our understanding of androgens' actions (**Figure 1**).
