Why does nail fungus cause thickened nails?

The fungus can affect the nail plate and the space beneath it. New nail may form unevenly, while loose keratin and other material collect under the plate. This can make the nail look raised, feel hard and become difficult to cut. Pressure from shoes may then cause tenderness or further separation. Thickened Nails is a very common presentation of Nail Fungus, but trauma, psoriasis and other nail disorders can produce a similar appearance. An examination helps separate these possibilities.

Soak briefly if needed, dry thoroughly and trim only the part that cuts easily. File sharp edges gently and choose footwear with enough room at the toes. Avoid forcing clippers under a thick plate or trying to remove it yourself, since this can injure the nail bed. Treatment may involve a product applied to the nail or medicine taken by mouth, depending on the diagnosis and the extent of involvement.

Arrange an appointment if thickening makes walking or shoe wear uncomfortable, the nail lifts, or several nails change. Seek prompt care for increasing pain, redness, warmth, swelling, drainage or fever. A new thick nail after a significant injury should also be examined. Earlier advice is sensible if diabetes, poor circulation or reduced immunity makes foot or hand problems more difficult to heal.

Does the thickening look fungal, or could injury or psoriasis explain it? Would testing change the treatment? How should I trim the nail safely, and when should discomfort or separation be reviewed?