Onychomycosis (Fungal Infection of the Nail): This is the most common cause of yellowing and thickening of the toenails, often leading to the nail falling off. It's a chronic infection that can be caused by various types of fungi.

Trauma: Physical injury to the toenail can cause it to turn yellow and eventually fall off due to damage to the nail bed or the nail itself.
Psoriasis: Nail psoriasis can cause changes in the nail's appearance, including yellowing, thickening, and separation from the nail bed, potentially leading to the nail falling off.
Eczema/Dermatitis: Certain skin conditions can affect the nails, leading to changes in color and texture, and potentially causing the nail to fall off.

Squamous Cell Carcinoma: Although rare, this type of skin cancer can manifest under the nail, causing changes in nail color and potentially leading to the nail falling off. It's crucial to diagnose and treat promptly.
Melanoma: A type of skin cancer that can occur under the nail, presenting with a dark streak or changing nail color. Early detection is vital for effective treatment.
Systemic Diseases (e.g., Diabetes, Thyroid Disorders): Certain systemic conditions can affect nail health, leading to changes in nail color and texture. Identifying and managing the underlying condition is essential.

Yellow Nail Syndrome: A rare condition characterized by yellowing of the nails, often associated with respiratory and lymphatic system abnormalities.
Nail Patella Syndrome: A genetic disorder that affects the development of nails and kneecaps, among other symptoms, and can cause nail abnormalities, including yellowing and falling off.
Koilonychia (Spoon Nails): Although more commonly associated with iron deficiency, in rare cases, it can present with yellowing of the nails.