Ridged fingernails are not a sign of thyroid issues, according to MedlinePlus. Two thyroid diseases, hyperthyroidism and hypothyroidism, cause the nails to crack or the nail bed to separate from the nail in some cases.
Ridges on the nail indicate other health concerns such as possible malnutrition, infection or other illnesses. A person can develop ridges on both their fingernails and toenails, so both areas cause concern if ridges exist. These small, raised lines run either horizontally or vertically along the nail. If the ridges run horizontally across the nail, known as Beau's lines, a doctor needs to be consulted to determine if any health issues exist, explains MedlinePlus.