Hair is a protein filament, primarily composed of keratin, that grows from follicles embedded beneath the skin’s surface. Its growth is not continuous but follows a genetically programmed, cyclical pattern. Understanding why hair stops growing requires looking at this life cycle, which dictates maximum length, and external or internal factors that can prematurely disrupt it. Cessation of growth can be a normal biological event, or it can signal an underlying hormonal or physiological issue affecting the follicle’s ability to produce a healthy strand.
The Natural Limits of Hair Growth
Hair stops growing at a certain length due to the inherent structure of the hair growth cycle, which consists of four phases. The Anagen phase is the first and longest, a period of active growth where cells in the hair bulb rapidly divide to form the hair shaft. For scalp hair, this phase lasts between two and seven years, determining the maximum possible length.
Once the Anagen phase concludes, the hair follicle enters the short Catagen phase, a transitional period lasting about two to three weeks. Growth ceases, and the lower part of the follicle shrinks and detaches from the dermal papilla, the structure that supplies nutrients. This detachment prepares the hair for the final stages of the cycle.
Following the transition, the hair enters the Telogen phase, a resting period that lasts two to four months. The hair remains anchored in the follicle but is no longer growing, becoming a club hair. The final step is the Exogen phase, where the old, resting hair is shed, making way for a new hair to begin the Anagen phase in the same follicle, restarting the cycle. The short Anagen phase of eyebrow and body hair explains why those hairs naturally stop growing at a much shorter length.
Hormonal and Genetic Influences on Follicle Decline
When hair growth stops permanently in a specific pattern, the cause is most often Androgenetic Alopecia, commonly known as pattern baldness. This condition is driven by genetics and hormones, leading to follicular miniaturization—the gradual shrinking of the hair follicle over successive growth cycles.
The key hormonal factor is Dihydrotestosterone (DHT), a potent androgen derived from Testosterone. An enzyme called 5-alpha reductase converts Testosterone into DHT, which is about five times more powerful than its parent hormone. In genetically predisposed individuals, specific hair follicles on the scalp are highly sensitive to DHT.
When DHT binds to receptors on these sensitive follicles, it signals a change in growth programming. The Anagen phase becomes progressively shorter with each cycle, and the hair produced is thinner, lighter, and shorter, transitioning from thick terminal hair to fine vellus hair. Eventually, the follicle miniaturizes to the point where it can no longer produce a visible hair shaft, resulting in permanent growth cessation.
External Triggers That Halt Growth
Hair growth can be abruptly halted or lead to excessive shedding due to external physiological or psychological stresses, a condition known as Telogen Effluvium (TE). This occurs when a significant number of growing hairs are prematurely shocked into the Telogen, or resting, phase. Hair growth stops immediately, but noticeable mass shedding begins two to four months after the initial triggering event.
Common triggers for this acute disruption include severe illness, major surgery, sudden weight loss, or high fever. Hormonal shifts, such as those experienced postpartum or during thyroid dysfunction, can precipitate a widespread shift into the resting phase. Psychological trauma or emotional stress is another factor that can trigger this temporary growth halt.
Nutritional deficiencies interfere with the follicle’s ability to sustain the Anagen phase. Insufficient levels of micronutrients like iron or zinc, needed for cellular division and protein synthesis, impede healthy hair production. Unlike pattern baldness, TE is temporary and reversible; once the trigger is resolved, the follicles resume normal growth and produce terminal hair within several months.