Essential Endocrinology and Diabetes. Richard I. G. Holt

Hypothalamus/median eminence Releasing and inhibiting hormones: Thyrotrophin‐releasing hormone (TRH) Peptide Somatostatin (SS; inhibits GH) Peptide Gonadotrophin‐releasing hormone (GnRH) Peptide Corticotrophin‐releasing hormone (CRH) Peptide Growth hormone‐releasing hormone (GHRH) Peptide Dopamine (inhibits prolactin) Tyrosine derivative Anterior pituitary Thyrotrophin or thyroid‐stimulating hormone (TSH) Glycoprotein Luteinizing hormone (LH) Glycoprotein Follicle‐stimulating hormone (FSH) Glycoprotein Growth hormone (GH) (also called somatotrophin) Protein Prolactin (PRL) Protein Adrenocorticotrophic hormone (ACTH) Peptide Posterior pituitary Vasopressin [also called antidiuretic hormone (ADH)] Peptide Oxytocin Peptide Thyroid Thyroxine (T4) and tri‐iodothyronine (T3) Tyrosine derivatives Calcitonin Peptide Parathyroid Parathyroid hormone (PTH) Peptide Adrenal cortex Aldosterone Steroid Cortisol Steroid Androstenedione Steroid Dehydroepiandrosterone (DHEA) Steroid Adrenal medulla Epinephrine (also called adrenaline) Tyrosine derivative Norepinephrine (also called noradrenaline) Tyrosine derivative Stomach^† Gastrin Peptide Pancreas (islets of Langerhans) Insulin Protein Glucagon Protein Somatostatin (SS) Protein Pancreatic polypeptide Protein Ghrelin Protein Small and large intestine^† Secretin Protein Glucagon‐like peptide 1 (GLP‐1) Protein Liver Insulin‐like growth factor I (IGF‐I) Protein Ovary Oestrogens Steroid Progesterone Steroid Testis Testosterone Steroid

      ^* The distinction between peptide and protein is somewhat arbitrary. Shorter than 50 amino acids is termed a peptide in this table.

      ^† The list is far from exhaustive for the gastrointestinal tract (see Chapter 11).

      Hormone responses can be widespread or incredibly focussed according to how widely the hormone’s receptor is distributed. For instance, thyroid hormone acts on many, if not all, of the more than 200 cell types in the body. The body’s entire metabolic rate increases if it is present in excess and declines if there is a deficiency (Chapter 8). Similarly, insulin acts on most tissues; an importance underlined by its pivotal role in the survival and growth of many cell types in laboratory culture. In contrast, other hormones may act only on one tissue. For instance, thyroid‐stimulating hormone (TSH), adrenocorticotrophic hormone (ACTH) and the gonadotrophins are secreted by the anterior pituitary and regulate specific cell‐types in the thyroid gland, the adrenal cortex and the gonads, respectively (Table 1.2).

Classification of hormones

There are three major groups of hormones, defined according to their biochemical synthesis (Box 1.4). Peptide or protein hormones are synthesized like any other cellular protein. The amino acid derivatives and steroid hormones originate from a cascade of intracellular enzymatic reactions.

      Peptide hormones

      The majority of hormones are peptides and range in size from tiny, only three amino acids [thyrotrophin‐releasing hormone (TRH)], to small proteins of >200 amino acids, such as TSH or luteinizing hormone (LH). While some peptide hormones are secreted directly, a typical feature is their storage in granules, the release from which is commonly controlled by another hormone, as part of a cascade, or by innervation.

Some peptide hormones have complex tertiary structures or comprise more than one peptide chain encoded by multiple genes. Oxytocin and vasopressin, the two posterior pituitary hormones, have ring structures linked by disulphide bridges. Despite being remarkably similar in structure, they have markedly different physiological roles (Figure 1.3). Insulin consists of α‐ and β‐chains linked by disulphide bonds. Like several hormones, it is synthesized as an inactive precursor that requires modification prior to release and activity. To some extent, this regulation protects the synthesizing cell from being overwhelmed by the action of its own hormone. The gonadotrophins, follicle‐stimulating hormone (FSH) and LH, TSH and human chorionic gonadotrophin (hCG) also have two chains. However, these α‐ and β‐subunits are synthesized separately from entirely different genes. The α‐subunit is