Hypothyroidism occurs when your thyroid isn't producing enough thyroid hormone to keep up with what your body needs. This can be due to a sluggish thyroid gland (for a variety of reasons), a lack of enough thyroid-stimulating hormone by your pituitary gland, a lack of adqueate nutrients the thyroid needs to function, a problem with receptor sites throughout the body, or an autoimmune disease like Hashimoto's Disease. There also may be a problem with thyroid hormone receptor sites throughout the body.
Hyperthyroidism occurs when your thyroid produces too much thyroid hormone. Simply put, the body has a regulator (thyroid) for metabolism and body maintenance and the dial gets turned up too high. This is a very serious problem. In hypothyroidism, where the thyroid is underfunctioning, you may feel miserable, but you can still live with it for a long time. Hyperthyroidism, on the other hand, is a great deal more serious.