Can substantial weight loss cause periods to become very light, even if there is still a lot of weight left to lose? can it cause spotting in between?

Rapid weight loss: Can cause light period. The main reason for light periods is a change in hormone levels. Causes for what seems to be a light period can include: thyroid problems, eating disorders, excessive exercise, initial periods for a girl, use of an intrauterine device, oral contraceptives, perimenopause ; implantation bleeding in early pregnancy which rarely occurs.