The back of the head as a witness

You run your hand over the back of your head: it’s thick, just like ten years ago. You move it up towards the crown, and there, under your fingers, it feels lighter. You’re not imagining that contrast. Dermatologists look for it too, and they use it to make sense of hair loss.

The back of the head is the area pattern hair loss affects least. That’s what makes it a good witness: when the rest changes, it gives you something to compare against.

A spared area

In male pattern hair loss, the usual pattern is well known: the hairline recedes, the top of the head thins, and the hair at the sides and back is spared¹. British dermatologists explain why: at the back and sides, hair is typically more resistant to miniaturisation by DHT, the hormone that gradually changes the follicles in pattern hair loss¹. The French Society of Dermatology puts it another way: the back of the head is generally not very sensitive to the hormones behind androgenetic alopecia².

Miniaturisation means that, in the affected areas, hairs first become thinner and shorter than those in unaffected areas, before they disappear¹. That’s why the comparison works: the back shows what your hair looks like when hormones aren’t interfering. For the details of the male pattern, read our article on the temples and the crown.

In women, the pattern is different but the logic is the same. The most common form is diffuse thinning of the centre and top of the head, with the front hairline kept³. The parting widens in the middle; the back usually stays fuller. We go into this in the article on hair loss in women.

The dermatologist’s reference point

Androgenetic alopecia is diagnosed mainly by examination². The doctor looks at your hair and scalp for hair loss that follows the typical pattern³. The examination also includes a pull test: the doctor tugs on a tuft of hair to see whether hairs come out easily³.

In doubtful cases, they move on to trichoscopy, a lit magnifier placed on the scalp³. They look for hairs of very uneven diameter, a high share of short fine hairs, follicles that carry only a single hair. Above all, they check whether these changes are more common at the front than at the back³. The European guideline lists this as one of the typical signs of pattern hair loss. The back isn’t examined for its own sake: it’s the yardstick.

What a study of 131 women showed

A team of dermatologists measured this difference. They reviewed the trichoscopy images of 131 women: 59 with androgenetic alopecia, 33 with chronic telogen effluvium (long-lasting shedding), and 39 with no hair problems, with diagnoses confirmed by examination and biopsy⁴. Hairs were measured at the front and at the back of the head.

The result fits in one line: without hair loss, hairs at the front and the back are almost the same thickness (0.061 and 0.058 mm). With androgenetic alopecia, the front hairs become thinner than the back ones (0.047 against 0.052 mm)⁴.

In women with no hair loss, as in those with chronic shedding, hairs at the front were slightly thicker than at the back. With androgenetic alopecia, the order was reversed⁴. The authors made this one of their main criteria, along with more than 10% very fine hairs (under 0.03 mm) at the front⁴.

When their criteria were met, they were rarely wrong: a specificity of 98%. But they missed more than one case in four, with a sensitivity of 72%⁴. The study is retrospective, includes only women, and relies on a device only a doctor uses⁴. It shows one simple thing: comparing the front with the back tells you something neither area tells you on its own.

The reserve for a transplant

Hair transplants rely on the same property. Follicles that aren’t sensitive to androgens keep their characteristics when moved to an area affected by pattern hair loss³. So they’re taken from the back and sides, where hair best resists DHT¹, as the French Society of Dermatology also describes².

To be a candidate, you need enough healthy hair to move to the area that needs it⁵. So the density at the back matters. And hair loss can continue in your original hair after a transplant⁵. The techniques are explained in our article on FUE, FUT and DHI.

When the back thins too

The back isn’t always untouched. The European guideline notes that, on top of the pattern, it’s not rare to see diffuse thinning of the upper sides and back of the head³.

Hair loss that affects the whole head evenly also points to something else. Telogen effluvium, temporary shedding that often follows an illness, childbirth or a shock, makes hair fall from all over the scalp⁶. You lose volume everywhere rather than in specific areas⁶. This kind of shedding doesn’t cause complete baldness⁷. It can, however, reveal pattern hair loss that was on its way⁶.

DermNet calls hair loss “diffuse” when it affects the scalp in a general distribution, as opposed to patchy loss⁸. The most common cause is telogen effluvium, with triggers such as a serious illness, childbirth, iron deficiency, a thyroid problem or major stress⁸. To tell them apart, a doctor can do a pull test, positive in temporary shedding and negative in pattern hair loss, and a blood test including ferritin and thyroid function⁸.

In practice, two situations are worth a doctor’s opinion: the back thinning at the same time as the top, and a clear, round or smooth patch appearing at the back. Heavier shedding than usual for a few weeks can also come from a treatable cause, such as iron deficiency⁶. Our article on how many hairs you lose a day helps you gauge what’s normal.

Comparing the top with the back at home

What a dermatologist does with a magnifier, you can do with photos, on your own scale. Photograph the back of your head along with the top, in the same conditions: same light, same distance, hair dry and styled the same way. The rules are in our guide to comparable photos.

Then compare the two areas over time. If the top thins from one photo to the next while the back stays the same, the difference is obvious at once. If both change together, that’s useful information too. Either way, you have dated images to show your doctor.

The back also helps you check your photos. If the back suddenly looks sparser than usual in one shot, the light or the angle has often changed. It tells you whether the photo of the top can be trusted.

The back of the head in your Calvap sessions

In Calvap, the back of the head is one of the areas you can track, along with the hairline, the temples, the crown and the sides. At each session, your last photo of the area appears see-through over the camera: you move until the two line up, and you get the same angle every week.

Your photos line up area by area on a timeline. Slide across the crown, then across the back, and see whether one is changing while the other holds. The photos stay on your phone, ready to show at an appointment.