Libido

Estrogen Receptor alpha

The estrogen receptor that dominates breast tissue, liver and the pituitary feedback loop. Gynecomastia and much of the negative feedback on LH run through it.

Local Brain Estrogen Synthesis

The brain makes its own estradiol from testosterone, in neurons and astrocytes, independently of what the testes supply. This single fact is why a blood estradiol reading does not tell you what the brain has, and why an aromatase inhibitor is a different kind of intervention from simply having low testosterone.

Medial Preoptic Area

The most aromatase-rich region in the male brain. Testosterone is converted to estradiol here before it acts, which means much of what looks like an androgen effect on sexual behaviour is an estrogen effect one step downstream.

Progesterone Receptor

The progesterone receptor. 19-nor steroids are structurally close enough to progesterone to activate it, which is why nandrolone and trenbolone produce progestogenic side effects that an aromatase inhibitor cannot touch.