Testosterone is an anabolic hormone naturally produced in response to resistance training and associated with muscle growth.
It is produced primarily by the testes in males and in smaller amounts by the ovaries and adrenal glands in females. Resistance exercise can cause short-term changes in circulating testosterone, although the size and duration of that response vary with age, sex, training status, exercise volume, intensity, and measurement timing. Testosterone acts through androgen receptors and can support muscle-protein synthesis and other processes involved in maintaining lean tissue.
The question is often oversimplified in two ways. First, a brief post-workout rise does not by itself guarantee greater long-term hypertrophy. Second, testosterone is only one part of the adaptation process; mechanical tension, nutrition, recovery, muscle-protein turnover, and other signaling pathways also matter. Cortisol is commonly described as catabolic, while insulin is primarily a metabolic hormone rather than the standard answer here.
Testosterone is also distinct from anabolic steroids used as drugs, which are synthetic or modified androgenic compounds and can produce effects beyond normal exercise-related fluctuations.