What would cause horizontal nail ridges and white spots on nails?

Nail ridges: The ridges can be caused by infections, rheumatoid arthritis or actual trauma to the nail bed.

Could be serious: Vertical ridges, especially in adult women are fairly common. In a male, and across rather than to the tip it can be a sign of serious conditions so have it checked. Horizontal ridges that are accompanied by white lines can be a symptom of arsenic poisoning. Horizontal depressions across the nails, called beau's lines, can be a sign of diabetes, malnutrition, or a circulatory problem. Best!

Horizontal nail : ridges (or Beau’s lines)run across nail & might be signs of diabetes, thyroid disease, kidney disease. They may occur in people who had syphilis or muscles or who had chemotherapy. These should be evaluated. Milk spots on nails are usually harmless & related to nail trauma (such as biting nails). Other causes can be Iron or protein deficiency, lead or arsenic poisoning, renal failure, cirrhosis,