Male hair loss is very common. By the age of 50 half of men will have noticeable hair loss, with 80% affected by age 70. Significant hair loss occurs in many younger men in their 20s.
There are may causes of hair loss, the most common being androgenetic hairless, or hair loss that is preprogrammed by our genes. It appears as a receding hairline with hair loss on the top and front and crown of the head. It is caused by the sensitivity of hair follicles to dihydroxytestosterone (DHT). Hair grows in a three phase growth cycle: anagen, catagen, and telogen. Anagene is the growth phase, catagen is an involution phase, and telogen is a resting phase. DHT shortens the growth phase and causes the miniaturization of hair follicles leading to hair loss.
Most hair is in the Anagen or growing phase of the hair cycles. About 90% are growing at any given time. This phase last for 2-6 years. Hair in this phase typically has a long and deep hair follicle that is well-pigmented.
Catagen or the involution phase of the hair cycle is short, lasting 1-2 weeks. The base of the follicle shrivels up leading to the next phase of the hair cycle. The hair remains in place.
Telogen or the resting phase follows catagen. The resting hair remains in place. At the end of telogen and at the start of anagen, the newly growing hair pushes the old hair out and grows to the surface of the skin and beyond.
Hair on the scalp grows in tufts of 3-4. Through a process called miniaturization, as we age, the tufts lose hair. As the members of the tufts are lost, the hair on the head starts to appear thinner.