“The most common disease that thickens and darkens toenails is nail fungus, which can also be seen in some chronic conditions.
When a fungus infects the toenail, the build-up of keratinous material causes thickening of the toenail and the accumulation of fungus or dirt causes discolouration of the toenail, which can manifest as darkening.
In the case of chronic diseases such as respiratory diseases and diabetes, the circulation to the limb is impaired and the blood supply to the ends is inadequate causing the toenail to grow slowly and the build up of keratinous material to manifest as a thickened and darkened toenail. It is important to diagnose the cause and treat it under the guidance of a doctor.”