Curekey medical reference·60 terms

Hair Loss Glossary

Plain-English definitions for the medical and biological terms used across our hair-loss guides. Each term links to the article that covers it in depth when one exists.

A

5-alpha reductase: The enzyme that converts testosterone into the more potent androgen DHT. Comes in two forms: Type I and Type II. Finasteride blocks Type II; dutasteride blocks both.
Alopecia areata: An autoimmune condition in which the immune system attacks hair follicles, producing round, well-defined patches of hair loss. Distinct from pattern baldness, it requires dermatology evaluation and a different treatment approach.
Anagen effluvium: Rapid, widespread shedding caused by a sudden insult to actively growing follicles. Most commonly seen with chemotherapy. Usually reversible once the trigger is removed.
Anagen phase: The active growth phase of the hair cycle, lasting two to six years for scalp hair. Most of your hair is in anagen at any given time. Treatments like minoxidil work in part by prolonging this phase.
Androgenetic alopecia (AGA): The clinical name for pattern hair loss in both men and women. Caused by genetically susceptible follicles miniaturizing in response to DHT. The most common cause of hair loss in adults and the type with the strongest medical treatments.

B

Biotin: A B-vitamin sometimes marketed as a hair-growth supplement. Helpful only if you're deficient, which is rare in adults eating a normal diet. Not a treatment for androgenetic alopecia.
Board-certified dermatologist: A physician who has completed a dermatology residency and passed the certification exam of the American Board of Dermatology. Curekey's clinical team includes board-certified physicians who review every treatment plan.

C

Catagen phase: The short transition phase between active growth and rest in the hair cycle. Lasts roughly two to three weeks. The follicle shrinks and detaches from the dermal papilla during this phase.
CCCA (Central centrifugal cicatricial alopecia): A scarring alopecia that begins at the crown and expands outward, most commonly affecting Black women. Early diagnosis matters because the follicle damage becomes permanent over time.
Contraindication: A specific reason a medication should not be used in a given patient. For example, finasteride is contraindicated in women who are pregnant or may become pregnant due to risks to a developing fetus.
Crown thinning: Hair loss at the vertex of the scalp, often the first visible sign of androgenetic alopecia in men. Can progress to a bald spot or expand to meet a receding hairline.

D

Dandruff (seborrheic dermatitis): A common inflammatory condition of the scalp characterized by flaking and itching. Doesn't cause hair loss directly but the inflammation can worsen existing thinning. Treated with medicated shampoos like ketoconazole.
Derma roller (microneedling): A handheld device with tiny needles that create controlled micro-injuries to the scalp. Studies suggest combining microneedling with topical minoxidil can improve outcomes. Not a standalone treatment.
Dermal papilla: The cluster of cells at the base of each hair follicle that signals the follicle to grow. Damage or signaling changes to the dermal papilla underlie most types of hair loss.
DHT (Dihydrotestosterone): A potent androgen hormone converted from testosterone by the enzyme 5-alpha reductase. In genetically susceptible follicles, DHT triggers the miniaturization that causes pattern baldness.
Diffuse thinning: Hair loss spread evenly across the scalp rather than concentrated in patches or a clear pattern. Common in women with androgenetic alopecia and in telogen effluvium.
Dutasteride: A 5-alpha reductase inhibitor similar to finasteride but blocking both enzyme types instead of one. More potent at lowering DHT, with a somewhat broader side-effect profile. Used off-label for hair loss in some cases.

E

Exogen phase: The phase of the hair cycle in which the old hair shaft is shed to make room for the new one growing underneath. Sometimes grouped with telogen rather than treated as a separate phase.

