Acanthosis Nigricans: Why Skin Darkens at the Neck, Armpits and Groin

What acanthosis nigricans actually is

Acanthosis nigricans is not a rash, an infection, or a hygiene problem. It is a change in skin texture and pigment that happens when insulin, at high enough levels for long enough, starts acting on receptors it does not normally occupy in large numbers.

Insulin and insulin-like growth factor-1 bind receptors on keratinocytes and fibroblasts in the skin, driving the epidermal thickening (acanthosis) and overgrowth of small, finger-like projections that give the skin its velvety texture, according to a 2004 pathophysiology review in the American Journal of Clinical Dermatology. Melanocytes respond to the same signal, which is why the thickened skin also darkens. The skin itself is not diseased — it is doing exactly what a skin cell does when insulin binds its receptor too often.

This form of acanthosis nigricans — the one tied to insulin resistance, obesity and PCOS — is a different condition entirely from the rare congenital form seen in some genetic syndromes and the rare paraneoplastic form seen with certain internal cancers. Knowing which one you have changes everything about what to do next, which is why the sudden-onset pattern gets its own warning later in this article.

Insulin resistance and high androgens tend to arrive together in PCOS, because the same hyperinsulinemia that pushes on skin receptors also pushes ovarian theca cells to make more testosterone. That overlap is why a small subset of women present with all three at once — marked insulin resistance, high androgens, and extensive acanthosis nigricans — a combination described in the dermatology literature as HAIR-AN syndrome. It is a more severe end of the same spectrum this article covers, not a separate disease.

What it looks like before you have a name for it

Most people notice acanthosis nigricans as a texture change before they notice a color change: skin that feels slightly rough or “dirty” no matter how hard it is scrubbed, in a fold that gets friction — the back of the neck, under the arms, the groin crease. The color shift follows, moving from a faint tan tinge to a distinct grey-brown or dark brown, with the surface taking on a velvety, almost suede-like feel under a fingertip. It does not itch, hurt, or spread the way a rash does, and soap and exfoliation do not remove it, because there is nothing sitting on top of the skin to wash off.

Where it shows up, and what a change at each site tells you

Not every site behaves the same way, and not every site is equally useful for tracking severity. A 1999 validation study examined 406 people at five body sites — the neck, axillae, elbows, knuckles and knees — and found that only the neck produced consistent agreement between examiners scoring its severity; the other four sites were too inconsistent to use for grading. That is one reason clinicians and researchers focus on the neck first.

The neck

The neck is where acanthosis nigricans is most likely to appear first and where it is easiest to track over time, because it is the one location a scoring system has actually been validated against. If your only patch is a faint discoloration at the nape of your neck, you are looking at the earliest, mildest version of the same process described in this whole article — not a separate, lesser condition.

The armpits

Armpit involvement tends to follow neck involvement rather than appear alone, and it is the site most likely to have small skin tags growing in the same fold — the two conditions share the same underlying driver, which is why PCOS skin tags and acanthosis nigricans so often show up together in the same person.

The groin and inner thighs

Groin involvement is common but under-reported, largely because it is covered by clothing and easy to miss during a routine skin check. The mechanism is identical to the neck and armpits: friction, warmth and moisture in a fold, acted on by the same elevated insulin signal. It is also the site people are most likely to mistake for a fungal infection, since both cause darkening and texture change in the same crease — a fungal patch typically has a distinct scaly, active border and responds to antifungal cream within one to two weeks, while acanthosis nigricans has a soft, faded edge and does not respond to antifungal treatment at all.

The face — less common, worth a closer look

Facial acanthosis nigricans is uncommon enough that it is worth flagging specifically. When it does appear — typically as a subtle darkening around the mouth or a general dulling of the forehead and cheeks — it tends to travel with a more severe combination of hyperinsulinemia and hyperandrogenism than neck-only involvement, which is one more reason a sudden or unusual pattern anywhere on the face deserves a doctor’s look rather than a home remedy.

