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Stimulating hormones

The pituitary gland is one of the main producers of stimulating hormones which are used to a target an endocrine gland into producing specific hormones and responses.

Structure

The pituitary gland is located immediately inferior to the hypothalamus and is connected to this structure via the hypophyseal stalk. The Pituitary gland has two embryologically distinct segments, the anterior pituitary and the posterior pituitary. The Posterior Pituitary gland secretes two hormones; Anti-Diuretic Hormone (ADH) and Oxytocin. The Anterior pituitary gland produces 7 stimulating hormones in response to hypothalamic regulatory hormones secreted from the hypothalamus.

Anterior Pituitary Hormones

Growth Hormone (GH)
Thyroid Stimulating Hormone (TSH)
Adrenocorticotropic Hormone (ACTH)
Prolactin
Follicle Stimulating Hormone (FSH)
Luteinising Hormone (LH)
Melanocyte Stimulating Hormone (MSH)

Anterior Pituitary Hormone Functions

The pituitary gland works in partnership with the hypothalamus via the hypothalamic-pituitary axis. As outlined in the summary table below hormones released from the hypothalamus will stimulate the secretion of subsequent stimulating hormones from the pituitary gland which will activate target endocrine glands throughout the body.


Hypothalamic hormone	Anterior Pituitary hormone	Target Gland	Effect
Growth Hormone Releasing Hormone (GHRH)	Growth Hormone (GH)	Bones and Muscles	Regulates metabolism and promotes tissue growth
Growth Hormone Inhibiting Hormone (GHIH)	Growth Hormone (GH)	Pituitary gland	Inhibits release of GH from pituitary gland
Thyroid Releasing Hormone (TRH)	Thyroid Stimulating Hormone (TSH)	Thyroid gland	Stimulates growth and activity of the thyroid gland and secretion of T3 and T4
Corticotropin Releasing Hormone (CRH)	Adrenocorticotropic Hormone (ACTH)	Adrenal cortex	stimulates adrenal cortex to secrete glucocorticoids
Prolactin Releasing Hormone (PRL)	Prolactin	Breasts	Stimulates the production of milk
Prolactin Inhibiting Hormone (PIH)	Prolactin	Pituitary gland	Inhibits the release of Prolactin from pituitary gland
Gonadotropin Releasing Hormone (GnRH)	Follicle Stimulating Hormone (FSH) Luteinising Hormone (LH)	Ovaries in females and testes in males	In females it stimulates the secretion of progesterone and oestrogen In males it stimulates the secretion of testosterone and production of sperm by the testes
	Melanocyte stimulating Hormone (MSH)	Skin	pigmentation of skin

Growth Hormone

Also known as somatotrophin is regulated by two hypothalamic hormones; Growth Hormone Releasing Hormone and Growth Hormone Inhibiting Hormone. Growth Hormone is a major anabolic hormone which stimulates cell division and growth in both bony and soft tissues throughout the body^[1]. Growth Hormone is of particular importance during childhood and adolescents. Although growth hormone is unable to continue stimulating long bone growth after the fusion of the epiphyses, it remains to have critical metabolic functions throughout life such as protein synthesis and fat breakdown for energy ^[1] .

Low blood glucose levels stimulates secretion of growth hormone from both the hypothalamus and the δ cells of pancreatic islets in order to rise glucose concentrations towards normal.

Thyroid stimulating hormone

Acts on the thyroid gland to produce thyroid hormones including T3 (triiodothyronine) and T4 (thyroxine). T3 and T4 have both metabolic and synthesis effects on various organs of the body such as;

Metabolic effects

increased oxygen consumption
increased BMR (basal metabolic rate)
carbohydrate metabolism through increasing insulin secretion and gluconeogenesis
lipid metabolism through lipolysis
protein metabolism
Thyroid Gland

Synthesis effects

increase heart rate and cardiac output
increase ventilation rate
increase appetite

Adrenocorticotrophic hormone

Acts on the adrenal gland to product 3 steroid hormones;

Aldosterone acts primarily on the distal convoluted tubule and the collecting duct of the kidneys, causing the reabsorption of sodium ions in exchange for potassium and hydrogen ions ^[2]. Water is also reabsorbed and these mechanisms control blood volume.

