1.2 The endocrine organs and their hormones*
| Gland | Hormone | Molecular characteristics |
|---|---|---|
| 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
