Thyroid problems cause the nails to split or feel brittle but are not a cause for ridges, according to MedlinePlus. Vertical nail ridges are common and are not a cause for concern, while horizontal nail ridges may indicate a health condition, notes Mayo Clinic.
Some causes of horizontal nail ridges are localized, while others are systemic or life threatening, according to HealthGrades. Localized causes of nail ridges include bacterial infection, fungal infection or trauma to the nail. Some causes of systemic nail ridges are chemotherapy, psoriasis and iron-deficiency anemia. Life-threatening causes of nail ridges can include myocardial infarction, arsenic poisoning or malnutrition caused by poor absorption of nutrients.