The cause of lines in the middle of the thumb needs to depend on the location of the lines. If it is a vertical line at the belly of the finger, it is mostly suggestive of malnutrition, and it is easy to have this kind of line when there is iron deficiency anemia. If it is a horizontal line at the base of the finger, it belongs to physiological individual differences, and some movements that require frequent clenching of the fist can also induce such changes. If there are horizontal lines on the nail of the thumb, it is mostly suggestive of a calcium deficiency, especially a phased calcium deficiency, which may occur due to phased nail development.