Understanding Thickened Nails

There are several causes of very thick or crumbling nails. They can be caused by injuries or pressure from shoes. Fungal infections are a common cause. Diabetes, psoriasis, or vascular disease are other possible causes.

Onychomycosis - Wikipedia

Comprehensive Toenail Problem Guide

What that weird dent or blotch on your nail might be saying about

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Thick toenails: Causes, symptoms, and treatments

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Difficult and Thickened Toenails - Cutting Edge Podiatry

Nail Fungus: Symptoms and Treatment