Cortisol increases blood glucose levels and the catabolism of fats and proteins. At the same time it also has a role in decreasing both the immune and inflammatory responses. Cortisol secretion displays a circadian rhythm, meaning the rate of secretion varies throughout the day, with the highest levels being in the morning, reaching a peak around 6am and then proceeding to fall during the day ^[2].

Androgens are hormones involved in the growth and development of the male and female genital tracts. They also stimulate muscle growth hence their use as illicit drugs in sports ^[2].

Follicle Stimulating Hormone and Luteinising Hormone

These are primarily hormones of the reproductive system. In females they stimulate the secretion of oestrogen by the ovaries, maturation of the ovarian follicles and secretion of progesterone from the corpus luteum.

In males they stimulate the secretion of testosterone and production of sperm in the testes.

Prolactin

Prolactin is mainly released during pregnancy to favour the development of the breasts ready for lactation and milk production, as well as part of the suckling reflex during breast feeding.

Melanocyte stimulating hormone

stimulates the synthesis of the dark pigment melanin in melanocyte cells in the skin and hair. Melanin is responsible for giving hair and skin pigmentation.

Posterior Pituitary Hormone Functions

The posterior pituitary produces two hormones; Anti-diuretic hormone and oxytocin . In contrast to the anterior pituitary hormones, the posterior pituitary hormones are stored. The posterior pituitary hormones are packaged into secretory vesicles, travel down the axon and are stored in Herring Bodies ^[3].

Anti-diuretic hormone secretion induces vasoconstriction and reabsorption of water from the collecting duct of the kidney to maintain equilibrium both intracellularly and extracellularly ^[3].

Oxytocin is excreted by females during childbirth with the purpose of allowing the uterus to contract and the release of milk from the breasts ^[3]. In males the hormone is secreted during ejaculation.

Clinical Diseases

Both hyper-secretion (increased production) and hypo-secretion (decreased production) of any of theses hormones from the pituitary gland can result in disease.

Hyper-secretion disorders

SIADH (syndrome of Inappropriate Anti-diuretic Hormone release) - $\uparrow$ Anti-diuretic hormone
Oxytocin toxicity - $\uparrow$ oxytocin
Acromegaly and gigantism - $\uparrow$ Growth Hormone
Hyperthyroidism - $\uparrow$ Thyroid Stimulating Hormone
Hyperprolactinaemia - $\uparrow$ Prolactin
Cushings' Syndrome - $\uparrow$ Adrenocorticotrophic hormone

Hypo-secretion disorders

Diabetes Inspidius - $\downarrow$ Anti-diuretic hormone
Oxytocin Insufficiency - $\downarrow$ Oxytocin
Hypothyroidism - $\downarrow$ Thyroid Stimulating Hormone
Addison's Disease - $\downarrow$ Adrenocorticotrophic Hormone
Dwarfism - $\downarrow$ Growth hormone
Infertility - $\downarrow$ Follicle Stimulating Hormone and Luteinising Hormone

SIADH

Syndrome of Inappropriate Anti-diuretic Hormone release. Increased production results in excessive water retention and hypervolaemic hyponatraemia ^[4]. Patients will also present with nausea, vomiting, headaches, restlessness and muscle weakness ^[5].

Oxytocin Toxicity

Results in an overactive uterus causing hypertrophy and limiting pregnancy due to insufficient space to hold the fetus ^[4].

Acromegaly and Gigantism

Children and adolescents with this disorder present with rapid, abnormal increase in height and rapid abnormal weight gain^[5]. Other features include increased large hands and feet, macrocephaly, excessive sweating and coarsening of the facial skin ^[5].

Hyperthyroidism

Patients with hyperthyroidism suffer the results of a thyroid in an overactive state and so present with;

increased appetite despite a decrease in weight.
tachycardia, sweating and palpitations
diarrhoea and alopecia
anxiety and restlessness

Hyperprolactinaemia

Patients with hyperprolactinaemia present in a variety of ways depending if they are a male, a pre-menopausal or post-menopausal woman ^[5]. Pre-menopausal women will present with; infertility, headaches, oligomenorrhea and galactorrhea ^[5]. Men with hyperprolactinaemia have hypogonadotropic hypogonadism and so similarly present with; infertility, decreased libido, impotence and gynecomastia ^[5].

