Estrogen becomes a real problem for people experimenting with anabolic steroids or even taking some of the TRT regimens which go past replacement doses and into performance enhancing territory.
Testosterone is converted into estradiol by aromatase, so people who boost their testosterone up to high levels get more estradiol as a result.
As an aside: Aromatase is present in body fat, so higher body fat will produce more sites for testosterone to convert to estradiol. This is one reason why higher body fat is correlated with lower testosterone. It’s also one reason why the decline of testosterone levels are correlated with the rise of obesity and you shouldn’t trust anyone who rants about declining testosterone levels without acknowledging that major correlation.
Back on topic: The increased estradiol production from excess testosterone can go above the normal range in men, which can cause a wide range of mental and physical problems. It can even promote growth of breast tissue that when left unchecked needs to be surgically removed later. Surgeons who deal with gynecomastia are seeing booming business right now due to all of the men going to TRT clinics and getting blasted with crazy doses of testosterone.
There are medications which reduce aromatase activity but they are very hard to get right. It’s a common story for men to suffer from excess estradiol after manipulating their testosterone, so they assume they can fix it with an aromatase inhibitor. They take slightly too much (which is very easy due to the dosing and duration of action) and crash their estradiol levels too low. Between the sudden swing in levels and the low level they can find themselves feeling a different kind of terrible. Someone I know become suicidal after taking an aromatase inhibitor at the ‘normal’ recommending broscience dosage. It took weeks to clear due to the dynamics of how everything returns to equilibrium. Very scary time.
Estradiol is highly active in the brain including regulation of key functions like MAO (the enzyme inhibited by MAOI antidepressants). One of many reasons why out-of-range levels or sudden fluctuations can make people feel bad in various ways.