Why does testosterone stimulate the growth of terminal hair on the face, axillae (armpits), and torso?

From the Guidelines

Testosterone causes hair growth on the face, under arms, and torso because it induces body and facial hair growth, as part of its masculinizing effects, as seen in individuals undergoing masculinizing hormone therapy 1. This process is a result of testosterone's ability to stimulate the development of terminal hair from vellus hair in androgen-sensitive regions.

• The effects of testosterone on hair growth are well-documented in the context of masculinizing hormone therapy, where individuals assigned female at birth experience increased body and facial hair growth as a result of testosterone treatment 1.
• The distribution and density of androgen receptors in different body areas determine the extent of hair growth, which can vary among individuals based on genetic factors.
• Testosterone therapy can lead to a range of physical changes, including thicker vocal cords, increased libido, clitoral growth, and an increase in muscle mass and reduction in fat mass 1.
• While testosterone can cause hair growth in certain areas, it can also lead to androgenic alopecia, or hair loss on the scalp, in genetically predisposed individuals 1.
• The most recent evidence suggests that testosterone therapy can have a range of effects on the body, including changes in haemoglobin levels, lipid profiles, and immune function 1.
• Overall, the relationship between testosterone and hair growth is complex and influenced by a range of factors, including genetics, hormone levels, and individual responses to hormone therapy.

From the FDA Drug Label

These effects include growth and maturation of the prostate, seminal vesicles, penis, and scrotum; development of male hair distribution, such as beard, pubic, chest, and axillary hair;

• Testosterone causes hair to grow on the face, under arms, and torso due to its role in development of male hair distribution.
• This is a result of the androgenic effects of testosterone, which include the growth and development of secondary sex characteristics, such as hair growth in these areas 2.

From the Research

Testosterone and Hair Growth

• Testosterone plays a crucial role in hair growth, particularly in the development of terminal hair on the face, underarms, and torso 3.
• The conversion of testosterone to dihydrotestosterone (DHT) is a key factor in terminal hair growth, with DHT acting on sex-specific areas of the body to convert small, straight, fair vellus hairs into larger, darker terminal hairs 3.
• The majority of hair follicles require the intracellular enzyme 5-alpha reductase to convert testosterone into DHT, which then binds to intracellular androgen receptors in the dermal papilla cells of the hair follicle 3.

Mechanism of Action

• The binding of DHT to androgen receptors in the hair follicle leads to the activation of various signaling pathways, resulting in the growth and maintenance of terminal hair 4, 3.
• The role of other hormones, such as estradiol, progesterone, and prolactin, in hair growth is also being researched, with evidence suggesting that they can influence the hair follicle growth and cycle 3.

Clinical Implications

• The use of 5-alpha reductase inhibitors, such as finasteride, can block the conversion of testosterone to DHT, leading to a decrease in prostate volume and an improvement in symptoms of benign prostatic hyperplasia 5.
• However, long-term use of finasteride and dutasteride may be associated with health risks, including non-alcoholic fatty liver disease, insulin resistance, type 2 diabetes, and dry eye disease 6.