Acanthosis nigricans: what that dark, velvety skin means for your health

Acanthosis nigricans is not a skin disease in itself — it is a sign written in skin, telling you that something systemic is driving abnormal skin cell growth. The velvety, dark, thickened plaques that collect in flexural creases — the back of the neck, armpits, groin, and skin folds — develop when circulating insulin or related growth signals bind receptors on skin cells and drive them to proliferate. Recognising the sign and finding its cause is the real clinical task.

The good news for most patients is that the commonest cause — obesity-related insulin resistance — is responsive to lifestyle change. The patches that took years to accumulate can substantially fade within six to twelve months of sustained metabolic improvement. The less comfortable news is that a rare but important cause is occult cancer, which demands very different and urgent action.

A. Core clinical — the AU general-practice framework

Who gets it and why

The Australasian College of Dermatologists describes eight recognised clinical types, but for general practice the landscape simplifies to a few common patterns and one rare emergency.

Obesity-associated is by far the commonest type. In Australian children living with obesity, prevalence runs between 50–74% in cohort studies (Brickman J Pediatr 2007). In adults with type 2 diabetes, prevalence approaches 74%. The driver is hyperinsulinaemia — the body produces more insulin to overcome insulin resistance, and at high concentrations insulin activates IGF-1 receptors on skin cells, triggering overgrowth (Hermanns-Lê Am J Clin Dermatol 2004).

Endocrine-associated cases arise from conditions that worsen insulin resistance or raise circulating growth factors: polycystic ovary syndrome, Cushing syndrome, acromegaly, and hypothyroidism. In women with PCOS, 20–50% have acanthosis nigricans (Monash PCOS guideline 2023).

Drug-induced acanthosis nigricans is caused by glucocorticoids, high-dose nicotinic acid, the oral contraceptive pill, growth hormone injections, and older protease inhibitors used in HIV treatment. Onset after starting a new drug is the key history point.

Familial acanthosis nigricans presents in childhood without metabolic disease, caused by FGFR3 gene variants — non-progressive and not harmful in itself.

Malignant acanthosis nigricans is detailed separately below — rare but the highest-stakes possibility.

The history that matters

The most important questions are about onset speed and distribution. Gradual darkening over years in an overweight person with a family history of type 2 diabetes points firmly to insulin resistance. Explosive spread over weeks, involvement of the mouth lining, lips, palms, or areolae, in someone who has lost weight unexpectedly — that pattern demands urgent cancer investigation.

Drug history is often overlooked. Ask specifically about corticosteroids (including inhalers used long-term at high doses), the contraceptive pill, niacin supplements, and any injectable medications.

In women, ask about menstrual regularity, acne, and unwanted body hair — these point toward PCOS as a driver. In any patient with new-onset hypertension, easy bruising, and central weight gain, consider Cushing syndrome.

Aboriginal and Torres Strait Islander patients develop insulin resistance and type 2 diabetes earlier in life and at lower BMI thresholds. Acanthosis nigricans is a clinically visible, free-of-cost screening sign that can prompt metabolic workup in settings where formal health assessments are underutilised (NACCHO).

Clinical assessment

The bedside examination takes seconds. Inspect the posterior neck, axillae, antecubital fossae, groin, and inframammary folds. Wipe a small patch with an isopropyl alcohol swab — terra firma-forme dermatosis (another cause of dark neck skin, entirely benign and due to retained sebum and dead cells) disappears with wiping; acanthosis nigricans does not.

For any adult over 40 with new acanthosis nigricans, actively survey the mouth, lips, palms, and nipple areolae. Mucosal or palmar involvement strongly raises the possibility of paraneoplastic cause even in the absence of other symptoms.

Investigations and workup

The baseline metabolic screen for acanthosis nigricans includes:

• Weight, BMI, waist circumference, blood pressure
• Fasting plasma glucose and HbA1c (MBS items 66536 and 66841)
• Fasting lipid profile and liver function tests
• TSH

In women with irregular periods, hirsutism, or acne, the Monash 2023 PCOS guideline recommends total testosterone, SHBG, DHEAS, LH/FSH, and pelvic ultrasound.

Overnight 1 mg dexamethasone suppression test is added if Cushingoid features are present. IGF-1 is checked if acromegaly is suspected.

Malignancy workup — CT chest/abdomen/pelvis, gastroscopy, tumour markers (CA 19-9, CEA) — is not routine for typical obesity-associated acanthosis nigricans. It is reserved for the red-flag pattern: explosive onset, mucosal or palmar involvement, unexplained weight loss, GI symptoms, or new-onset acanthosis nigricans in someone over 40 without an obvious metabolic cause.

