Endocrinology is a branch of medicine dealing with disorders of the endocrine system and its specific secretions called hormones.

Function of endocrine organs, hormones and receptors

Hormones are molecules that act as signals from one type of cells to another. Most hormones reach their targets via the blood.

All multicellular organisms need "coordinating systems to regulate and integrate the function of differentiating cells." Two mechanisms perform this function in higher animals: the nervous system and the endocrine system. The endocrine system acts through the release (generally into the blood) of chemical agents and is vital to the proper development and function of organisms.

Endocrinology is concerned with the study of the biosynthesis, storage, chemistry, and physiological function of hormones and with the cells of the endocrine glands and tissues that secrete them.

The endocrine system consists of several glands, in different parts of the body, that secrete hormones directly into the blood rather than into a duct system. Hormones have many different functions and modes of action; one hormone may have several effects on different target organs, and, conversely, one target organ may be affected by more than one hormone.

In the original 1902 definition by Bayliss and Starling, they specified that, to be classified as a hormone, a chemical must be produced by an organ, be released (in small amounts) into the blood, and be transported by the blood to a distant organ to exert its specific function. This definition holds for most 'classical' hormones, but there are also paracrine mechanisms (chemical communication between cells within a tissue or organ), autocrine signals (a chemical that acts on the same cell), and intracrine signals (a chemical that acts within the same cell). A neuroendocrine signal is a 'classical' hormone that is released into the blood by a neurosecretory neuron (see article on Neuroendocrinology).

Hormones act by binding to specific receptors in the target organ. A receptor has at least two basic constituents:

a recognition site, to which the hormone binds

an effector site, which precipitates the modification of cellular function

Between these is a "transduction mechanism" in which hormone binding induces allosteric modification that, in turn, produces the appropriate response.

Amines
Amines, such as norepinephrine, epinephrine, and dopamine, are derived from single amino acids, in this case tyrosine. Thyroid hormones such as 3,5,3�-triiodothyronine (T3) and 3,5,3�,5�-tetraiodothyronine (thyroxine, T4) make up a subset of this class because they derive from the combination of two iodinated tyrosine amino acid residues.

Peptide and Protein
Peptide hormones and protein hormones consist of three (in the case of thyrotropin-releasing hormone) to more than 200 (in the case of follicle-stimulating hormone) amino acid residues and can have molecular weights as large as 30,000. All hormones secreted by the pituitary gland are peptide hormones, as are leptin from adipocytes, ghrelin from the stomach, and insulin from the pancreas.

Steroid
Steroid hormones are converted from their parent compound, cholesterol. Mammalian steroid hormones can be grouped into five groups by the receptors to which they bind: glucocorticoids, mineralocorticoids, androgens, estrogens, and progestagens.