Horizontal nail ridges may indicate several health conditions but have no bearing on thyroid disease prevalence, notes WebMD. However, brittle or dry nails that split or crack frequently may indicate thyroid disease. If the development of a yellowish color accompanies splitting and cracking, a fungal infection may be the cause.
Horizontal ridges, or Beau’s lines, run from side to side on the nail, notes WebMD. Underlying health problems, previous injury and arsenic poisoning may cause Beau’s lines. Vertical ridges on the nail are generally innocuous and a normal result of the aging process. A medical professional should evaluate dark lines beneath a nail immediately, since they may indicate melanoma, a dangerous skin cancer.