The endocrine glands produce circulating substances called hormones that work at distant sites from the point of production. These glands include the thyroid gland, the parathyroid glands, the adrenal glands and the pancreas. Thyroid hormone, parathyroid hormone, circulating steroids and insulin are some of the hormones produced by these glands. Excessive activity can produce severe illness that may require surgical intervention. Excessive production of thyroid hormone produces a state called hyperthyroidism and this may require surgical intervention in addition to medical therapy. Endocrine surgery usually involves the surgical removal of a portion of a gland or glands to limit or eliminate the effects of overproduction.