Underarm hair that gradually thins or stops growing altogether is usually driven by shifting hormone levels, and it happens more often than most people realize. Androgens, the hormones responsible for triggering underarm hair at puberty, can decline with age, medical conditions, or certain medications, and when they do, those follicles can quietly go dormant. The change is rarely dangerous on its own, but it sometimes signals something worth investigating, from a routine part of aging to an underlying endocrine condition that needs attention.
How Hormones Control Underarm Hair
Underarm hair is what dermatologists call “androgen-dependent” hair. Before puberty, the skin under your arms is covered with fine, nearly invisible vellus hairs. Around puberty, rising levels of androgens, particularly testosterone and its more potent derivative dihydrotestosterone (DHT), transform those fine hairs into the thicker, darker terminal hairs you recognize as underarm hair.1PubMed Central. Hormonal Effects on Hair Follicles This conversion depends on the follicles having enough androgen receptors and the bloodstream carrying enough of the hormones to activate them.
The critical detail is that different body sites require different androgen thresholds to produce terminal hair. Pubic hair follicles are more sensitive and respond to relatively low androgen levels, which is why pubic hair tends to appear first during puberty. Underarm follicles need stronger or more prolonged androgen stimulation to make the same switch.2PubMed. Androgens and hair growth This means underarm hair is often the first androgen-dependent hair to thin when hormone levels drop, because those follicles were the last to come online and sit closer to the threshold where a modest hormonal dip makes a visible difference.
Aging and Natural Hormone Decline
For many people, the explanation is simply getting older. Androgen levels do not stay at their peak indefinitely. In women, the transition into perimenopause and menopause brings a steep decline in ovarian hormone production, and the adrenal glands, which also produce a share of androgens, gradually reduce their output too. The result can be noticeably thinner or absent underarm and pubic hair over a span of years. This is common enough that many postmenopausal women consider it unremarkable.
In men, testosterone levels decline more gradually, typically dropping by roughly one to two percent per year after age 30. Most men retain enough androgen activity to keep body hair going well into old age, but some experience more pronounced declines, especially if other health factors are in play. By the 70s and 80s, reduced underarm hair is not unusual in men either, though it tends to be less dramatic than in women because baseline androgen levels started higher.
The follicles themselves also change with age, independent of hormones. Hair follicles can miniaturize over time, shrinking in size and producing thinner, shorter hairs that eventually become vellus-like again. The growth phase of each hair cycle shortens, so individual hairs spend less time actively growing and more time resting or shedding.3PubMed Central. Following historical “tracks” of hair follicle miniaturisation in patterned hair loss While that particular finding relates to scalp hair, the same general principle of follicular miniaturization applies to body hair follicles as they age.
Medical Conditions That Reduce Body Hair
When underarm hair loss happens earlier than expected or comes on suddenly, a medical condition may be involved. Several categories of disease can interfere with the hormonal signals or the follicles themselves.
Adrenal Insufficiency
Addison’s disease and other forms of adrenal insufficiency reduce the adrenal glands’ ability to produce androgens. Since the adrenal glands are a major source of androgens in women (and contribute in men as well), people with adrenal insufficiency often lose underarm and pubic hair while the hair on their scalp remains relatively unaffected. This pattern, where body hair thins but scalp hair does not, can be an early clue that something is off with adrenal function. Other signs typically include fatigue, low blood pressure, and skin darkening.
Thyroid Disorders
Both an underactive thyroid (hypothyroidism) and an overactive thyroid (hyperthyroidism) can affect hair growth across the body. Thyroid hormones help regulate the hair cycle, and when they are out of balance, hair can become sparse, dry, or slow-growing. The outer third of the eyebrows is classically the first to go in hypothyroidism, but underarm and body hair can thin as well. Thyroid-related hair changes are usually reversible once hormone levels are corrected with medication.
