The medical term for the toe cap is toenail, and the cause of the thickening and hardening of the toenail of the big toe may be onychomycosis, psoriasis, or congenital thick nails. 1. Onychomycosis: Onychomycosis, also known as gray nails, is a thickening and hardening of the toenail due to a fungus that causes a constant buildup of keratin on the big toe. 2. Psoriasis: commonly known as psoriasis, the surface of the toenail is uneven, the nail plate is thickened and becomes dirty brown, and the scales under the nail bed cause the nail plate to bow up or buckle, making the toenail thicken and harden. 3. Congenital thick nails: the nail plate of congenital thick nails is thicker and harder the more it reaches the free edge. There may be other reasons for the thickening and hardening of the big toe cap, such as aging, thyroid or pituitary gland dysfunction, etc. You should go to a regular hospital and have your doctor determine the cause of the condition.