Androstendione, or androstone, or simply "andro" is a hormone found in all animals as well as some plants. It is a metabolite of DHEA and serves as a direct precursor of testosterone, the principal male sex hormone. 

Androstendione is produced in the gonads and the adrenal glands and is required for the growth and maturation of the male sex organs. It also stimulates the growth of muscles and is used by athletes to enhance their athletic performance.

Many males are taking andro as a testosterone that helps to stimulate and restore libido, increase energy, and a general sense of well being.