source: KidsHealth summary/abstract:
What is the Thyroid?
The thyroid is a small gland below the skin and muscles at the front of the neck, at the spot where a bow tie would rest. It’s brownish red, with left and right halves (called lobes) that look like a butterfly’s wings. It weighs less than an ounce, but helps the body do many things, such as get energy from food, grow, and go through sexual development.
What is Hyperthyroidism?
Thyroid hormone problems happen when the thyroid gland makes either too much or too little hormone for the body.
If the thyroid is overactive, it releases too much thyroid hormone into the bloodstream, causing hyperthyroidism. The body use up energy more quickly than it should, and chemical activity (like metabolism) in the cells speeds up.
If the thyroid is underactive, it makes too little thyroid hormone, causing hypothyroidism. The body uses up energy more slowly, and chemical activity (metabolism) in the cells slows down.