It sounds like you should see your doctor. It could be a lack in certain vitamins, it could be that something happened to your foot and you didn't realize, it could be a sort of infection. There's no way of knowing but by seeing your doctor. I hope everything works out. Good luck.
It could be the beginning of toenail fungus. I'd start with a cheap but natural treatment of toenail fungus.
How to Use Epsom Salt as Toenail Fungus Treatment
It's called Melanonychia. It could be a nail"freckle or mole". It's common in people with darker skin. Or it could be an indication of a malignant melanoma if you are caucasian.
You should get it checked out to rule out anything serious.
I have read all the scary answers like could be a sign of hearth disease and can be melanoma ( skin cancer very rare) but I have found the most liekly reason for me : psoriasis. If you have nail psioriasis like me having thin line like this is a common sign and it is not related to the previous reasons that are also true, but less likely if you also have psioriasis.
Psoriatic Onychia of the type of subungual hemorrhages has 2 types of hemorrhages.
The hemorrhagic spots of the first type, which are translucent through the nail, can be located in the region of the nail lune. They are formed due to the extended capillaries of the matrix unguis and disappear with pressure.
The second type of subungual hemorrhages is characterized by the striata hemorrhages, becoming yellowish-brown or black in color. Sometimes the thin arrow-shaped strips in the distal part of the nail produce the impression of splinters.
All the damages of the nails are accompanied by the redness, infiltration, damage of near-ungual roller (Psoriatic Paronychia). Rear ungual skin is infiltrated, with the abundant peeling, which is absent with Onychomycoses and Candidiasis.
And see this link :
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