Human hair is in a constant state of renewal. Each hair follicle cycles through distinct phases, beginning with a growth phase where the hair actively lengthens. This is followed by a brief transition and then a resting period before the hair is shed and a new one begins to grow. This entire cycle is heavily influenced by hormones, which regulate the timing and duration of each phase.
The Role of Androgens in Hair Growth
Androgens are hormones that stimulate the growth of certain hair types. They are responsible for converting fine, light-colored vellus hairs into thicker, darker terminal hairs during puberty. The primary androgen is testosterone, but its effects on hair are mediated by its conversion into the more potent dihydrotestosterone (DHT). This conversion happens within the hair follicle, facilitated by an enzyme called 5-alpha reductase.
DHT binds to androgen receptors within the hair follicle, which alters gene expression related to hair growth. This interaction triggers the growth of hair on the face, chest, and underarms. The sensitivity of hair follicles to androgens varies across the body, explaining why these hormones stimulate growth in some areas but not others.
These hormones exhibit an “androgen paradox.” While DHT drives the growth of thick body hair, it has the opposite effect on the scalp in individuals with a genetic predisposition for baldness. In these people, DHT causes scalp follicles to miniaturize, shortening the growth phase and producing finer hairs. This process eventually leads to male and female pattern baldness.
Other Key Hormones Influencing Hair
Thyroid hormones are fundamental for regulating the body’s metabolism, including the metabolic activity within hair follicle cells. Proper levels of these hormones are necessary to sustain the energy required for consistent hair production. Both an overactive and an underactive thyroid can disrupt the normal hair growth cycle.
The hormone estrogen also has a distinct influence on hair by extending the anagen, or active growth, phase. This is why many women experience hair that feels thicker and fuller during pregnancy when estrogen levels are significantly elevated. By keeping more hairs in the growth phase for longer, estrogen reduces the rate of shedding and contributes to increased hair volume.
How Hormonal Imbalances Affect Hair
Elevated levels of cortisol, a hormone released in response to stress, can push many hair follicles prematurely from the growth phase into the resting phase. This shift results in a diffuse shedding of hair across the scalp, a condition known as telogen effluvium. This shedding typically becomes noticeable a few months after a stressful event.
In women, an overproduction of androgens can lead to hirsutism, characterized by coarse, dark hair growth in a male-like pattern on the face, chest, and back. This imbalance is frequently associated with Polycystic Ovary Syndrome (PCOS). Conversely, an underactive thyroid, or hypothyroidism, can lead to hair that is dry, brittle, and sparse across the scalp due to a disrupted growth cycle.
Gender Differences in Hormonal Hair Growth
Differences in hair patterns between men and women result from their hormonal profiles. Males produce higher levels of androgens, particularly testosterone, which drives the development of extensive facial and chest hair after puberty. These high androgen levels are the primary reason for more prominent body hair in men.
Females have higher levels of estrogen, which promotes longer growth phases for scalp hair. This contributes to the lower incidence of androgen-driven scalp hair loss in women before menopause. Hormonal shifts, such as the decline in estrogen during menopause, can alter these hair patterns. This reduction in estrogen can unmask the effects of androgens on scalp follicles, often leading to hair thinning in later life.