An exogenous or from outside the body method of replacing or supplementing what the body is lacking or deficient in. Replacement therapy can have many different forms. Some deficiencies can be treated using oral medications, such as pills or liquids, others may need to be given subcutaneously, intramuscularly, or intravenously.
In the case of diabetes, when we give insulin, it is a replacement therapy, as we give injections to replace or possibly supplement the natural insulin the body is lacking. Addison's disease, Hypothyroidism, and Pancreatic insufficiency are also treated by replacement therapy--providing the hormones or enzymes the body needs to function properly from outside sources.
Fluids given to combat dehydration, whether they are given orally or subcutaneously, are a form of replacement therapy, because it is a way to replace the lost and necessary fluids and electrolytes. Prescribed vitamin supplements or injections are another form of replacement therapy. Blood transfusions can also be thought of as replacement therapy.
Replacement therapy is carried out until the body has or can produce what it needs on a regular basis in enough quantities to keep it healthy.