what cause brittle nails
1 Kidney disease can cause a build-up of nitrogen waste products in the blood, which can damage nails. 2  Liver disease can damage nails.  Thyroid diseases such as hyperthyroidism or hypothyroidism may cause brittle nails or splitting of the nail bed from the nail plate (onycholysis).
Conditions we more often associate with skin, like psoriasis, can also cause brittle nails. Some companies sell vitamins that are meant to help strengthen nails, most notably using a b-complex vitamin called biotin, but there is very little evidence to suggest that these vitamins actually improve brittle nails.