Hair Loss Causes & Risk Factors

Interesting facts about hair

  • By week 22 of fetal life, a developing baby has all hair follicles formed. We do not generate new hair follicles anytime during the course of our lives.
  • There are approximately 100,000 hair follicles on the scalp.
  • Hair is the second fastest growing tissue in the human body, the first being bone marrow.
  • The rate of hair growth is about 1.25 cm (0.5 inches) per month. With age the speed of hair growth significantly slows down, and may turn out to be only 0.25 cm (0.1 inch) a month.
  • Baldness becomes visible only when the amount of lost hair approaches to 50%.
  • Hereditary hair loss (androgenetic alopecia) is the most common form of hair loss in men, and is responsible for more than 95% of all cases of baldness.
  • Since the television age began, the United States did not have a bald President.
  • Hair are a protein product of follicles and so are sensitive to changes within the body, and hair loss is often the result of an internal disorder.

What is hair loss?

Hair loss (alopecia) is a tendency of follicles to stop producing hair growth, leading to a decrease in the amount of hair. Although alopecia can occur anywhere on the body, it is especially distressing when it affects the scalp. It usually develops gradually and may be patchy or diffuse.

Androgenic alopecia

The major cause of hair loss is Androgenic alopecia - the genetic predisposition for hair loss inherited from either or both of parents. Almost all people have some degree of androgenic alopecia. Visible changes occur in most people by the age of 50 years.

Androgenetic alopecia is more common in men than women. About two-thirds of men experience some degree of appreciable hair loss by the time they are 35 years old, and about 85% have significantly thinning hair by age 50. And nearly 40% of women experience some degree of androgenetic alopecia in their lifetime.

The main cause of androgenic alopecia in both men and women involves a genetic sensitivity to a dihydrotestosterone (DHT), a derivative of the male hormone testosterone. DHT causes hair follicles to shrink and induces miniaturization of hair. Shrinking follicles produce thinner hair, and eventually fail to produce new hairs at all. Stress may trigger [4] onset of androgenic alopecia by inducing hormonal changes.

Androgenetic alopecia in men (male pattern baldness) is generally characterized with the onset of a receding hairline and thinning crown. Hair on the forehead area or on the top of the head appear to be the most sensitive to DHT. Hair on the sides and back of the head do not possess this genetic trait and therefore are not affected.

In women androgenetic alopecia (female pattern baldness) can begin at puberty, but is most often occurs after menopause. Women develop a diffuse thinning of the hair throughout the scalp while the frontal hairline generally remains intact.

Other causes

In addition to the common male and female patterns other possible hair loss causes include:

Telogen effluvium

Telogen effluvium (delayed shedding from stress) is a diffuse hair shedding, often with an acute onset, caused by a major body stress. It is the most common cause after androgenic alopecia.

Understanding the telogen effluvium requires knowledge about hair growth cycle.

All hair has a growth phase (anagen), and a resting phase (telogen). On the scalp, growth phase lasts for 2 to 6 years (on average 3 years), while resting phase lasts about 3 months. During telogen, the resting hair remains in the follicle until it is pushed out by new growing anagen hair.

In most people, 80-90% of the hair on the scalp is in the growth phase, and 5-15% in telogen phase at any given time.

Telogen effluvium is triggered when a stressful event induces a large number of hairs to enter the resting stage at one time. A few months later, after the stressful event, all resting follicles begin to shed their hairs at almost the same time. Because the stressful event happened months ago, most people do not connect it with their hair loss. Telogen effluvium is a temporary condition, and new hairs begin to grow within a few months.

Stressful events include:

  • Physical stress - surgery, very high fever, major illness, rapid weight change.
  • Emotional stress - mental illness, divorce, death of a loved one, job loss.

Anagen effluvium

Anagen effluvium (anagen hair loss) is the abrupt loss of hairs that are in their growing phase (anagen) due to insult of hair follicle[15]. Anagen effluvium may be caused by radiation and toxic chemicals. This condition is reversible, however hair regrowth occurs after a delay of 1-3 months.

Pregnancy and childbirth

After pregnancy many women experience excessive hair loss. During pregnancy, increased levels of estrogen prolong the growing phase of the hair cycle. However, after giving birth, estrogen levels come down and many hair enter the resting phase. Some women experience increased shedding several months after delivering a baby. This condition usually resolves completely as the hair growth cycle returns to normal.

