Thyroid hormones are most closely associated with metabolism or the process by which fuel (in the form of food) is converted by the body into cellular structures such as proteins and fats or else excreted from the body. Everybody understands the lay person's definition of a fast metabolism making it easy for an individual to stay slim or how a slow metabolism can make it harder to lose weight.
When we think of a metabolism being fast, it is associated with elevated levels of T3 and T4, the major thyroid hormones while a slow metabolism is associated with much lower levels of T3 and T4. Where it becomes a medical issue is when an individual suffers from hyperthyroidism (excess thyroid hormone production) or hypothyroidism (insufficient thyroid production). As you might expect the former is associated with rapid loss of weight while the latter with obesity.
Thyroid hormone treatment is commonly prescribed to remedy both situations with the use of levothyroxine (T4), liothyronine sodium (T3) and Armour Thyroid (a combination drug containing T3 and T4) all used widely.