Excessive hair growth on the body can be a source of significant social and personal concern. In the fields of dermatology and aesthetics, it is crucial to distinguish between different types of hair overgrowth, as their causes and biological mechanisms differ fundamentally. While both conditions involve "superfluous hair"—hair that appears in areas where individuals prefer to be hairless, such as the face, abdomen, or limbs—they are categorized into two distinct medical phenomena: hirsutism and hypertrichosis.
The Biology of Hair: Terminal vs. Vellus
To understand these conditions, one must first understand the types of hair the human body produces. Hair is a keratinized structure growing from a follicle, which is a downward extension of the epidermis.
There are three primary types of hair:
- Lanugo: Fine, downy fetal hair that is typically shed around the 7th to 8th month of development.
- Vellus: Short, fine, and soft hair that lies close to the skin surface and lacks a medulla.
- Terminal: This is the thick, long, and pigmented hair that forms the bulk of the hair we see.
Terminal hair is characterized by its depth and diameter (up to 0.6mm) and contains a medulla, which is often only present in thick terminal strands. Terminal hair is categorized by its hormonal relationship: it can be asexual (scalp/eyebrows), ambisexual (axilla/pubis), or sexual (beard/chest), the latter of which is driven by increased androgenic hormone production.
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