F

FDA-approved: Cleared by the U.S. Food and Drug Administration for a specific medical indication after clinical trials. Minoxidil and oral finasteride are both FDA-approved for hair loss. Some treatments used in the field are FDA-approved for one use and used off-label for another.
Female pattern hair loss (FPHL): Androgenetic alopecia in women. Typically presents as diffuse thinning along the part rather than a receding hairline. Treatment options differ from those used in men.
Finasteride: A 5-alpha reductase inhibitor that blocks the conversion of testosterone to DHT. Taken orally at 1 mg/day or used as a topical formulation. FDA-approved for male pattern hair loss; one of the most evidence-supported treatments.
Follicle miniaturization: The progressive shrinking of hair follicles in response to DHT in genetically susceptible individuals. Each cycle produces a thinner, shorter, lighter hair until the follicle stops producing visible hair entirely. The biological mechanism behind androgenetic alopecia.
Frontal fibrosing alopecia (FFA): A scarring alopecia that causes a band-like recession of the frontal hairline, most often in postmenopausal women. Often accompanied by eyebrow loss. Requires dermatology evaluation; permanent if not addressed early.
FUE (Follicular Unit Extraction): A hair transplant technique that extracts individual follicular units from a donor area and implants them in thinning regions. An alternative or complement to medical treatment, generally considered after pattern hair loss has stabilized.

G

Generic medication: A medication sold under its chemical name rather than a brand name, typically at lower cost. Generic minoxidil and finasteride are chemically identical to their brand-name versions and equally effective.

H

Hair bulb: The rounded base of the hair follicle deep in the dermis. Contains the cells that divide to form the hair shaft and the dermal papilla that controls follicle activity.
Hair density: The number of hair strands per unit of scalp area. Reduced density is one of the measurable changes in androgenetic alopecia. Treatments aim to preserve or restore density as well as shaft caliber.
Hair growth cycle: The repeating cycle each follicle moves through: anagen (active growth), catagen (transition), telogen (rest), and exogen (shedding). Disruption to this cycle underlies most types of hair loss.
Hair shaft: The visible portion of a hair above the scalp. Made primarily of keratin. The diameter of the shaft (caliber) shrinks as a follicle miniaturizes.
Hairline: The forward-most edge of scalp hair across the forehead and temples. Recession of the hairline at the temples is often the first visible sign of pattern hair loss in men.

I

Initial shed (startup shed): A temporary increase in shedding that often occurs in the first 2-3 months after starting minoxidil or finasteride. Caused by older hairs being pushed out as follicles synchronize into a new growth cycle. A sign treatment is working, not failing.

K

Ketoconazole: An antifungal medication available as a shampoo (1% over-the-counter, 2% by prescription). Has some evidence as a supportive treatment for pattern hair loss when combined with minoxidil or finasteride. Not a primary treatment on its own.

L

Low-level laser therapy (LLLT): Devices like laser combs and caps that deliver low-intensity red light to the scalp. Some studies suggest modest benefits for androgenetic alopecia. Generally used as an adjunct to medical treatment rather than a replacement.
Ludwig scale: A staging system for female pattern hair loss with three grades based on the width of the central part. The female equivalent of the Norwood scale used for men.

M

Maintenance phase: The period after initial regrowth when treatment continues to preserve what's been gained. Stopping treatment in this phase generally reverses progress within 3-6 months because the underlying biology resumes.
Male pattern baldness (MPB): The colloquial name for androgenetic alopecia in men. Follows the Norwood scale: typically begins at the temples and crown, can progress to the horseshoe pattern in advanced cases.
Microneedling: Use of fine needles to create controlled micro-injuries in the scalp, triggering wound-healing and growth-factor responses. Most effective when combined with topical minoxidil, based on the available studies.
Minoxidil: Originally developed as an oral antihypertensive, minoxidil was found to grow hair as a side effect and was reformulated as a topical treatment. Works by prolonging the anagen phase and increasing follicle activity. Available as a topical solution, foam, and (off-label) as a low-dose oral medication.

N

Norwood scale (Hamilton-Norwood): A seven-stage classification system for male pattern baldness, ranging from Stage I (no recession) to Stage VII (only a horseshoe of hair around the sides and back). Used clinically to track progression and stage treatment plans.

O

Off-label prescription: Prescribing a medication for a use other than its FDA-approved indication. Common in dermatology and considered legitimate medical practice when supported by clinical evidence. Topical finasteride and low-dose oral minoxidil are examples in hair-loss care.
Oral minoxidil (low-dose): Minoxidil prescribed in low oral doses (typically 0.25 to 5 mg/day) as an alternative to or alongside topical minoxidil. Used off-label for hair loss. Requires medical supervision because of its blood-pressure-lowering origins.