Treatment — treat the driver, support the skin

Weight management is the cornerstone for obesity-associated cases. A structured programme combining a Mediterranean-style or lower-carbohydrate dietary pattern with at least 150 minutes per week of aerobic exercise plus resistance training achieves the 7–10% weight reduction that visibly fades the plaques over six to twelve months. The GPCCMP (chronic condition management plan) under MBS item 965 enables direct referral to a dietitian and exercise physiologist within the plan.

Metformin 500 mg twice daily, titrated to 1 g twice daily, is the first-line pharmacotherapy for insulin-resistance-driven acanthosis nigricans. A paediatric RCT demonstrated improvement in acanthosis nigricans severity scores on metformin beyond lifestyle alone (Marquette JCEM 2008). Metformin is PBS Authority Streamlined for type 2 diabetes and authority-listed for PCOS; it is used off-label for isolated acanthosis nigricans without a formal diabetes or PCOS diagnosis.

GLP-1 receptor agonists (semaglutide, dulaglutide) produce superior weight loss to metformin and parallel improvements in acanthosis nigricans, but are PBS-listed in Australia only for type 2 diabetes. Private-cost access for obesity or PCOS is available but expensive.

Addressing the specific driver is essential: levothyroxine for hypothyroidism, appropriate PCOS management (combined oral contraceptive + spironolactone), cessation of the offending drug for drug-induced cases.

Topical adjuncts provide modest cosmetic benefit when used alongside metabolic correction. The most studied combination is tretinoin 0.05% cream at night with ammonium lactate 12% lotion in the morning (Blobstein Cutis 2003). Urea 10–20% cream (available over the counter) softens the thickening. Calcipotriol and salicylic acid preparations are alternatives. Laser treatments are available privately but should wait until metabolic correction is established — relapse is common otherwise.

B. Insulin resistance — what the biology means for patients

The pathophysiology is worth understanding because it reframes the skin findings as metabolic feedback, not cosmetic bad luck.

In insulin-resistant tissue — muscle, liver, adipose — cells respond poorly to insulin’s normal signal. The pancreatic beta cells compensate by secreting progressively more insulin to achieve the same glucose disposal. This compensatory hyperinsulinaemia produces the metabolic effects we try to treat (fatty liver, dyslipidaemia, elevated glucose, hypertension) but also spills over to activate IGF-1 receptors on keratinocytes and dermal fibroblasts. These receptors, unlike the metabolic insulin receptor, are not resistant — they respond avidly to excess insulin, driving epidermal hyperplasia and papillomatosis (Hermanns-Lê 2004).

This means the skin is reporting insulin load in real time. As metabolic control improves — through weight loss, exercise, metformin, or GLP-1 receptor agonists — circulating insulin falls, IGF-1R stimulation on keratinocytes decreases, and the plaques fade. Conversely, worsening metabolic control or rapid weight gain can cause the patches to darken or spread.

In malignant acanthosis nigricans, the driver is entirely different. Tumour-secreted transforming growth factor-α acts on epidermal growth factor receptors in skin, producing identical histology without any underlying insulin resistance. This explains why these patients are often thin and why the acanthosis nigricans may improve if the tumour is resected.

C. Malignant acanthosis nigricans — the must-not-miss

Malignant acanthosis nigricans represents under 1% of all presentations but carries serious consequences if missed. The distinguishing features are:

• Explosive onset — develops over weeks rather than creeping over years
• Atypical distribution — mouth and lip lining, vermilion border, palms (“tripe palms”), periorbital skin, areolae
• Associated B-symptoms — weight loss, fatigue, anorexia, dysphagia
• Age over 40 without an obvious metabolic driver
• Co-occurrence with the sign of Leser-Trélat — sudden eruption of multiple seborrhoeic keratoses

Around 70–90% of cases are caused by gastric adenocarcinoma, with the remainder attributable to lung, colorectal, pancreatic, ovarian, and breast cancers (Krawczyk Postepy Dermatol Alergol 2014).

The workup when malignant acanthosis nigricans is suspected is urgent: CT chest/abdomen/pelvis with contrast, gastroscopy with biopsies, colonoscopy, tumour markers (CA 19-9, CEA, CA 125), and haematology. Therapeutic Guidelines — dermatology recommends treating this as an equivalent to a two-week-wait cancer pathway. If acanthosis nigricans is paraneoplastic, treating the tumour is the priority — the skin findings often improve or remit with successful cancer treatment and may relapse if the tumour recurs.

D. Australian operations

General practice access points:

• GPCCMP (MBS item 965) enables direct allied health referral — dietitian, exercise physiologist, psychologist — for obesity or type 2 diabetes as the underlying driver. PCOS, hypothyroidism, and Cushing syndrome also qualify.
• Mental Health Care Plan (MBS item 2715) is appropriate where body image distress, depression, or anxiety complicates management — particularly in adolescents experiencing school avoidance or bullying.
• Aboriginal and Torres Strait Islander health assessment (MBS item 715) from age 15 onwards is a well-suited framework for the metabolic workup that acanthosis nigricans prompts in ATSI patients who have a disproportionate burden of early-onset insulin resistance (NACCHO).

