The exact genetic mutation that causes congenital circumscribed, localized, and nevoid hypertrichosis is unknown.

Acquired hypertrichosis lanuginosa is commonly present with cancer. This condition is also linked to metabolic disorders, such as anorexia, hormone imbalances, such as hyperthyroidism, or as a side effect of certain drugs.

Acquired generalized hypertrichosis may be caused by cancer. The resulting hair growth is known as malignant down. The mechanism behind cancer induced hypertrichosis is unknown. Oral and topical minoxidil treatments are also known to cause acquired generalized hypertrichosis.

There is controversy over whether auricular hypertrichosis is a Y-linked or autosomal trait, or perhaps both (in different families). It was proposed also that this phenotype results from the interaction of two loci, one on the homologous part of the X and Y and one on the nonhomologous sequence of the Y.

Lee et al. (2004), by Y-chromosomal DNA binary-marker haplotyping, suggested that a cohort of southern Indian hairy-eared males carried Y chromosomes from many haplogroups of the Y-phylogeny. According to a hypothesis of Y linkage, it would require multiple independent mutations within a single population. No significant difference between the Y-haplogroup frequencies of hairy-eared males and those of a geographically matched control sample of unaffected males was established. They concluded that the auricular hypertrichosis is not Y-linked in southern India, but it is unlikely to be same in any population.

Genetic forms of localized autosomal recessive hypotrichosis include:

Individuals affected by certain ED syndromes cannot perspire. Their sweat glands may function abnormally or may not have developed at all because of inactive proteins in the sweat glands. Without normal sweat production, the body cannot regulate temperature properly. Therefore, overheating is a common problem, especially during hot weather. Access to cool environments is important.

Sabinas brittle hair syndrome is inherited as an autosomal recessive genetic trait.

In a study by Howell et al. patients were located and studied by means of complete histories and physical examinations, analyses of serum trace metals, ceruloplasmin concentration, urine and serum amino acids, and routine metabolic urine screens. In addition, serum and urine luteinizing hormone (LH) and follicle-stimulating hormone (FSH) values were determined, and were interpreted in conjunction with total plasma estrogen, estradiol, and testosterone levels. Close examination demonstrated the scalp hairs were very brittle, coarse, wiry in texture, and broke off quite easily with mechanical trauma such as combing and brushing. Some hairs could be visualized in their follicles, which were broken off at the skin line. Most patients had accompanying hyperkeratosis (thickening of the skin) of moderate degree on exposed surfaces. Maxillary hypoplasia (midfacial retrusion) was significant in many patients. The brittle, short hair, reduced eyelashes, crowded teeth, and dull appearance created a characteristic facial appearance. Post-pubertal patients had development of secondary sexual characteristics consistent with their age, except for sparse pubic escutcheons. All cases studied demonstrated some degree of mental deficiency; I.Q.'s ranged between 50–60. A deficiency in eye–hand coordination was also noted.

Hypotrichosis ("" + "" + "") is a condition of abnormal hair patterns, predominantly loss or reduction. It occurs, most frequently, by the growth of vellus hair in areas of the body that normally produce terminal hair. Typically, the individual's hair growth is normal after birth, but shortly thereafter the hair is shed and replaced with sparse, abnormal hair growth. The new hair is typically fine, short and brittle, and may lack pigmentation. Baldness may be present by the time the subject is 25 years old.

Hypotrichosis is a common feature of Hallermann–Streiff syndrome as well as others. It can also be used to describe the lack of hair growth due to chemotherapy.

The opposite of hypotrichosis is hypertrichosis, where terminal hair (thick) grows in areas that would otherwise normally have vellus hair (thin), for example abnormally thick facial hair growth in women.

Several studies have examined salivary flow rate in individuals and found parotid and submandibular salivary flow ranging from 5 to 15 times lower than average. This is consistent with the salivary glands being of ectodermal origin, although some findings have suggested that there is also mesodermal input.

Ear hair generally refers to the terminal hair arising from follicles inside the external auditory meatus in humans. In its broader sense, "ear hair" may also include the fine vellus hair covering much of the ear, particularly at the prominent parts of the anterior ear, or even the abnormal hair growth as seen in hypertrichosis and hirsutism. Medical research on the function of ear hair is currently very scarce.

