The answer: HORMONE IMBALANCES.
When you start putting exogenous hormones into your body it throws a very complex and intricately connected system into flux.
The steroid itself doesn’t cause gyno. The problem lies in the imbalance of hormones that they create. High aromatizers, or those with high amounts of the enzyme aromatase (responsible for converting testosterone to estradiol (E2), or estrogen) can throw their ratio of testosterone:estrogen out of whack and contribute to gynecomastia. Other hormone imbalances (progesterone, SHBG, DHT, etc) can be influenced by certain steroid compounds and can lead to the dreaded moobs. Depending on the severity, men can build a functioning mammary system and even lactate!
Puberty is also a time where the endocrine system is trying to find homeostasis. There are many boys who develop breast tissue during these years. I’ve met trainers who, on their journey to lose weight, reveal this breast tissue as it becomes more apparent with low body fat. Unless developing gyno is addressed immediately with other drugs, the tissue is usually permanent and must be removed surgically. Puberty gyno found later in life has to be removed by knife.
Then there is fat, which is a fascinating tissue. It’s not just a storage dump for reserve calories. It’s a very biologically active tissue and a endocrine producer too. Fat creates aromatase, which is why males who start getting too fat may also start growing boobs. It’s one of the huge reasons why those who choose to get on gear should drop as much weight as possible BEFORE going on cycle (15% bf or less is what is recommended). That extra fat-produced aromatase paired with extra testosterone means lots of conversion to estrogen and a perfect recipe for man boobs.
Individuals who are transitioning from male to female use this to their benefit. They jack up their estrogen and, voila, begin to grow breasts.
To sum it up, being on steroids does not automatically cause gyno. A neglected or imbalanced endocrine system is the cause. Most enhanced trainers I know are careful to take blood work often (or use other common symptoms of ele