<p>Testosterone, discovered during the endocrine gold rush of the 1930s, was the first hormone chemically synthesized for replacement therapy. In both men and women, testosterone functions directly through the androgen receptor (AR) and indirectly as a prohormone, converted by aromatase into 17β-oestradiol (oestradiol), which activates the oestrogen receptors ERα and ERβ. Testosterone is also metabolized to dihydrotestosterone—a potent, non-aromatizable AR agonist—through steroid 5α-reductases. Testosterone and its metabolites signal through AR- and ER-mediated genomic and rapid non-genomic actions. Long recognized for its role as a sex hormone, mounting evidence underscores the importance of testosterone in the regulation of systemic metabolism in both male and female organisms. Here, we highlight key milestones in the history of testosterone’s discovery and therapeutic applications. Additionally, we synthesize the current understanding of testosterone as a key messenger promoting metabolic homeostasis in preclinical models and humans.</p>