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General endocrinology

Last updated: June 4, 2021

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Endocrinology is the study of the endocrine system (i.e., the hypothalamus, pituitary gland, thyroid gland, adrenals, and gonads), metabolic diseases, and certain aspects of nutritional medicine. The endocrine glands are responsible for producing and secreting hormones, which influence the function of cells in certain tissues of the body. Hormone secretion is controlled by highly regulated pathways, the most important of which is the hypothalamic-pituitary axis. The hypothalamus secretes and stores nontropic hormones (e.g., ADH, oxytocin) and releasing hormones (e.g., TRH, CRH, GnRH). The pituitary gland is composed of the anterior pituitary, which secretes tropic hormones (e.g., ACTH, TSH, FSH, LH) and whose function is controlled by hypothalamic releasing hormones, and the posterior pituitary, which stores ADH and oxytocin. The gonads are the ovaries in female individuals and testicles in male individuals. Their function is controlled via the hypothalamic-pituitary-gonadal axis, as well as the secretion of sex hormone-binding globulin (SHBG). Disruption of the hypothalamic-pituitary axis can result in the development of various endocrine disorders, which are classified according to the level of pathway disruption: primary (disorders of the peripheral endocrine gland), secondary (pituitary dysfunctions), and tertiary (hypothalamic disorders). An understanding of these hormone pathways is important for the diagnosis and management of endocrine disorders, particularly when interpreting changes in hormone levels and the results of suppression and/or stimulation tests.

For more information, see the articles "Thyroid gland and parathyroid glands" and “Adrenal gland.”

This article focuses on the hypothalamic-pituitary axis. Other important structures, hormones, and metabolic diseases are discussed in their respective articles.

Hormones

Overview of the most important types of hormones
Type of hormone Description Example
Based on signaling pathways
Paracrine hormones
Autocrine hormones
  • Affect the secreting cell itself
Endocrine hormones
  • Secreted into the bloodstream to reach their targets
Based on chemical nature
Steroid hormones
Amine hormones
Peptide/protein hormones
Based on solubility
Lipophilic hormones
Hydrophilic hormones

Hydrophilic hormones (e.g., catecholamines) are stored in secretory granules and released when needed. Lipophilic hormones (e.g., adrenocortical steroid hormones) pass into the bloodstream once synthesized without being stored in cells.

Feedback control mechanisms

Hormone secretion is controlled by the following feedback mechanisms: [2]

Diagnosis of endocrine diseases

Hypothalamus

Pituitary gland (hypophysis)

B-FLAT”: Basophils secrete FSH, LH, ACTH, and TSH.

PiG on Acid”: Prolactin and GH are secreted by Acidophils.

Hypothalamus and anterior pituitary

Tropic hormones

Tropic hypothalamic hormones and their effects
Axis Hypothalamus Pituitary gland Endocrine target organ
Hypothalamic-pituitary-adrenal axis
  • CRH
    • Stimulates ACTH, MSH, and β-endorphin secretion from proopiomelanocortin (POMC) precursor
    • CRH levels decrease after long-term steroid treatment via negative feedback.
Hypothalamic-pituitary-thyroid axis
Hypothalamic-pituitary-gonadal axis
  • Gonadotropins
    • LH
    • FSH: stimulates the maturation of germ cells in both male and female individuals

Nontropic hormones

Nontropic hypothalamic hormones and their effects
Axis Hypothalamus Pituitary gland
Hypothalamic-pituitary-somatotropic axis
Hypothalamic-pituitary-prolactin axis
Hypothalamic-melanocortin system
  • Melanocyte-inhibiting hormone: inhibits release of MSH

“No PRO-BLAM:” Derivatives of PROopiomelanocortin are Beta-endorphin, ACTH, and MSH.

Hypothalamus and posterior pituitary

Hormones of the posterior pituitary gland
Hormone Regulation [9] Main effects Clinical relevance
Antidiuretic hormone
  • Elevated in SIADH despite plasma hypoosmolality
  • ADH deficiency or resistance can lead to diabetes insipidus.
  • Alcohol inhibits ADH release and causes diuresis. [12]
Oxytocin
  • Involved in the neuromodulation of social and reproductive behavior, fear, anxiety, and depression

Overview of hypothalamic and pituitary drugs
Drug class Examples Mechanism of action Indications Side effects
GnRH agonists
GnRH antagonists
  • Degarelix
Somatostatin analogs
GHRH analogs [15]
  • Tesamorelin
  • Stimulate GH receptors
GH receptor antagonists
  • Pegvisomant
  • Block GH receptor
  • Hepatotoxicity
Dopamine agonists

ADH antagonists [17][18]

ADH analogs
Oxytocin
  • Mediates calcium influx → uterine contraction

Overview

Physiological effects of LH and FSH

Hormone Target gonad
Ovary Testicle
FSH
LH

Physiological effects of sex hormones

Female sex hormones

Male sex hormones

Feedback control mechanisms

Overview of reproductive system drugs
Drug class Examples Mechanism of action Indications Side effects
Selective estrogen receptor modulators (SERMs)
  • Tamoxifen
  • Hot flashes
  • Multiple simultaneous pregnancies
  • Visual disturbances [23]
Synthetic estrogens [24]
  • Ethinyl estradiol
  • Mestranol
  • Diethylstilbestrol (DES)
Aromatase inhibitors
  • Anastrozole
  • Letrozole
  • Exemestane
Androgen agonists
Antiandrogens
  • Steroid binding; achieved via nonsteroidal competitive inhibition at androgen receptors
  • Cyproterone
Anabolic steroids [24]
  • Methyltestosterone
  • Oxandrolone
  • Fluoxymesterone
Progestins [24]
Antiprogestins [24]
  • Mifepristone
  • Ulipristal

Overview [31][32][33]

Regulation of appetite

Neuroendocrine regulation of appetite
Neurotransmitter/hormone Regulation Site of production Effects
Ghrelin [31][33]
  • ↑ Appetite
  • GH release (via GH secretagogue receptor)
Endocannabinoid
  • Hunger [34]
  • Intake of fatty and sweet food
  • Unclear [35]
Neuropeptide Y
  • Hunger
  • ↑ Appetite
  • Regulation of anxiety-related behavior
  • Increased neuronal excitability

Regulation of satiety [31][33]

Neuroendocrine regulation of satiety
Hormone Regulation Source Effects
Leptin
Cholecystokinin
  • ↓ Appetite (short term)
  • ↓ Gastric emptying
GLP-1
Peptide YY
  • ↓ Appetite (short term)
  • ↓ Gastric emptying
Amylin [37]
  • Glucose (cosecreted with insulin)
  • ↓ Appetite (short term)
  • ↓ Gastric emptying
  • Glucagon release

Ghrelin makes you Gain weight. Leptin makes you Lose weight.

Appetite regulators

Appetite-regulating drugs
Drug class Example Mechanism of action Indication Side effects

Cannabinoid receptor agonist [38][39]

  • Activation of CB1 and CB2 receptors
Synthetic progestin
  • Unclear [40]
GLP-1 agonist [43]
  • Activation of GLP-1 receptor
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