Autoimmune Conditions
Alopecia areata, an autoimmune condition where the immune system attacks hair follicles, can target any hair-bearing area of the body, including the underarms. In its more extensive forms (alopecia totalis or alopecia universalis), body hair loss can be widespread. Another condition called frontal fibrosing alopecia, which is a scarring form of hair loss most commonly seen in postmenopausal women, can also cause loss of axillary hair alongside its more recognizable pattern of receding the hairline and thinning the eyebrows.4PubMed Central. Male Frontal Fibrosing Alopecia with Generalized Body Hair Loss When frontal fibrosing alopecia causes scarring in the underarm follicles, the hair loss is permanent because the follicle itself is destroyed.
Pituitary Problems
The pituitary gland orchestrates much of the body’s hormone production. A tumor, injury, or other dysfunction affecting the pituitary can cause a cascading drop in multiple hormones, including the signals that tell the adrenal glands and gonads to produce androgens. When this happens, body hair loss is often one piece of a broader pattern that includes fatigue, changes in menstruation, and other hormonal symptoms.
Medications That Thin or Stop Body Hair
Several commonly prescribed drugs can reduce body hair as either a direct goal or a side effect. If you started a new medication and then noticed your underarm hair thinning, the timing may not be coincidental.
Anti-androgen medications are probably the most straightforward cause. Spironolactone, for instance, is frequently prescribed for conditions like polycystic ovary syndrome and hormonal acne. It works by blocking androgen receptors and reducing androgen production, which means body hair that depends on those androgens can thin or stop growing.5PubMed. Innovative use of spironolactone as an antiandrogen in the treatment of female pattern hair loss Other anti-androgens, including finasteride and flutamide, can have similar effects. Hormonal contraceptives with anti-androgenic progestins may also reduce body hair over time.
Chemotherapy is a more dramatic example. Many chemotherapy agents target rapidly dividing cells, and hair follicle cells are among the fastest-dividing cells in the body. The resulting hair loss typically affects the scalp most visibly, but underarm, pubic, and limb hair can also fall out. Hair usually regrows after treatment ends, typically starting within four to six weeks of completing chemotherapy, though the regrowing hair may differ in texture or color for a while.6PubMed Central. Scalp cooling versus standard cold-cap in preventing chemotherapy-induced alopecia in patients with localized breast cancer In a small percentage of cases, some degree of hair loss persists long-term.
Hormone replacement therapy can also shift body hair patterns, particularly in transgender individuals. Estrogen-based therapy reduces androgen activity, so trans women often experience significant thinning of body hair over months to years. Conversely, testosterone therapy in trans men typically increases body hair, including underarm hair. For anyone on hormone therapy who notices unexpected changes, the medication itself is a likely explanation.
Past Hair Removal Could Be the Reason
If you spent years removing underarm hair through waxing, epilating, or laser treatments, the follicles themselves may have been damaged enough that hair growth slowed or stopped on its own. Repeated waxing pulls hair out at the root, and over many years, some follicles simply give up. The hair that does regrow can come in finer and sparser than before.
Laser hair removal is designed to do this permanently. Modern laser devices can achieve dramatic and lasting reductions in hair density, with studies reporting an average reduction of around 80% in treated areas.7SAIMSARA Journal. Laser Epilation and Laser Hair Removal from Cosmetic Use to Clinical Applications and Safety If you had laser treatments on your underarms years ago, even a partial course of sessions, you might have enough follicular damage that the remaining hairs are now too fine or too few to notice. Some people forget about treatments they had a decade earlier and are puzzled when the hair seems to stop growing “on its own.”
Electrolysis, which destroys individual follicles with an electric current, is the most permanent form of hair removal and can eliminate growth entirely in treated areas. Even methods you might not think of as permanent, such as years of daily shaving, do not actually damage follicles (shaving only cuts hair at the surface), so they would not explain a genuine reduction in growth. The distinction matters: if you only ever shaved and your underarm hair has disappeared, something else is going on.