Cushing's Syndrome

Patients with Cushing's Syndrome present with a number of characteristic features such as a buffalo hump, moon face, easy bruising, stretch marks, glucose intolerance and growth retardation.

Diabetes Inspidius

This is the passage of large volumes (3L per day) of dilute urine, due to impaired water resorption by the kidneys ^[6].Patients present with polydipsia, polyuria and nocturia.

Oxytocin Insufficiency

Decreased levels of oxytocin results in a halt in uterine contractions and milk ejection during the process of giving birth ^[4].

Hypothyroidism

Patients of hypothyroid present with symptoms resulting from an under active state of the thyroid gland;

Bradycardia
decreased appetite but an increase in weight
constipation
depression
dry and cold skin

Addisons Disease

Patients with Addison's disease characteristically present with chronic, worsening fatigue, weight loss and hyperpigmentation of the skin which is most visible on skin folds, elbows, knees, knuckles and lips ^[7]. Due to the loss of salt patients also tend to crave salty foods ^[7].

Dwarfism

Dwarfism is defined as an adult height 4 feet 10 inches or under as result of a medical or genetic condition ^[8] . Characteristics of dwarfism include; a large head with a prominent forehead, flattened bridge of the nose, protruding jaw, crowded and misaligned teeth, forward curvature of the lower spine, bowed legs and 'double-jointedness' ^[8].

References

^ ^a ^b McGEOWN, J G (2007). Physiology. Eslevier limited. p. 247.
^ ^a ^b ^c Murphy, Richard (July 2012). Crash Course Endocrinology E-Book. Mosby. p. 52. ISBN 9780723437703.{{cite book}}: CS1 maint: year (link)
^ ^a ^b ^c Patel, Hiran; Tiwari, Vivekanand (2018), "Physiology, Posterior Pituitary", StatPearls, StatPearls Publishing, PMID 30252386, retrieved 2019-01-22
^ ^a ^b ^c Patel, Hiran; Tiwari, Vivekanand (2018), "Physiology, Posterior Pituitary", StatPearls, StatPearls Publishing, PMID 30252386, retrieved 2019-01-28
^ ^a ^b ^c ^d ^e ^f Shahid, Zainab; Singh, Gurdeep (2018), "Physiology, Hypothalamus", StatPearls, StatPearls Publishing, PMID 30571001, retrieved 2019-01-28
^ Wilkinsom, Ian B. (2017). Oxford handbook of Clinical Medicine. OXFORD UNIVERSITY PRESS. p. 240. ISBN 978-0-19-968990-3.
^ ^a ^b "Adrenal Insufficiency (Addison's Disease) | Disorders | Knowledge Base". Pituitary Network Association. Retrieved 2019-01-28.
^ ^a ^b "Dwarfism: Types, Causes, Treatments, and More". WebMD. Retrieved 2019-01-28.

[:0-1] McGEOWN, J G (2007). Physiology. Eslevier limited. p. 247.

[:1-2] Murphy, Richard (July 2012). Crash Course Endocrinology E-Book. Mosby. p. 52. ISBN 9780723437703.{{cite book}}: CS1 maint: year (link)

[:2-3] Patel, Hiran; Tiwari, Vivekanand (2018), "Physiology, Posterior Pituitary", StatPearls, StatPearls Publishing, PMID 30252386, retrieved 2019-01-22

[:3-4] Patel, Hiran; Tiwari, Vivekanand (2018), "Physiology, Posterior Pituitary", StatPearls, StatPearls Publishing, PMID 30252386, retrieved 2019-01-28

[:4-5] ^ ^a ^b ^c ^d ^e ^f Shahid, Zainab; Singh, Gurdeep (2018), "Physiology, Hypothalamus", StatPearls, StatPearls Publishing, PMID 30571001, retrieved 2019-01-28

[6] Wilkinsom, Ian B. (2017). Oxford handbook of Clinical Medicine. OXFORD UNIVERSITY PRESS. p. 240. ISBN 978-0-19-968990-3.

[:5-7] "Adrenal Insufficiency (Addison's Disease) | Disorders | Knowledge Base". Pituitary Network Association. Retrieved 2019-01-28.

[:6-8] "Dwarfism: Types, Causes, Treatments, and More". WebMD. Retrieved 2019-01-28.

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