The endocrine system regulates vital body functions by means of hormonal messengers. Hormones are defined as circulating messengers, with action at a distance from the organ (gland)of origin of the hormone. Hormones can be regulated by nerve cells; endocrine agents can serve as neural messengers. There is also a relationship between the endocrine system and the immune system; autoantibodies may cause an organ to produce an excess or deficiency of a hormone. Manifestations of an endocrine disorder are related to the response of the peripheral tissue to a hormone excess or deficiency.
Hormone action also may be paracrine (acting on adjacent neighboring cells to the cell of origin of the hormone) or autocrine (acting on the cell of origin of the hormone itself); agents acting in these ways are called factors rather than hormones (Fig. 170-1). Hormones generally are regulated in a feedback loop so that the production of a hormone is linked to its effect or its circulating concentration. Endocrine disorders generally manifest from one of four ways: 1.
By excess hormone: In Cushing syndrome, there is an excess of glucocorticoid present; if the excess is secondary to autonomous glucocorticoid secretion by the adrenal gland, the trophic hormone adrenocorticotropic hormone (ACTH) is suppressed.
2.
By deficient hormone: In glucocorticoid deficiency, the level of cortisol is inadequate; if the deficiency is at the adrenal gland, the trophic hormone is elevated (ACTH).
3.
By an abnormal response of end organ to hormone: In pseudohypoparathyroidism, defects in the gene (GNAS1) encoding the alpha subunit of the stimulatory G protein (Gsa) is manifested by elevated PTH levels in the face of deficient PTH function (PTH resistance).
4.
By gland enlargement that may have effects as a result of size rather than function: With a large nonfunctioning pituitary adenoma, abnormal visual fields and other neurologic signs and symptoms result even though no hormone is produced by the tumor.
Peptide hormones act through specific cell membrane receptors; when the hormone is attached to the receptor, the complextriggers various intracellular second messengers that cause the biologic effects. Peptide hormone receptor number and aviditymay be regulated by hormones. Steroid hormones exert their effects by attachment to intranuclear receptors, and the hormone-receptor complex translocates to the nucleus, where itbinds with DNA, and leads to further gene activation.
The interpretation of serum hormone levels must be related totheir controlling factors. For example, a given value of parathyroidhormone may be normal in a eucalcemic patient but inadequatein a hypocalcemic patient with partial hypoparathyroidism orexcessive in a hypercalcemic patient with hyperparathyroidism.