Simplify the following text for a Grade 11 native English-speaking person.
Original: Graves ' disease is an autoimmune disease where the thyroid is overactive , producing an excessive amount of thyroid hormones ( a serious metabolic imbalance known as hyperthyroidism and thyrotoxicosis ) .
Simplified: Graves ' disease ' is an autoimmune disease of the thyroid . In people affected by the disease , the thyroid produces a bigger quantity of hormones than normal , which is commonly known as hyperthyroidism .