I had something similar as part of adrenal insufficiency - brown line down nails, not diffuse, but i also had hyperpigmentation in other areas including knuckles, elbows, unexposed areas, wounds turning brown after healing, as well as an overall 'tan' that i hadn't realized i had. jaad.org/article/S0190-9622....
I hope you don't have Addison's (autoimmune adrenal insufficiency) but perhaps worth exploring. Signs & symptoms include high potassium levels, nausea/vomiting, weakness and others, see for example my.clevelandclinic.org/heal...
Apparently low protein in diet, low vit D, and low B12 can also cause this. As well as the usual suspects like a fungal infection and so on webmd.com/skin-problems-and...
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