Toenails most commonly turn white from fungal infection or decreased blood supply to the nail bed. Parallel lines extending all the way across the nail may be a sign of low protein in the blood.
The decreased blood supply causes 'Terry's nails,' which often have dark or reddened tips. The condition could be indicative of liver cirrhosis, heart failure, diabetes, iron deficiency, chemotherapy, malnutrition or an over-active thyroid.
While small white spots and streaks are normal and nothing to worry about, nail changes in color, shape, texture or thickness not resulting from nail bed detachment should be addressed with a doctor.