When the thyroid gland doesn’t produce enough thyroid hormone, this is known as hypothyroidism, or an underactive thyroid. The thyroid can also produce too much hormone, which is known as hyperthyroidism, or an overactive thyroid. The study authors suggest that there may be a link between thyroid hormone levels and negative personality traits. This suggests that increases in ‘darker traits’ may be specific to having an overactive thyroid, rather than thyroid problems in general. The study authors suggested that when thyroid hormone levels are high, the body goes into overdrive, speeding up metabolism and increasing brain activity.