causes of 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).
1 Chronic picking or rubbing of the skin behind the nail can cause median nail dystrophy, which gives a lengthwise split or ridged appearance of the thumbnails.  Long-term exposure to moisture or nail polish can cause nails to peel and become brittle.