The thyroid gland is responsible for almost all of the metabolic activity of the body. The hormones in the thyroid gland can affect the body negatively in one of two ways: hyperthyroidism or hypothyroidism (http://women.webmd.com/guide/understanding-thyroid-problems-basics).
In hyperthyroidism, the thyroid produces too many hormones. However, there are many different diseases associated with hyperthyroidism. Graves' disease is caused by overproduction of thyroid hormone. Subacute thyroiditis is a short-term inflammation of the thyroid, causing hormones to “leak” out of the thyroid. Toxic adenomas are the nodes that develop on the thyroid for a short period of time, again causing too many hormones to “leak” out for a short period of time. These nodes upset the body's chemical balances for a time.
On the opposite end of the spectrum, there is hypothyroidism, which is when the thyroid gland does not secrete enough hormones, or does not produce thyroid hormones at all. Hypothyroidism leads to decreased amounts of energy and a general slowing down of the body (http://www.healthscout.com/ency/68/741/main.html). One of the most serious disorders is called Hashimoto’s thyroiditis. It is an autoimmune disorder where the body attacks the thyroid, causing the thyroid tissue to die and therefore stop producing hormones. Exposure to certain chemicals, such as iodide and lithium, can cause hypothyroidism as well.
Although thyroid disorders can throw off other bodily functions, they are usually treatable with medicine prescribed by a doctor. If you have any questions about thyroid disorders, please contact your doctor.