Density vs. Diameter — Why Both Matter

When people talk about thinning hair, they usually mean density — how many hairs are on the scalp. But density is only half the story. The other variable, one that gets far less attention, is diameter: how thick each individual hair actually is.

Picture two people who each have about 100,000 hair follicles. One person’s hairs average 70 microns across; the other’s average 40 microns. Same follicle count, very different amount of visible hair — thicker individual strands mean more volume and less scalp showing through, even with an identical number of follicles (Price, 1999; Liu et al., 2025).

That’s the mechanism behind follicle miniaturization. With certain types of hair loss, particularly androgenetic alopecia, genetically susceptible follicles gradually shrink over successive growth cycles (Price, 1999; Liu et al., 2025) — producing hair that’s thinner, shorter, finer, and sometimes less pigmented (Price, 1999; Dakkak et al., 2024).

Over time this becomes noticeable: a widening part, more scalp showing through, a smaller ponytail, hair that just doesn’t feel as full as it used to.

This is the part that catches people off guard: you don’t have to see a dramatic increase in shedding to have progressive thinning. Often the shedding barely changes at all — it’s that each follicle is quietly producing less hair than it used to (Price, 1999).

So the more useful question isn’t just how much hair you’re losing — it’s what’s happening to the individual hairs your follicles are still producing. That distinction ends up mattering a lot once you start figuring out what’s actually going on.

The Four Things That Actually Cause Hair Loss

When someone tells me they’re losing their hair, I don’t jump to one diagnosis — I think through four different things that can be happening to the hair or the follicle itself.

1. Excessive Shedding — Too Many Hairs Leave the Cycle

The first possibility is that your follicles are producing hair normally, but too many of them are entering the resting-and-shedding part of the cycle at the same time — a condition called telogen effluvium (Mirmirani & Fu, 2021). A variety of physical or physiologic stressors can trigger this shift, including:

• significant illness
• surgery
• major psychological stress
• rapid weight loss
• inadequate calorie or protein intake
• certain nutritional deficiencies
• hormonal changes
• some medications

(Mirmirani & Fu, 2021; Dakkak et al., 2024)

One of the frustrating things about telogen shedding is that it often doesn’t happen immediately after the trigger. You may experience a significant stressor in January and not notice substantial shedding until March or April — that two-to-four-month delay reflects the follicle’s own timeline (Dakkak et al., 2024). That’s why evaluating increased shedding usually means looking back over the previous several months, not just the last few weeks.

2. Progressive Miniaturization — The Follicle Produces Less Hair

The second possibility is follicle miniaturization: instead of shedding dramatically more hair, the follicle gradually shrinks and produces progressively finer hairs (Price, 1999; Liu et al., 2025) — a hallmark of androgenetic hair loss.

Androgens, including DHT (dihydrotestosterone), affect genetically susceptible follicles differently depending on their location. DHT is produced locally from testosterone by the enzyme 5α-reductase, binds the androgen receptor on dermal papilla cells, and activates genes that gradually transform terminal follicles into miniaturized ones (Price, 1999; Liu et al., 2025; Dakkak et al., 2024). Repeated cycles of this can turn a thick terminal hair into an increasingly fine one, showing up as a widening part, decreased ponytail thickness, thinning at the crown, and increased scalp visibility (Dakkak et al., 2024).

This is why someone can tell you, “I’m not actually shedding that much — my hair just isn’t as thick as it used to be.” They may be describing miniaturization rather than excessive shedding.

3. Shaft Breakage — The Follicle May Be Completely Fine

The third possibility isn’t hair loss from the follicle at all — it’s breakage. The hair follicle is alive, but the visible hair shaft is primarily a hardened, keratinized structure (Koch et al., 2020), and once it emerges from your scalp it’s exposed to everything you do to it: heat styling, bleaching, coloring, chemical treatments, tight hairstyles, friction, brushing, environmental exposure. Repeated exposure to any of these can weaken the shaft (Koch et al., 2020), showing up as split ends, shorter broken hairs, roughness, frizz, or reduced volume — which can look like hair loss even when the follicle itself is working fine.

That’s the real distinction: shedding is when a hair comes out from the follicle, while breakage is when it snaps somewhere along the shaft — two different problems that call for different fixes.

4. Follicle Destruction — When the Follicle Is Actually Damaged

The fourth category is less common but particularly important: follicle destruction. Some inflammatory and autoimmune conditions damage the follicle itself. In certain types of scarring alopecia, inflammation destroys the follicular structure — including the stem cells in the bulge region — and replaces it with fibrous scar tissue (Harries & Paus, 2010; Hajizadeh et al., 2023; Lin et al., 2019).

This is fundamentally different from temporary shedding or miniaturization. A miniaturized follicle is still there and may keep producing hair; a permanently destroyed one can’t be “restarted” with a vitamin or a new shampoo (Harries & Paus, 2010; Lim et al., 2019). That’s one reason persistent or unusual hair loss deserves a closer look, especially with scalp symptoms like significant itching, burning, tenderness, scaling, or redness (Dakkak et al., 2024).