Biotin deficiency is a rare condition that occurs when the body does not have enough biotin, a B-vitamin essential for various metabolic processes. Symptoms may include hair loss, skin rashes, and brittle nails. It can also lead to neurological issues such as depression and fatigue. This deficiency can result from inadequate dietary intake, certain medical conditions, or prolonged use of specific medications. Foods rich in biotin include eggs, nuts, and whole grains. Most people get enough biotin from their diet, making severe deficiency uncommon.