Hair growth within the ear canal is often observed to increase in older men, together with increased growth of nose hair. Visible hair that protrudes from the ear canal is sometimes trimmed for cosmetic reasons. Excessive hair growth within or on the ear is known medically as "auricular hypertrichosis". Some men, particularly in the male population of India, have coarse hair growth along the lower portion of the helix, a condition referred to as "having "hairy pinnae"" ("hypertrichosis lanuginosa acquisita").

Sabinas brittle hair syndrome, also called Sabinas syndrome or brittle hair-mental deficit syndrome, is an autosomal recessive congenital disorder affecting the integumentary system.

Focal dermal hypoplasia has been associated with PORCN gene mutations on the X chromosome. 90% of the individuals who are affected with the syndrome are female: the commonly accepted, though unconfirmed, explanation for this is that the non-mosaic hemizygous males are not viable.

The differential diagnosis of focal dermal hypoplasia (Goltz) syndrome includes autosomal recessive Setleis syndrome due to TWIST2 gene mutations. It associated with morning glory anomaly, polymicrogyria, incontinentia pigmenti, oculocerebrocutaneous syndrome, Rothmund-Thomson syndrome and microphthalmia with linear skin defects (also known as MLS) syndrome because they are all caused by deletions or point mutations in the HCCS gene.

Pure hair-nail type ectodermal dysplasia is a genetic mutation in the "hair matrix and cuticle keratin KRTHB5 gene" that causes ectodermal dysplasia of hair and nail type. Manifestations of this disorder include onychodystrophy and severe hypotrichosis. It represents as an autosomal dominant trait.

Trichorrhexis may have a genetic basis but appears to be precipitated by environmental factors. Among Caucasians the defect often appears at the ends of the hair shaft with splitting of the ends, thinning and whitish discoloration.

These conditions are directly related to environmental causes such as "perming", blow drying, aggressive hair brushing, and excessive chemical exposure.

In some cases, trichorrhexis nodosa may be caused by an underlying disorder such as argininosuccinic aciduria, Menkes' kinky hair syndrome, Netherton's syndrome, hypothyroidism, or trichothiodystrophy.

The hair is normal in quantity and is usually silvery-blond or straw-colored. It is disorderly, it stands out from the scalp, and cannot be combed flat. The underlying structural anomaly is longitudinal grooving of the hair shaft, which appears triangular in cross section. This is caused by mutations in one of three possible genes; "PADI3", "TGM" or "TCHH3." The characteristic hair shaft anomaly can be demonstrated in asymptomatic family members by scanning electron microscopy. To be noticeable, 50% of hairs must be affected by the structural abnormality. Improvement often occurs in later childhood. An autosomal dominant mode of inheritance has been suggested though an autosomal recessive pattern with varying degrees of penetrance has also been noted. The stiffness of the uncombable hair has been reasoned to be due to the triangular form of the hair shaft in cross section. It has been suggested that the condition may result from premature keratinization of the inner root sheath.

Focal dermal hypoplasia (also known as "Goltz syndrome") is a form of ectodermal dysplasia. It is a multisystem disorder characterized primarily by skin manifestations to the atrophic and hypoplastic areas of skin which are present at birth. These defects manifest as yellow-pink bumps on the skin and pigmentation changes. The disorder is also associated with shortness of stature and some evidence suggests that it can cause epilepsy.

Uncombable hair syndrome, also known as Pili trianguli et canaliculi, Spun-glass hair, and Cheveux incoiffables, is a rare structural anomaly of the hair with a variable degree of effect. It was first reported in the early 20th century and was described in the 1970s. It becomes apparent from as little as 3 months to up to 12 years of age.

Patients with CHH usually suffer from cellular immunodeficiency. In the study of 108 Finnish patients with CHH there was detected mild to moderate form of lymphopenia, decreased delayed type of hypersensitivity and impaired responses to phytohaemagglutinin. This leads to susceptibility to and, in some more severe cases, mortality from infections early in childhood. There has also been detected combined immunodeficiency in some patients

Patients with CHH often have increased predispositions to malignancies.

