Some possible causes of splitting toenails are an injury, harsh cosmetic nail treatments, exposure to moisture and certain medical issues. Another cause can be vitamin insufficiency or malnutrition. Nail splitting can occur either in a horizontal or a longitudinal direction, states Skinsight.
Nails that split in a horizontal direction are referred to as onychoschizia, while onychorrhexis is longitudinal splitting of nails. These conditions are related to brittle nails. Causes, such as repeated exposure to harsh soaps, using nail hardener and strong nail polish remover, can lead to brittle nails that split, notes the American Osteopathic College of Dermatology.
Split toenails can also be a symptom of medical conditions, such as fungal infections, psoriasis, hypothyroidism and nutritional deficiencies, according to Foot Vitals.