Armpit hair, medically known as axillary hair, is a type of terminal hair that signals the onset of puberty. It is thick, coarse, and darkly pigmented, a significant change from the fine, light vellus hair covering the skin during childhood. The common perception that this hair seems to grow back almost immediately after removal is rooted in a specific, accelerated biological process. Armpit hair follicles respond uniquely to internal signals, driving a high rate of cell division that leads to rapid visible growth. This article explores the physiological mechanisms, particularly the hair growth cycle and hormonal influences, that account for the speed of armpit hair growth.
Understanding the Hair Growth Cycle
All hair on the human body follows a recurring, cyclical pattern that determines its ultimate length and growth rate. This cycle has four distinct phases: anagen, catagen, telogen, and exogen. The anagen phase is the growth phase, where the cells in the hair matrix rapidly divide and push the hair shaft upward. The duration of this active phase is the primary factor dictating how long a hair can grow before it sheds.
Following the growth phase is the catagen phase, a brief transitional period where the hair follicle shrinks and detaches from the blood supply, which only lasts about two to three weeks. The hair then enters the telogen, or resting, phase, which can last for several months, during which the hair is not actively growing. Finally, the exogen phase sees the old hair shed, making way for the cycle to restart with a new anagen hair.
The speed and maximum length of any hair are directly related to the length of its anagen phase. Scalp hair, for example, has a long anagen phase that can last for years, allowing it to grow very long. In contrast, body hair, including that in the armpits, has a significantly shorter anagen phase, typically lasting only a few months. This shorter growth period explains why armpit hair will not grow to the same length as head hair.
The Specific Influence of Androgens
The defining characteristic that makes armpit hair grow so quickly is the influence of sex hormones, specifically a group called androgens. Androgens, which include testosterone and its more potent derivative, dihydrotestosterone (DHT), are the main regulators of hair follicles in this region. At puberty, the increase in androgen levels stimulates the previously dormant vellus hair follicles in the armpits to transform into active terminal hair follicles.
The armpit follicles possess a high sensitivity and a large concentration of receptors for these androgens. This hormonal stimulation acts on the dermal papilla cells, essentially signaling the hair follicle to enter a state of aggressive, accelerated cell proliferation. This results in a highly active anagen phase, where the hair grows thicker, darker, and at a visibly rapid pace compared to the hair on the rest of the body.
The duration and activity of the growth phase in the armpit are entirely dependent on the presence of androgens. Although the anagen phase for axillary hair is short in terms of overall duration, the high hormonal activity during this time causes the hair to grow at a high daily rate until it reaches its genetically determined maximum length, which is usually less than six inches. The constant stimulation by these hormones is the core biological reason for the perceived speed of regrowth in this area.
Non-Hormonal Factors That Influence Speed
While hormones are the primary driver, other factors contribute to the variability in hair growth speed observed among individuals. Genetic predisposition plays a substantial role, as an individual’s DNA influences the inherent sensitivity of their hair follicles to androgens and determines the maximum duration of the anagen phase. If a person inherits a gene for a slightly longer or more active anagen phase in the armpits, their hair will grow noticeably faster than others.
Hair growth rates also naturally fluctuate throughout a person’s life, peaking between the ages of 15 and 30 before gradually slowing down with age. Overall health and metabolic function also have an impact, as conditions like thyroid dysfunction or nutritional deficiencies can disrupt the hair growth cycle and slow the rate of cell division. Hair requires a steady supply of nutrients, such as iron, zinc, and B vitamins, to support rapid growth.
The Shaving Misconception
It is a common misconception that shaving causes hair to grow back faster or thicker. Removing the hair shaft with a razor only creates a blunt tip on the remaining stubble, which makes the regrowth appear darker and coarser as it emerges. Shaving does not affect the biological activity of the hair follicle root beneath the skin, meaning the actual rate of growth remains unchanged.