Acanthosis nigricans
Velvety, darkened patches of skin, often at the neck, armpits, or groin. A visible sign of high circulating insulin.
Acanthosis nigricans is a thickening and darkening of the skin that shows up in folds and creases: the back and sides of the neck, the armpits, the groin, sometimes the knuckles. The patches feel velvety rather than rough.
It is not a skin condition in its own right. It is a marker of chronically elevated insulin. Insulin acts on receptors in the skin that drive cell growth, so when insulin runs high for a long time, these areas thicken and darken. In PMOS, where insulin resistance is common, acanthosis nigricans is one of the few signs of high insulin you can see without a blood test.
If you notice it, it is worth asking a clinician about insulin testing (fasting insulin or HOMA-IR), not just fasting glucose.