What “mild” acanthosis nigricans actually means

Mild acanthosis nigricans is not a different condition from the more visible version — it is an earlier point on the same process, most often a faint discoloration confined to the neck with minimal thickening and no involvement anywhere else. It is easy to dismiss as sun exposure, a tan line, or residue that will wash off. It will not wash off, because it is not on the surface. Mild presentations are also the ones most likely to fade with treatment, since less time has passed for the epidermal thickening to build up — a point covered in full in what actually fades it.

Does everyone with PCOS get it?

No — and the rate depends heavily on body weight. Among 339 PCOS patients with a normal BMI (under 23 kg/m²), a 2013 study in the Journal of Dermatology found acanthosis nigricans in 33 women, or 9.7%; even at that low rate, its presence still predicted insulin resistance with 92.6% specificity, though it missed most cases (18.6% sensitivity) — meaning a normal-BMI PCOS patient without visible acanthosis nigricans can still have insulin resistance underneath. In heavier PCOS populations the rate climbs well above that; the same androgen-dependent skin findings study that examined acne, hirsutism and skin tags in PCOS patients found that HOMA-IR — the standard insulin resistance measure — was significantly higher in the PCOS group with acanthosis nigricans than the group without it.

What it means for your risk of impaired glucose tolerance

A 2005 study of 121 PCOS women found that 42.9% overall had abnormal glucose tolerance — 32.3% impaired glucose tolerance, 9.1% type 2 diabetes, and 1.6% impaired fasting glucose — and that acanthosis nigricans independently predicted which women fell into that group, with an odds ratio of 2.7. That is not a reason for alarm; it is a reason to ask for the right test. A visible patch is a prompt to request an oral glucose tolerance test or a fasting insulin and glucose panel rather than to guess, and the full panel worth asking for is covered here.

Who this does NOT mean has PCOS, diabetes or a serious problem

Acanthosis nigricans by itself does not confirm PCOS, diabetes, or any specific diagnosis, and a few groups get it for reasons that have nothing to do with insulin resistance at all:

• Naturally deeper skin tone in a fold is not acanthosis nigricans. Normal pigmentation variation in body creases is common and does not have the thickened, velvety texture change that defines the condition.
• A rare hereditary form exists in some children and adolescents with no metabolic disease at all, distinct from — and unrelated to — the insulin-driven form this article covers.
• Certain medications (high-dose nicotinic acid, systemic corticosteroids, some hormonal contraceptives) can trigger it independent of a person’s underlying insulin status.
• Obesity alone, without PCOS, drives the same insulin pathway and produces the identical skin change — acanthosis nigricans is a marker of insulin resistance broadly, not a PCOS-specific sign, which is exactly why it appears on this site’s symptom list rather than its diagnosis list.
• Type 2 diabetes and prediabetes, independent of any ovarian or hormonal cause, produce the same texture change through the same elevated-insulin pathway — which is why acanthosis nigricans shows up in dermatology clinics far beyond gynecology and PCOS care.

None of this means the patch is meaningless — it means the patch tells you insulin is elevated, not which specific condition elevated it. Sorting out which one applies to you is a question for blood work, not for the mirror.

Note: in May 2026, PCOS was renamed polyendocrine metabolic ovarian syndrome, or PMOS, by a global consensus of more than 50 organisations. Same condition, same insulin mechanism behind acanthosis nigricans — only the label on the diagnosis changed. This article uses PCOS, since that is still the term most readers search.

Your next step

If you have noticed a velvety, darkened patch at your neck, armpits or groin, the useful next move is not a scrub or a bleaching cream — it is a fasting insulin and glucose test, because the patch is downstream of that number, not the other way around. Insulin resistance and what actually moves it are covered here, and if fading the visible skin change itself is the goal, the treatment evidence — what works, what doesn’t, and the realistic timeline — is laid out separately.