Birth control pills

Birth control pills can cause shedding and trigger genetic hair loss (androgenic alopecia). Excess hair loss can occur with starting, switching, or discontinuing the pills.

Women experiencing hair shedding or thinning, while taking birth control pills, usually have a genetic predisposition for hair loss. The hormonal changes that occur trigger the onset of the androgenic alopecia. The pills that are higher in progesterone compared to estrogen are more likely to contribute to this process.

After stopping the pills, woman may notice hair shedding two or three months later. This may continue for six months when it usually stops. In some cases the process cannot be reversed and the woman may not regrow all lost hair.

If you have a history of female pattern loss in the family, you should consult with your doctor before going on the pill.

Alopecia areata

Alopecia areata is patchy hair loss. It is considered an autoimmune disease, but the cause is unknown.

Sudden loss of hair from small patches on the head is a common symptom. The patch of alopecia areata is round or oval, and is completely bald and smooth. In the vast majority of cases the condition is temporary and goes away all by itself within 6-7 months, and hair growth in the bald patch resumes. About 10% of those who have an episode of alopecia areata experience long-term hair loss, or develop new patches as old patches resume hair growth.

About 2% of people experience an episode of alopecia areata at some point in their lives. This form of hair loss affects both sexes equally and occurs at all ages, but is more common in children and adolescents.

Iron deficiency

Iron deficiency is one of the possible causes of excessive hair loss, especially in women[2]. Adequate levels of iron are essential to hair growth and maintenance. Some people don't have enough iron in their diets or may not fully absorb iron. Iron depletion is common to women during menstruation and pregnancy and can be corrected through proper diet or iron supplements.

Protein deficiency

Severe protein malnutrition may also cause hair loss. The body will save protein by shifting growing hairs into the resting phase. Some people who go on crash diets that are low in protein, or have severely abnormal eating habits, may develop protein malnutrition. This condition can be reversed and prevented by eating the proper amount of protein and, when dieting, maintaining adequate protein intake.

Thyroid disease

Both hyperthyroidism and hypothyroidism can cause hair loss. Thyroid hormones directly modulate multiple hair biology parameters[3] -- from hair follicles cycling to pigmentation. About 30% of persons with thyroid disease suffer from hair loss.

Traction alopecia (hair pulling)

Traction alopecia is the loss of hair from constant pulling, often the result of tightly braided hair styles. Braiding, corn-rows, tight pony-tails, and hair extensions are the most common styling causes.

Tinea capitis

Tinea capitis (also known as Scalp ringworm) is caused by fungi called dermatophytes. They can invade the hair and skin of the scalp, leading to hair loss. Tinea capitis is especially common in children, and rarely occurs after puberty. Once infection is treated, hair generally regrow.

Drug-induced alopecia

Drug-induced alopecia is a diffuse hair loss caused by toxic effect of the drug. This condition is usually reversible after discontinuation of the offending drug. Also, medications can trigger the onset of androgenic alopecia, cause temporary or even permanent hair loss.

List of drugs that may cause hair loss:

  • Lithium - about 12-19% of patients taking lithium developed hair thinning[5].
  • Anticonvulsants: valproic acid/divalproex precipitates alopecia in up to 12% of patients in a dose-dependent relationship; carbamazepine[5], lamotrigine, topiramate
  • Anticoagulants (blood thinners): coumarin, heparin, warfarin, enoxaparin [8].
  • Beta blockers: atenolol, metoprolol, nadolol, propranolol[6], timolol
  • NSAIDs and salicylates[6]
  • Anti-tuberculosis drugs: isoniazid, thiacetazone, and ethionamide [7]
  • Dopaminergic drugs[13]: levodopa, bromocriptine
  • Interferons [9]
  • Antipsychotic medicines: haloperidol[10], olanzapine [11], zotepine [12]
  • Thyroid medications: iodine, thiocyanate, thiouracil, thiamazol, methylthiouracil, propylthiouracil, carbimazol [16]
  • ACE inhibitors[17]
  • Androgenic agents
  • Retinoids: isotretinoin [18], acitretin[20], etretinate
  • Cholesterol-lowering drugs: atorvastatin, lovastatin
  • Colchicine [14] may cause anagen effluvium
  • Amiodarone [19]
  • Vasopressin
  • Immunosuppressants
  • Antidepressants
  • Amphetamines
  • Anabolic steroids

Sources & References:

