The perception of “hairiness” is highly personal, but medically, excessive hair growth falls into two main categories: hypertrichosis and hirsutism. Hypertrichosis describes excessive hair growth anywhere on the body, affecting both men and women, often unrelated to sex hormones. Hirsutism, by contrast, is specific to women, characterized by the growth of coarse, dark hair in areas where men typically grow hair, such as the face, chest, and back.
The Biology of Hair Types and Cycles
Hair growth is a cyclical process governed by the activity of the hair follicle, consisting of three primary phases: anagen, catagen, and telogen. Anagen is the active growth period, lasting from a few years for scalp hair to a few months for body hair. This is followed by catagen, a short transitional stage lasting two to three weeks, where the follicle shrinks and growth slows.
The telogen phase is the resting period, which typically lasts two to four months before the old hair is shed and a new anagen phase begins. The duration of the anagen phase dictates the maximum length a hair can reach.
The body is covered by two types of hair: vellus and terminal hair. Vellus hair is the short, fine, light-colored hair, often called “peach fuzz,” that covers most of the body. Terminal hair is the longer, thicker, darker, and more coarse hair found on the scalp, eyebrows, and pubic regions after puberty. The transformation of vellus hair into terminal hair is strongly influenced by hormones, particularly androgens.
Genetic and Natural Factors Influencing Hair Density
Variation in body hair density and distribution is often due to inherited traits and normal physiology rather than a medical condition. Genetic background plays a substantial role in determining hair color, texture, and natural density. Individuals of certain ethnic backgrounds, such as those of Mediterranean, South Asian, or Middle Eastern descent, may naturally have more prominent terminal hair in areas like the arms, legs, and chest. This is a normal trait and not a sign of a hormonal issue.
Normal life stages also cause physiological shifts in hair growth patterns. Puberty triggers the conversion of vellus hair to terminal hair in androgen-sensitive areas like the underarms and pubic region. For women, hormonal changes during perimenopause, menopause, or pregnancy can alter the hair cycle and density. Hair density naturally decreases with age, but sometimes the remaining hair becomes coarser or more noticeable, particularly on the face, due to changes in hormone sensitivity.
Hormonal Imbalances Leading to Excessive Hair Growth
When excessive hair growth follows a male-pattern distribution in women, it is classified as hirsutism, caused nearly always by an increased effect of androgens on the hair follicles. Androgens, like testosterone and DHEA-S, are male sex hormones that women produce in smaller amounts, primarily in the ovaries and adrenal glands. Sensitive hair follicles in areas such as the upper lip, chin, chest, and abdomen react to these hormones by transforming fine vellus hair into thick, pigmented terminal hair.
The most frequent cause of hirsutism is Polycystic Ovary Syndrome (PCOS), a hormonal disorder affecting up to 10% of women of reproductive age. PCOS causes the ovaries to overproduce androgens, leading to hirsutism in roughly 70% of cases. Hirsutism from PCOS is often accompanied by other symptoms of androgen excess, including acne, irregular menstrual periods, and male-pattern hair thinning on the scalp.
Adrenal gland disorders, though less common, can also lead to androgen-driven hair growth. Congenital Adrenal Hyperplasia (CAH) is an inherited condition that causes the adrenal glands to overproduce androgens, often leading to hirsutism and irregular cycles. Conditions like Cushing’s syndrome, caused by excessive cortisol, can also stimulate androgen production, resulting in hirsutism.
Medications and Other Medical Conditions
Excessive hair growth can be an acquired side effect of certain medications, typically resulting in hypertrichosis rather than hirsutism. These drugs directly affect the hair follicle’s cycle, often by prolonging the anagen (growth) phase. Immunosuppressive drugs like cyclosporine, used after organ transplants, are known to cause generalized hair growth.
Anti-seizure medications, such as phenytoin, and the vasodilator minoxidil are also associated with increased hair growth. Discontinuing the medication often reverses the hypertrichosis, though this should only be done under medical supervision. The hair growth caused by these medications is generally not dependent on androgen levels.
Other, rarer systemic conditions can also manifest as acquired hypertrichosis. Certain neurological disorders or metabolic conditions like Porphyria Cutanea Tarda, which causes extreme light sensitivity, can be linked to generalized hair growth. In rare instances, a sudden onset of fine, downy hair, known as acquired hypertrichosis lanuginosa, can precede the diagnosis of an internal malignancy. If hair growth is rapid, sudden, or accompanied by symptoms like voice deepening or changes in muscle mass, prompt medical evaluation is important.