Some diseases are also associated with changes in the nails, which can include brittleness. Thin and brittle nails can be a sign of hypothyroidism, for example. The term onychoschizia refers to splitting of the fingernails as well as brittle or soft nails. Taking biotin (a vitamin) supplements can help in some cases of brittle nails, and application of moisturizers after soaking in water can also be of benefit. Medically Reviewed by a Doctor on 3/30/2015.