  • 1. R. Weller, H. Hunter, MW. Mann Clinical Dermatology. 5th Ed. 2015, Wiley-Blackwell. pp.
  • 2. Moeinvaziri M, Mansoori P, Holakooee K, Safaee Naraghi Z, Abbasi A. Iron status in diffuse telogen hair loss among women. Acta Dermatovenerol Croat. 2009;17(4):279-84. PubMed
  • 3. van Beek N, Bodó E, Kromminga A, Gáspár E, Meyer K, Zmijewski MA, Slominski A, Wenzel BE, Paus R. Thyroid hormones directly alter human hair follicle functions: anagen prolongation and stimulation of both hair matrix keratinocyte proliferation and hair pigmentation. J Clin Endocrinol Metab. 2008 Nov;93(11):4381-8. PubMed
  • 4. Schmidt JB. Hormonal basis of male and female androgenic alopecia: clinical relevance. Skin Pharmacol. 1994;7(1-2):61-6.
  • 5. Mercke Y, Sheng H, Khan T, Lippmann S. Hair loss in psychopharmacology. Ann Clin Psychiatry. 2000 Mar;12(1):35-42.
  • 6. Llau ME, Viraben R, Montastruc JL. Drug-induced alopecia: review of the literature. Therapie. 1995 Mar-Apr;50(2):145-50.
  • 7. Dixit R, Qureshi D, Mathur S. Alopecia caused by isoniazid. J Pharmacol Pharmacother. 2014 Apr;5(2):155-7.
  • 8. Wang YY, Po HL. Enoxaparin-induced alopecia in patients with cerebral venous thrombosis. J Clin Pharm Ther. 2006 Oct;31(5):513-7.
  • 9. Taliani G, Biliotti E, Capanni M, Tozzi A, Bresci S, Pimpinelli N. Reversible alopecia universalis during treatment with PEG-interferon and ribavirin for chronic hepatitis C. J Chemother. 2005 Apr;17(2):212-4.
  • 10. Kubota T, Ishikura T, Jibiki I. Alopecia areata associated with haloperidol. Jpn J Psychiatry Neurol. 1994 Sep;48(3):579-81.
  • 11. Leung M, Wrixon K, Remick RA. Olanzapine-induced hair loss. Can J Psychiatry. 2002 Nov;47(9):891-2.
  • 12. Kubota T, Ishikura T, Jibiki I. Three cases of alopecia areata induced by zotepine. Acta Neurol (Napoli). 1993 Jun;15(3):200-3.
  • 13. Langan EA, Lisztes E, Bíró T, Funk W, Kloepper JE, Griffiths CE, Paus R. Dopamine is a novel, direct inducer of catagen in human scalp hair follicles in vitro. Br J Dermatol. 2013 Mar;168(3):520-5. PubMed
  • 14. Hsia CW, Shui HA, Wang CY, Yu HM, Ho MY, Cheng KT, Tseng MJ. Proteomics demonstration that histone H4 is a colchicine-induced retro-modulator of growth and alkaline phosphatase activity in hair follicle dermal papilla culture. J Proteomics. 2011 May 16;74(6):805-16. PubMed
  • 15. Kanwar AJ, Narang T. Anagen effluvium. Indian J Dermatol Venereol Leprol. 2013 Sep-Oct;79(5):604-12.
  • 16. Wilburne M. Hair loss and pigmentation due to thiouracil derivatives. JAMA. 1951;147:379–80.
  • 17. Steckelings UM, Artuc M, Wollschläger T, Wiehstutz S, Henz BM. Angiotensin-converting enzyme inhibitors as inducers of adverse cutaneous reactions. Acta Derm Venereol. 2001 Oct-Nov;81(5):321-5.
  • 18. Kmieć ML, Pajor A, Broniarczyk-Dyła G. Evaluation of biophysical skin parameters and assessment of hair growth in patients with acne treated with isotretinoin. Postepy Dermatol Alergol. 2013 Dec;30(6):343-9
  • 19. Samuel LM, Davie M, Starkey IR. Amiodarone and hair loss. Postgrad Med J. 1992 Sep;68(803):771.
  • 20. Lee CS, Koo J. A review of acitretin, a systemic retinoid for the treatment of psoriasis. Expert Opin Pharmacother. 2005 Aug;6(10):1725-34. PubMed

Created: May 03, 2015
Last updated: February 24, 2016

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