P

PCOS (Polycystic ovary syndrome): A hormonal condition in women involving elevated androgens. Can contribute to hair thinning on the scalp and excess hair growth elsewhere. Treatment often includes addressing the underlying hormonal imbalance.
Postpartum shedding: A form of telogen effluvium that occurs 2-6 months after childbirth as a large proportion of follicles synchronously enter the resting phase. Almost always resolves on its own within 6-12 months.
PRP (Platelet-rich plasma): An in-office procedure in which the patient's own platelets are isolated and injected into the scalp. Theory: growth factors in platelets stimulate follicle activity. Evidence is mixed; typically used as an adjunct to medical treatment.

R

Receding hairline: Progressive retreat of the frontal hairline, usually starting at the temples. The classic early sign of male pattern baldness. Maps to Norwood Stage II-III when bilateral.

S

Scarring alopecia (cicatricial alopecia): A category of hair loss disorders in which inflammation destroys the follicle, replacing it with scar tissue. Permanent and progressive without treatment. Requires dermatology evaluation; not amenable to standard pattern-hair-loss treatments.
Sebaceous gland: The small oil-producing gland attached to each hair follicle. Overactivity contributes to oily scalp and dandruff. Generally not the primary driver of hair loss but can contribute to scalp inflammation.
Sebum: The oily substance secreted by sebaceous glands. Lubricates the hair shaft and scalp. Excess sebum can contribute to dandruff and scalp irritation but doesn't cause pattern hair loss directly.
Shedding: Normal loss of 50-100 hairs per day as follicles cycle out of anagen and into telogen. Increased shedding (over weeks or months) can signal telogen effluvium, the early phase of starting treatment, or progression of an underlying condition.
Side effect: An unintended effect of a medication beyond its primary use. Most hair-loss medications have well-characterized side-effect profiles; weighing them against benefits is part of any prescribing decision.
Spironolactone: An aldosterone antagonist with anti-androgen effects, prescribed off-label for female pattern hair loss in some patients. Not used in men due to its hormonal effects. Requires medical supervision and lab monitoring.

T

Telehealth: The delivery of medical care via secure video, phone, or asynchronous online consultation. Curekey is a telehealth service; the assessment is reviewed by a U.S.-licensed physician without requiring an in-person visit.
Telogen effluvium: A temporary diffuse shedding triggered by a physiological stressor: illness, surgery, childbirth, major weight loss, severe stress, or medication change. Typically begins 2-3 months after the trigger and resolves within 6-12 months.
Telogen phase: The resting phase of the hair cycle, lasting about three months. Roughly 10-15% of scalp hairs are in telogen at any time. At the end of telogen, the hair is shed and a new anagen cycle begins.
Testosterone: The principal male sex hormone, also present in smaller amounts in women. Converted into the more potent androgen DHT by 5-alpha reductase. Total testosterone levels are not predictive of pattern hair loss; follicle sensitivity to DHT matters more.
Thinning hair: A reduction in either the diameter of individual hair shafts (caliber) or the number of strands per area (density), or both. Often the earliest noticeable change in pattern hair loss, before any visible recession or bald spot.
Topical finasteride: A liquid formulation of finasteride applied directly to the scalp rather than taken orally. Aims to deliver the DHT-blocking effect locally with lower systemic exposure. An off-label alternative for patients sensitive to oral finasteride.
Traction alopecia: Hair loss caused by chronic mechanical tension on the follicle, often from tight ponytails, braids, weaves, or extensions. Reversible if the tension is removed early; can become permanent if follicles are scarred.
Tretinoin: A vitamin A derivative used in some compounded hair-loss formulations to enhance the absorption of minoxidil. Best known as a topical treatment for acne and photoaging.
Trichotillomania: A behavioral condition involving compulsive hair pulling, producing patchy or irregular hair loss. Distinct from medical hair loss; addressed through behavioral therapy rather than dermatology.

Looking for the broader medical guides?

Back to the hair-loss hub

Curekey patient outdoors after starting treatment

Get thicker, fuller hair in 3–6 months

Prescribed by board-certified dermatologists. Delivered to your door.

Start my assessment

Takes 2 minutes · Free to start