Onychomycosis (Fungal Infection of the Nail): This is the most common cause of thickened nails, especially in older adults. It's caused by fungal infections such as dermatophytes, yeast, or molds, leading to nail thickening, discoloration, and sometimes pain.

Psoriasis: Nail psoriasis can cause thickening of the nails, along with other symptoms like pitting, nail lifting, and discoloration. It's a common condition that affects the skin and nails.
Eczema (Dyshidrotic Eczema): Eczema can lead to thickened nails, especially if the condition affects the hands or feet. It's characterized by itchy, dry, and cracked skin, which can also involve the nail bed.
Trauma: Repeated trauma to the nails, such as from ill-fitting shoes or excessive exposure to water, can cause thickening of the nails as a protective response.

Squamous Cell Carcinoma: Although rare, squamous cell carcinoma can present with thickened nails, especially if it involves the nail bed. It's crucial to diagnose and treat this condition early to prevent serious complications.
Melanoma: Subungual melanoma is a type of skin cancer that can cause thickening and darkening of the nail. It's essential to investigate any new or changing nail pigmentation to rule out this potentially deadly condition.

Yellow Nail Syndrome: A rare condition characterized by thickened, yellowish nails, often associated with lymphedema, respiratory problems, or other systemic diseases.
Nail-Patella Syndrome: A genetic disorder that affects the development of nails, knees, and other parts of the body, leading to thickened or absent nails.
Darier's Disease: A rare genetic disorder that can cause thickened nails, along with skin and other systemic symptoms.