What converts testosterone to DHT?

The main androgen hormone secreted in the body is testosterone but in order to be carried to tissue such as that in the brain or the skin, testosterone bonds with the enzyme 5-alpha reductase and produces DHT.

stress is linked to hair loss is because stress causes the release of cortisol hormone, which increases adrenaline, testosterone and DHT

DHT is responsible for male characteristics such as increased muscle mass and a deep voice and during fetal development it plays a role in the development of the penis and prostate. In normal levels, DHT regulates hair growth. If there is increased 5-alpha reductase in the body, the amount of testosterone converted into DHT will increase, which may lead to hair loss. This may be because increased levels cause an acceleration of the life cycle of the hair and increase in the hairs resting phase — this gradually shrinks the hair follicle by reducing blood circulation so the hair follicle is not properly nourished in order to survive.
DHT also seems to trigger an autoimmune response in the body that destroys the hair follicle. Most treatments for hair loss include blocking the conversion (e.g. finasteride) of testosterone to DHT in order to delay the progression of alopecia. (Ustuner, 2013)
DHT can be influenced by both external and internal factors. For example, the idea that stress is linked to hair loss is because stress causes the release of cortisol hormone, which increases adrenaline, testosterone and DHT. Endocrine issues such as an underactive thyroid can also influence hormone levels and lead to hair loss. People with an enlarged prostate will also show increased DHT levels and there’s even a theory (Bhargava, 2014) that the mechanism behind male pattern baldness is the body’s way to protect men from prostate cancer.