Thick skin under the toenails, often caused by a fungal condition known as onychomycosis, can make the toenails themselves appear thicker than normal and even take on a yellow or brown discoloration Onychomycosis can cause the skin to thicken along the outer edge of the nail and directly underneath, according to Columbia University 1 Other health issues that yellow nails indicate involve severe thyroid, lung disease, psoriasis, and diabetes 3 White Nails If your nails are white with darker rims they may be warning you about your liver's health This is a good sign of hepatitis Fingers can also become jaundiced, another sign of liver stress Nail infection (paronychia) Paronychia is an infection of the skin of the fingers or toes, at the place where the skin folds down to meet the nail Acute, or sudden onset, paronychia is caused by the staphylococcus bacteria The organism can gain entry if the nail is cracked, broken, bitten, or trimmed too closely
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