The sudden cessation of underarm hair growth can be an unexpected and confusing change, prompting questions about underlying causes. Hair growth patterns are dynamic biological processes governed by genetics, hormones, and external factors. When hair in the armpit stops growing, it suggests an interruption in the normal cycle of the hair follicles. The reasons can range from natural shifts in hormone levels to the effects of medications or physical damage.
Understanding the Hair Growth Cycle
All hair on the human body follows a predictable pattern of growth and rest, divided into three phases. The first phase, Anagen, is the active growth period where the hair shaft is produced. For underarm hair, this active phase is relatively short, typically lasting only 30 to 45 days. This short duration explains why axillary hair does not grow as long as scalp hair.
Next is the Catagen phase, a short transitional stage lasting only a few days where growth stops and the hair follicle shrinks. Finally, the hair enters the Telogen phase, a resting period lasting a few months before the old hair is shed and the cycle begins anew. Because the Anagen phase is short, any disruption that prematurely shifts follicles into the Telogen phase becomes noticeable quickly.
Hormonal Shifts That Affect Growth
The growth of underarm hair is controlled by androgens, such as testosterone and dihydrotestosterone (DHT). Androgens stimulate the fine vellus hairs in the armpit to transform into the thicker, darker terminal hairs observed after puberty. Therefore, a reduction in the level or effectiveness of these androgens is the most common reason for slowing or cessation of growth.
Major life stages naturally alter androgen levels. As people age, the production of androgens gradually declines, which can lead to thinning or a complete halt of armpit hair growth, often noticeable in the 50s. Women undergoing menopause also experience a significant decrease in hormone production, sometimes resulting in lighter or thinner axillary hair.
The thyroid gland produces hormones that regulate metabolism and significantly influence the hair cycle. An underactive thyroid (hypothyroidism) can cause general hair thinning and fragility, including in the underarms, by prolonging the resting phase. Hormonal imbalances like Polycystic Ovary Syndrome (PCOS) can cause excessive hair growth due to high androgens. However, treatment for such conditions often adjusts hormone levels, which can lead to reduced growth in androgen-dependent areas.
Underlying Health Conditions and Drugs
When hormonal causes are ruled out, specific medical conditions or pharmaceutical interventions may be responsible. Alopecia areata, an autoimmune disorder, can cause patchy hair loss anywhere on the body, including the armpits, as the immune system mistakenly attacks the hair follicles. Although often presenting as sudden, circular bald patches, it can affect any hair-bearing skin.
Certain nutritional deficiencies can also disrupt the hair cycle by depriving the follicles of necessary building blocks. Low levels of nutrients like iron, zinc, or protein can lead to telogen effluvium, where a large number of hairs prematurely enter the resting phase and shed. While typically causing diffuse thinning across the scalp, this systemic disruption can affect all body hair.
Several common medications interfere with the hair growth cycle, causing hair loss as a side effect. Chemotherapy drugs target rapidly dividing cells and often induce Anagen effluvium, causing severe loss of body hair, including in the armpits, within days or weeks of starting treatment. Other medications, such as anticoagulants, high doses of Vitamin A derivatives (retinoids), hormonal contraceptives, and certain antidepressants, can trigger Telogen effluvium, leading to increased shedding a few months after starting the drug.
Physical Damage and Lifestyle Factors
External factors related to hair removal or general health can cause a reduction in underarm hair growth. Repeated, long-term hair removal methods that physically stress the follicle, such as plucking or waxing, can lead to permanent damage over time. This trauma may cause the follicle to cease production or replace the terminal hair with a finer vellus hair.
Modern hair removal techniques like laser or electrolysis are designed to intentionally destroy the hair follicle, resulting in a permanent cessation of growth. If a person has undergone these treatments, the lack of hair is the intended outcome. Even without targeted removal, chronic inflammation, such as severe folliculitis, can scar the follicle, preventing hair from growing back.
Temporary disruptions, such as severe psychological stress, major surgery, or rapid weight loss, can trigger a widespread shift of hair into the resting phase. This phenomenon, Telogen Effluvium, causes increased shedding and temporary thinning across the body, including the underarms, which typically resolves once the underlying stressor is addressed. If the hair follicle is destroyed by scarring or physical trauma, regrowth is impossible; however, if the follicle is merely dormant due to hormones or stress, growth can resume.