• Clinical science

General endocrinology

Abstract

Endocrinology is the field of medicine concerned with endocrine tissue (e.g., the pituitary gland, thyroid gland, adrenals, testicles, and ovaries), metabolic diseases, and to an extent, nutritional medicine. Endocrine tissue is responsible for producing and secreting hormones, which influence the function of certain cells and organs. Hormone secretion is controlled by highly regulated pathways that involve cell signaling and postive or negative feedback. Disruption to these pathways can lead to an imbalance of hormones, resulting in various pathological conditions involving hyperactive or hypoactive glands (e.g., hyperthyroidism or hypothyroidism, respectively). One complex pathway is known as the hypothalamic-pituitary axis, which is the main focus in this learning card. An understanding of these hormone pathways is important for determining the next best step of management, particularly when interpreting changes in hormone levels and the results of suppression or stimulation tests.

Overview of endocrinological diseases

This learning card focuses on the hypothalamic-pituitary axis. Other important hormones and metabolic diseases are discussed in their respective learning cards.

Basics of endocrinology

Hormone

Hydrophilic hormones (e.g., catecholamines) are stored in secretory granules and are released when needed. Lipophilic hormones (e.g., adrenocortical steroid hormones) pass into the bloodstream once synthesized. They are not stored by cells.

Degradation of hormones

Feedback control mechanisms

Hormones are controlled by different feedback mechanisms. Take the simplified feedback mechanism of the antidiuretic hormone (ADH) as an example:

  1. Receptors in the hypothalamus measure plasma osmolality (measuring system).
  2. If the osmolality exceeds a set point, the neural pituitary gland excretes ADH (controlled variable).
  3. ADH increases renal reabsorption of water (process unit).
  4. Receptors in the hypothalamus detect falling osmolality and reduce ADH secretion, which decreases the amount of water reabsorption in the kidneys.

Diagnosis of endocrine diseases

The following methods may be used:

References:[1][2]

Hypothalamus and pituitary gland

Hypothalamus

Pituitary gland (hypophysis)

Overview of hypothalamic-(anterior) pituitary axis

Tropic hormones

Hypothalamus Pituitary gland Endocrine target organ
CRH (corticotropin-releasing hormone)
TRH (thyrotropin-releasing hormone)
GnRH (gonadotropin-releasing hormone)
  • Gonads
    • : Ovaries
    • : Testicles

Nontropic hormones

Hypothalamus Pituitary gland Main effect

GHRH (growth hormone-releasing hormone)

  • Stimulates growth and has an anabolic effect on the body
  • Direct effects of GH:
    • ↓ Glucose uptake into cells
    • Lipolysis
    • ↑ Protein synthesis in muscle
    • ↑ Production of IGF
Somatostatin (growth hormone–inhibiting hormone)
  • Inhibits release of GH
  • Opposes effects of GHRH
  • GI tract: suppresses release of: gastrin, cholecystokinin, secretin, VIP

MSH-RH (melanocyte-stimulating hormone-releasing hormone)

  • Skin: hyperpigmentation due to stimulation of melanocytes
Melanocyte-inhibiting hormone

TRH (thyrotropin-releasing hormone, thyroliberin)

  • Breast tissue: growth and lactation
Dopamine, also called PIH (prolactin-inhibiting hormone)

Hormones of the hypothalamus-(posterior) pituitary axis

Important diseases associated with the hypothalamus and pituitary gland

References:[3][4][5][6][7][8][9]

Adrenal cortex

The adrenal cortex consists of three distinct layers

For more information see the learning card for hormones of the adrenal cortex

Thyroid gland

Thyroid hormones

  • The thyroid gland secretes two thyroid hormones: T3 (triiodothyronine) and T4 (thyroxine, tetraiodothyronine).
  • More T4 is produced than T3 ; (ratio of T4:T3 is about 20:1) but T4 is less potent.
    • Peripheral 5'-deiodinase in the blood converts T4 into the biologically active T3.
    • T4 is therefore considered a hormonal precursor (prohormone).
    • Half of the T4 is processed into biologically inactive T3 (reverse T3).
  • The half-life of T3 is about one day (∼ 20 hours), whereas the half-life of T4 is about one week (∼ 190 hours). This longer half life makes T4 suitable for use as a depot form that can be used replacement therapy.
  • Furthermore, the C cells of the thyroid gland produce calcitonin, which regulates calcium balance.

Physiological effects of thyroid hormones

Feedback control mechanisms

Important diseases associated with the thyroid gland

References:[7][10][11][12]

Gonads

Physiological effects of LH, FSH, and sex hormones

Feedback control mechanisms

Important diseases associated with the gonads

References:[11]

Regulation of appetite