Hyperthyroidism, or overactive thyroid, is the term used for when there are too many thyroid hormones in the blood. Causes of hyperthyroidism may include: Graves Disease, Toxic Multinodular Goiter, or a viral infection causing inflammation of the thyroid itself. Hyperthyroidism may cause weight loss, irregular periods, feeling too hot, nervousness, feeling on edge, and a rapid heart rate Be sure to watch for our next post on Monday about Hypothyroidism, yes there is a difference in these.