Psoriasis is indicated when arthritis and skin scaling at the elbow accompany pitting. A large spot under the nail is caused by the nail separating from the nail bed and may reflect a number of illnesses. But not all nail color changes reflect disease. Some whitening and thickening is normal with aging, beginning around age 20. Vertical lines called longitudinal ridges and tiny white spots are usually regarded as normal.
Race is a factor as well.
Pigmentation bands in the nail of a Caucasian person could indicate cancer, while in a non-Caucasian, pigmentation bands are more likely to be normal. It's a question of degree and how rapidly the condition has arisen. Know what your nails look like when you're healthy, so you have a baseline for comparison. A major stroke, surgery, or medications like tetracycline can change the growth rate or color of nails. A horizontal furrow or bowline will eventually grow out.