What Causes A Light Period?
An Overactive Thyroid

The thyroid gland is located in your neck and is responsible for various bodily functions. The thyroid helps the process of energy and oxygen being transported to your cells, tissues, and organs. An overactive thyroid results from too much thyroid hormone and is referred to as hyperthyroidism. It can cause a host of serious issues for many parts of the body, including the heart and muscles.
Hyperthyroidism causes abnormally light periods and missed periods. It's more common to suffer from hypothyroidism, which is when you don’t have enough of the thyroid hormone. Some sufferers can feel the nodules pushing against their neck. Many women are unaware that an untreated thyroid condition can affect their periods. See your doctor for treatment if you suspect a problem with your thyroid.