The condition affects 0.1%–0.2% of the population, and occurs equally in both males and females. Alopecia areata occurs in people who are otherwise healthy and have no other skin disorders. Initial presentation most commonly occurs in the late teenage years, early childhood, or young adulthood, but can happen at any ages. Patients also tend to have a slightly higher incidence of conditions related to the immune system, such as asthma, allergies, atopic dermatitis, and hypothyroidism.

This condition is self-limiting. Improvements in grooming techniques and in environmental conditions will correct the abnormality.

Danforth (1921) reported that middigital hair was present in men more often than in women. Caucasians were found to have a higher incidence of middle phalangeal hair than other ethnic groups, including Afro-Americans, American Indians, and Japanese.

Saldanha and Guinsburg (1961) studied the presence or absence of middigital hair in a white population of Sao Paulo, Brazil, including 131 males and 158 females, and compared their findings with those of previous reports. The frequencies of individuals without midphalangeal hair showed striking population differences. The range among northern Europeans varied from 20 to 30%, and among Mediterraneans, from 30 to 50%. Among Japanese, American Indians, and blacks, the figures varied between about 60% and 90%. The trait was virtually absent among Eskimos.

- Midphalangeal hairiness (%)

A unibrow is part of normal human variation, but can also stem from developmental disorders. A unibrow is a recognized feature of Cornelia De Lange syndrome, a genetic disorder whose main features include moderate to severe learning difficulties, limb abnormalities such as oligodactyly (fewer than normal fingers or toes) and phocomelia (malformed limbs), and facial abnormalities including a long philtrum (the slight depression/line between the nose and mouth).

Other medical conditions associated with a unibrow include:

- Waardenburg Syndrome;

- Patau Syndrome;

- Smith-Lemli-Opitz Syndrome;

- Sanfilippo Syndrome;

- 3p Deletion Syndrome;

- Chromosome Deletion Dillan 4p Syndrome (Wolf–Hirschhorn Syndrome);

- Gorlin Syndrome (Basal Cell Nevus Syndrome);

- Frontometaphyseal Dysplasia;

- ATRX Syndrome;

- Chromosome 9q34 Microdeletion Syndrome or Kleefstra syndrome.

Many medications are being studied, including abatacept, MEXIS/M6S, triamcinolone, secukinumab, tralonkinumab, apremilast, botulinum toxin, INCB018424, bimatoprost, clobetasol, AS101, autologous platelet-rich plasma, topical minoxidil, and nitric oxide gel. Some of these medications are approved for other diseases, others are not available outside of studies.

In 2014, preliminary findings showing that oral ruxolitinib, a drug approved by the US Food and Drug Administration (FDA) for bone marrow disorder myelofibrosis, restored hair growth in three individuals with long-standing and severe disease. The medicine costs almost USD $10,000 a month.

In humans, hair is commonly present on all the basal segments of the digits and invariably absent from all the terminal ones. On the middle segments, there is wide fluctuation with apparent familial and racial tendencies. Hair is present on the middle segment of the fingers more frequently than on the middle segment of the toes. Hair is most often found on the middle segment of the fourth finger.

Cartilage–hair hypoplasia (CHH), also known as McKusick type metaphyseal chondrodysplasia, is a rare genetic disorder. It is a highly pleiotropic disorder that clinically manifests by form of short-limbed dwarfism due to skeletal dysplasia, variable level of immunodeficiency and predisposition to malignancies in some cases. It was first reported in 1965 by McKusick et al. Actor Verne Troyer is affected with this form of dwarfism, as was actor Billy Barty, who was renowned for saying "The name of my condition is Cartilage Hair Syndrome Hypoplasia, but you can just call me Billy."

Tricho-hepato-enteric syndrome is estimated to affect 1 in 300,000 to 400,000 live births in Western Europe. This syndrome was first reported in 1982 with a report on 2 siblings, and as of 2008 there were around 25 published cases in medical journals. There seem to be no racial differences in its occurrence. It might be more common, as many genetic diseases, in areas with high levels of consanguinity.