Thick, yellow toenails are often caused by a fungal infection known as onychomycosis, but there are other potential causes as well. Fungal infections typically cause the nail to become brittle, crumbly, and may lead to a foul odor. In contrast, psoriasis, a skin condition, can also affect the nails, causing them to thicken and change color, but without the odor associated with fungal infections. Another possibility is trauma to the nail, which can cause thickening and discoloration.
To differentiate, examine the nail closely: a fungal infection often starts at the tip of the nail and can cause the nail to separate from the nail bed. If the entire nail is affected uniformly, it might be a systemic condition like psoriasis. Measuring the thickness of your toenail with a caliper can provide a baseline to monitor changes over time.