Nutritional Deficiencies and General Health
Your body needs adequate nutrition to maintain hair growth anywhere on the body, and deficiencies can slow or halt the process. Iron deficiency is one of the most studied nutritional links to hair loss, with research exploring the relationship between low iron stores and various forms of hair thinning.8PubMed. The diagnosis and treatment of iron deficiency and its potential relationship to hair loss Other nutrients that play roles in hair health include zinc, biotin, vitamin D, and protein. Severe calorie restriction, crash dieting, or eating disorders can trigger a type of widespread hair shedding called telogen effluvium, where a large proportion of hairs shift into the resting phase at once. Body hair, including underarm hair, can be affected alongside scalp hair.
Chronic illness of many kinds, from autoimmune diseases to liver or kidney problems, can also thin body hair as a downstream effect of the body redirecting resources away from non-essential functions. Hair growth is biologically expensive and is one of the first things the body scales back under metabolic stress. If your underarm hair loss coincides with other changes like fatigue, weight shifts, or feeling generally unwell, the hair may be a symptom rather than the problem itself.
Natural Variation and Genetics
Not everyone grows the same amount of underarm hair, and some people never grow very much to begin with. Genetics and ethnicity play a large role in determining hair density, thickness, and distribution across the body.2PubMed. Androgens and hair growth People of East Asian descent, for example, tend to have less body hair overall compared to people of Mediterranean or Middle Eastern descent, and that includes underarm hair. These are normal variations, not signs of hormonal problems.
Individual follicle sensitivity to androgens also varies from person to person, even among siblings. Two people with identical testosterone levels can have very different amounts of body hair based purely on how responsive their follicles are to those hormones. This sensitivity is largely genetically determined, and it can change somewhat with age. Someone who always had moderate underarm hair might find it gradually becoming sparser in their 40s, not because of any disease but because their follicles were never strongly androgen-responsive to begin with and are now losing what sensitivity they had.
When to Talk to a Doctor
Reduced underarm hair on its own is rarely a medical emergency, but certain patterns should prompt a conversation with your doctor. The key question is whether the hair loss is isolated or accompanied by other changes. Gradual thinning in someone over 50 who feels otherwise fine is almost certainly age-related. But underarm hair loss combined with fatigue, weight changes, skin darkening, menstrual irregularities, or loss of hair in other unexpected areas (outer eyebrows, pubic area) suggests a hormonal or autoimmune issue worth investigating.
A few specific red flags to watch for:
- Sudden onset: Hair that falls out quickly over weeks rather than gradually thinning over years is more likely to have a medical cause, from alopecia areata to a medication side effect.
- Patchy loss: Smooth, well-defined bald patches rather than overall thinning can point to alopecia areata or a fungal infection.
- Scarring or skin changes: If the skin where hair has been lost looks scarred, shiny, or discolored, a scarring alopecia like frontal fibrosing alopecia may be involved, and early treatment matters because scarred follicles do not regrow hair.
- Other body hair disappearing too: Loss of underarm hair alongside pubic hair, eyebrow thinning, or limb hair loss points toward a systemic cause rather than a local one.
Basic blood work, including a complete blood count, thyroid panel, iron studies, and hormone levels (testosterone, DHEA-S, cortisol), can identify or rule out the most common medical causes. If an autoimmune condition is suspected, a dermatologist may recommend a skin biopsy of the affected area.
What Underarm Hair Actually Does
It is worth noting that underarm hair, while often treated as an inconvenience, appears to have served a biological purpose. The apocrine sweat glands concentrated in the underarms produce odor-carrying secretions, and the hair in that area acts as a wick to help disperse those substances into the air.9PubMed Central. Human hair – an evolutionary relic? In our evolutionary past, this likely played a role in signaling sexual maturity and individual identity through scent. The hair also reduces friction between the arm and torso during movement, which is why some athletes who shave their underarms report increased chafing.
From a practical standpoint, losing your underarm hair is unlikely to cause any health problems. The apocrine glands still function without the hair, and most people in modern life actively remove the hair anyway. But understanding the biology helps explain why the hair appears when it does (puberty, when androgens rise), why it might vanish (when androgens fall), and why its presence or absence tracks so closely with hormonal status throughout your life. If nothing else, that vanishing underarm hair is your body’s way of telling you something about its hormonal landscape, and it is usually worth listening.