Hair growth is a complex biological process. Hair naturally grows at about 0.5 inches (13 mm) per month, but various factors can impede this process. Understanding these influences helps explain why hair might not be growing as expected.
Understanding the Hair Growth Cycle
Hair growth follows a cyclical pattern, consisting of three primary phases: anagen, catagen, and telogen. Each hair follicle on the body operates independently, ensuring that not all hair sheds simultaneously. The anagen phase is the active growth period, where hair cells rapidly produce new strands, typically lasting two to seven years for scalp hair. Approximately 85% to 90% of scalp hairs are in this growth phase at any given time.
Following the anagen phase, hair enters the catagen phase, a short transitional period lasting about two to three weeks. During this phase, hair follicles shrink and detach from their blood supply, signifying the end of active growth. The final stage is the telogen phase, a resting period of around three to four months, during which the hair remains in the follicle but does not grow. This phase concludes with the exogen phase, where the old hair is shed to make way for new growth, with typical shedding being 50 to 100 hairs per day. Disruptions to this cycle, such as a shortened anagen phase or an increased number of follicles prematurely entering the telogen phase, can lead to noticeable reductions in hair growth or increased shedding.
Internal Influences on Hair Growth
Genetics play a role in hair growth patterns. Androgenetic alopecia, commonly known as male or female pattern baldness, is a genetically determined condition characterized by progressive thinning of scalp hair. This condition involves an increased sensitivity of hair follicles to dihydrotestosterone (DHT), a potent androgen. DHT causes hair follicles to miniaturize, leading to thinner, shorter hairs and eventually permanent hair loss in affected areas.
Hormonal imbalances can impact the hair growth cycle. Both an overactive (hyperthyroidism) and underactive (hypothyroidism) thyroid can disrupt normal hair growth, leading to thinning or shedding. Conditions like Polycystic Ovary Syndrome (PCOS) in women can cause elevated androgen levels, which may result in scalp hair thinning. Natural hormonal shifts during pregnancy, postpartum, or menopause can induce temporary or longer-term hair loss due to changes in estrogen and progesterone levels.
Nutritional deficiencies compromise hair health by depriving follicles of necessary building blocks. Insufficient intake of iron, zinc, or protein can lead to hair thinning and reduced growth. These nutrients are crucial for keratin production and supporting metabolic processes within hair follicles.
Systemic medical conditions can manifest as hair growth problems. Autoimmune diseases, such as alopecia areata, involve the immune system mistakenly attacking healthy hair follicles, leading to patchy hair loss. Chronic illnesses can disrupt the hair growth cycle, influencing hair health and density.
External and Lifestyle Contributions to Stunted Growth
Physical damage to hair can impede its growth. Excessive heat styling (e.g., flat irons, curling irons) weakens the hair shaft, leading to breakage and preventing full length. Chemical treatments (e.g., perms, relaxers, dyes) compromise hair integrity, making it more fragile and susceptible to damage. Tight hairstyles (e.g., braids, ponytails, buns) create constant tension on hair follicles, a condition known as traction alopecia. This continuous pulling can damage the follicles over time, potentially leading to permanent hair loss if not addressed early.
Stress can contribute to stunted hair growth. Severe physical or emotional stress can trigger a condition called telogen effluvium, where a large number of hair follicles prematurely enter the resting (telogen) phase. This sudden shift results in increased hair shedding, typically occurring two to three months after the stressful event. The physiological mechanism involves stress hormones disrupting the normal hair growth cycle.
Certain medications are known to cause hair loss as a side effect. Chemotherapy drugs target rapidly dividing cells, including hair follicle cells, often leading to hair loss. Other medications (e.g., blood thinners, antidepressants, anti-seizure drugs) can induce hair shedding, usually by pushing more hair into the telogen phase. In most cases, medication-induced hair loss is temporary and hair growth resumes once the medication is stopped.