PBS access:

• Metformin — PBS Authority Streamlined for type 2 diabetes; PBS Authority (specialist-initiated) for PCOS. Off-label for isolated insulin-resistance acanthosis nigricans — private script, available at low cost.
• GLP-1 receptor agonists (semaglutide, dulaglutide, liraglutide) — PBS Authority for type 2 diabetes only; private cost for obesity or PCOS ($130–460/month depending on agent and dose).
• Tretinoin 0.05% cream — PBS General Schedule (prescription required; verify current listing). Calcipotriol — PBS General Schedule for psoriasis; off-label for acanthosis nigricans.
• NPS MedicineWise provides up-to-date prescribing guidance for all these agents.

Documentation note: record at each visit whether mucosal, palmar, or explosive-onset features are absent. Explicit negative documentation of red-flag features provides medico-legal protection if a paraneoplastic cancer presents later in the same patient.

E. Special populations

Children and adolescents. Acanthosis nigricans is the single most visible physical sign of insulin resistance in young people. The RACGP recommends a full metabolic screen — fasting glucose, HbA1c, lipids, liver function — in any child with acanthosis nigricans, regardless of BMI percentile. Paediatric endocrinology input is appropriate for severe cases, particularly in adolescent females where HAIR-AN syndrome (hyperandrogenism, insulin resistance, acanthosis nigricans) overlaps with PCOS.

Pregnancy. Gestational insulin resistance can worsen existing acanthosis nigricans. The underlying condition (pre-gestational diabetes, PCOS) should already be managed through obstetric channels. Topical tretinoin is contraindicated in pregnancy (category X).

Aboriginal and Torres Strait Islander patients. Earlier onset, faster progression, and higher absolute risk of type 2 diabetes and cardiovascular disease mean that acanthosis nigricans appearing in an ATSI teenager or young adult should trigger the same workup applied to older non-Indigenous patients. Culturally safe care and engagement with local Aboriginal Community Controlled Health Services support adherence to long-term management.

People with HIV on antiretroviral therapy. Protease inhibitors from earlier treatment generations (lopinavir, ritonavir, older regimens) are established causes. Modern integrase inhibitor-based regimens have a much lower risk. If acanthosis nigricans appears after a regimen change, note the timing and review drug history with the treating infectious diseases team.

When to escalate

Refer or escalate when:

• Suspected malignant acanthosis nigricans — urgent gastroenterology and surgical oncology review within days
• Severe insulin resistance syndromes (HAIR-AN, lipodystrophy, type B anti-receptor antibodies) — endocrinology
• Severe adolescent acanthosis nigricans with confirmed insulin resistance and failed lifestyle intervention — paediatric endocrinology
• Persistent cosmetic distress despite metabolic correction — dermatology for topical review or consideration of laser treatment
• Cushing syndrome or acromegaly suspected — endocrinology for confirmatory testing
• PCOS with virilisation, severe hyperandrogenism, or fertility concerns — gynaecology or reproductive endocrinology

What this article is and is not

This is general health information drawn from Australian general practice guidelines — Therapeutic Guidelines, RACGP, Australasian College of Dermatologists, Monash PCOS guideline 2023, AMH, and NPS MedicineWise. It is not personal medical advice and does not create a doctor–patient relationship. Decisions about investigation and treatment are made in consultation with your own GP and treating clinicians.

For consumer-friendly information: HealthDirect — acanthosis nigricans, Better Health Channel — insulin resistance, Australasian College of Dermatologists patient leaflet.

If you have concerns about sudden-onset skin changes with weight loss or gut symptoms, seek prompt assessment from your GP rather than waiting for a scheduled appointment.

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Sources cited

1. RACGP — Skin manifestations of internal disease (AFP 2014)
2. Therapeutic Guidelines (eTG) — Dermatology
3. Australasian College of Dermatologists — Acanthosis nigricans
4. Monash University — International evidence-based guideline for PCOS 2023
5. Australian Diabetes Society
6. Australian Medicines Handbook (AMH)
7. NPS MedicineWise
8. HealthDirect — Acanthosis nigricans
9. Better Health Channel — Insulin resistance
10. NACCHO — chronic disease in ATSI populations
11. Krawczyk et al — Malignant acanthosis nigricans (Postepy Dermatol Alergol 2014)
12. Brickman et al — AN as marker of paediatric insulin resistance (J Pediatr 2007)
13. Hermanns-Lê — Pathophysiology of acanthosis nigricans (Am J Clin Dermatol 2004)
14. Marquette et al — Metformin for acanthosis nigricans (JCEM 2008)
15. Blobstein — Tretinoin + ammonium lactate (Cutis 2003)