Some people are born with it. It is often found in people with diabetes or a tendency towards diabetes and is most common among people of African descent. There are many other causes, including:
Obesity
Addison's disease, a condition caused by a deficiency of hormones from the adrenal gland
Disorders of the pituitary gland within the brain
Growth hormone therapy
Hypothyroidism (low levels of thyroid hormone caused by decreased activity of the thyroid gland)
Oral contraceptives