People notice dark patches on their neck, armpits, or groin, search for "hyperpigmentation," and land in a world of brightening serums, exfoliating acids, and skincare routines. They try those products. Nothing happens. The patches stay. Sometimes they try harder, scrubbing or using stronger actives, which irritates the skin without changing the patches.
Acanthosis nigricans is not hyperpigmentation in the conventional sense. It is not excess melanin deposited in response to inflammation, UV, or hormones. It is a thickening and darkening of the skin driven by growth factor signalling, most commonly associated with insulin resistance and metabolic dysfunction. The darkness is part of a structural skin change, not just a pigment change.
Recognising acanthosis nigricans means recognising that the appropriate response is not a topical. It is a blood test.
What it looks like
Acanthosis nigricans presents as dark, velvety, thickened patches of skin. The texture is the key distinguishing feature. Where hyperpigmentation is flat, acanthosis nigricans has a tactile quality: it feels soft, slightly raised, and velvety or suede-like when you run your finger across it.
The colour ranges from brown to dark brown to near-black, and it tends to be uniform within the affected area rather than patchy or variable. The darkening develops gradually, often so slowly that people do not notice it until it is well established.
The edges are diffuse rather than sharply defined. The affected area blends into surrounding skin without a clear border.
Where it appears
Acanthosis nigricans has a strong preference for skin folds and flexural areas where skin-on-skin contact occurs.
The most common locations are the back and sides of the neck, the armpits (axillae), the groin, and under the breasts. It can also appear on the elbows, knuckles, knees, and in the skin folds of the abdomen.
The neck is often the first place people notice it, because it is visible without effort. The darkening across the back of the neck is one of the most recognisable presentations and is frequently what prompts the initial search for information.
The location itself is a strong identification marker. Dark, velvety patches in skin folds behave and look very different from the flat pigment marks of PIH, the symmetrical facial patches of melasma, or the scattered spots of sun damage. If the darkening is in a fold and has a velvety texture, acanthosis nigricans should be the primary consideration.
What causes it
Acanthosis nigricans is driven by growth factor stimulation of keratinocytes and fibroblasts in the skin. The most common driver is elevated insulin, which is why the condition is so closely associated with insulin resistance and type 2 diabetes.
Insulin resistance is the primary cause in the majority of cases. When the body's cells become less responsive to insulin, the pancreas produces more insulin to compensate. That excess circulating insulin binds to insulin-like growth factor (IGF) receptors on skin cells, stimulating them to multiply and thicken. The darkening and velvety texture are the visible result of this buildup.
Obesity is a major risk factor, both because it is associated with insulin resistance and because skin folds in people with higher body fat create more of the flexural environment where acanthosis nigricans tends to appear.
Polycystic ovary syndrome (PCOS) is another common association, because insulin resistance is a feature of many PCOS presentations.
Medications can occasionally trigger or worsen acanthosis nigricans. Insulin injections (particularly at high doses), oral contraceptives, corticosteroids, niacin, and growth hormone therapy have all been associated. If the onset correlates with starting a medication, that association is worth raising with the prescriber.
Rare causes include certain cancers (malignant acanthosis nigricans, most commonly associated with gastric adenocarcinoma), genetic syndromes, and endocrine disorders beyond insulin resistance. Malignant acanthosis nigricans tends to be more severe, more widespread, and more rapid in onset than the metabolic form. Any acanthosis nigricans that appears suddenly, spreads rapidly, or is unusually extensive warrants urgent medical evaluation.
Why topical treatment does not work
Brightening products target melanin production pathways. Tyrosinase inhibitors, exfoliants, and retinoids work by reducing melanin synthesis, accelerating turnover, or fading deposited pigment.
Acanthosis nigricans is not primarily a melanin problem. The darkening results from the thickening and structural change of the skin driven by growth factor signalling. The excess melanin present is secondary to the epidermal hyperplasia (thickening), not the primary process.
Applying brightening products to acanthosis nigricans is targeting a downstream effect while the upstream cause continues. Even aggressive exfoliation or chemical peels will not resolve it because the thickened, darkened skin will regenerate from the same growth factor stimulation.
The most effective "treatment" for acanthosis nigricans is addressing the underlying insulin resistance. When insulin levels normalise, the growth factor stimulation decreases, the skin gradually thins back toward normal, and the darkening lightens. Weight loss, dietary changes, exercise, and in some cases medication (metformin) to improve insulin sensitivity can all produce visible improvement in the skin over months.
This is not a skincare problem. It is a metabolic one.
What blood tests to discuss with your doctor
If you recognise the acanthosis nigricans pattern on your skin, the next step is a conversation with your doctor about metabolic screening. The specific tests your doctor will consider include:
- Fasting glucose to assess blood sugar levels.
- Fasting insulin to assess insulin levels directly. Glucose can be normal while insulin is already elevated (compensated insulin resistance), which is why both matter.
- HbA1c (glycated haemoglobin) to assess average blood sugar control over the preceding 2 to 3 months.
- Lipid panel because insulin resistance frequently coexists with dyslipidaemia.
- Thyroid function because thyroid disorders can contribute to metabolic dysfunction.
For women with acanthosis nigricans who also have irregular periods, excess hair growth, or acne, PCOS screening (including androgen levels and pelvic ultrasound) is also worth discussing.
These tests are not about the skin. They are about identifying the metabolic pattern that the skin is signalling. The darkened patches on the neck are, in a sense, the body's visible warning system.
When to see a doctor
If you recognise the pattern. Dark, velvety, thickened patches in skin folds are distinctive enough that self-recognition is common. If this describes what you are seeing, a metabolic screen is the appropriate first step.
If the patches appeared suddenly or are spreading rapidly. Gradual onset associated with weight gain is the typical metabolic presentation. Rapid onset, rapid spread, or unusually severe involvement can indicate a paraneoplastic process (cancer-associated acanthosis nigricans) and requires prompt medical evaluation.
If you are already managing diabetes or insulin resistance and the patches are not improving. This may indicate that metabolic control needs reassessment, or that the insulin resistance is more significant than current management is addressing.
If you have been treating it as cosmetic hyperpigmentation without improvement. Months of brightening products on acanthosis nigricans is months spent addressing the wrong problem.
The dark patch on your neck is not asking for a better serum. It is asking for a blood test.
