Hyperthyroidism is a condition in which the thyroid gland makes too much thyroid hormone. It is also referred to as an "overactive thyroid".
This essentially causes a 'speeding up' of a lot of the bodies processes - you metabolise food faster, become more anxious, your skin becomes more dry, your reflexes get brisker, your heart goes faster and your blood pressure increases. It also causes diarrhoea, and can cause either menorrhagia, or oligomenorrhoea.