4. pituitary gland

PITUITARY GLAND
PHYSIOLOGY: ENDOCRINE

ANTERIOR PITUITARY GLAND
OVERVIEW
‣ The pituitary gland acts as a higher level of control in
endocrine physiology, regulating the function of several
other endocrine organs
‣ The pituitary gland consists of two lobes: the anterior and
posterior pituitary
‣ Differing in embryological origin and histology, these
effectively function as two separate endocrine glands

https://teachmephysiology.com/endocrine-system/pituitary-gland/anterior-pituitary-gland/
Anterior Pituitary Posterior Pituitary
Growth Hormone (GH) Antidiuretic Hormone (ADH)
Thyroid Stimulating Hormone (TSH) Oxytocin
Follicle Stimulating Hormone (FSH)
Luteinising Hormone (LH)
Adrenocorticotrophic Hormone (ACTH)
Prolactin (PRL)

OVERVIEW OF AXES
‣ The hypothalamus releases hormones, which stimulate or inhibit the secretion of anterior
pituitary hormones
‣ The anterior pituitary hormones have a subsequent trophic (“feeding”) effect on a further
endocrine organ in the body, stimulating it to release a final hormone in the pathway, known as
the peripheral hormone
‣ This peripheral hormone travels in the blood stream to exert metabolic actions at a variety of
tissues in the body
‣ The rise in levels of this peripheral hormone will usually have a positive and/or negative
feedback effect on the pituitary and hypothalamus
‣ i.e. will stimulate or inhibit the further release of the respective hypothalamic and/or
pituitary hormones
‣ This circuit between the hypothalamus, anterior pituitary and an endocrine gland is known as
an axis

https://teachmephysiology.com/endocrine-system/pituitary-gland/anterior-pituitary-gland/

OVERVIEW OF AXES
‣ Note that in some cases, the anterior pituitary hormone itself may be the peripheral hormone in the
pathway or may act on tissues in addition to stimulating a target endocrine gland to release the
peripheral hormone
‣ There are 5 anterior pituitary axes summarized below:
‣ Hypothalamic-Pituitary-Adrenal Axis 
Involving Adrenocorticotrophic Hormone (ACTH)
‣ Growth Hormone Axis 
Involving Growth Hormone (GH)
‣ Hypothalamic-Pituitary-Thyroid Axis 
Involving Thyroid Stimulating Hormone (TSH)
‣ Hypothalamic-Pituitary-Gonadal Axis 
Involving Follicle Stimulating Hormone (FSH) and Luteinizing Hormone (LH)
‣ Prolactin Axis 
Involving Prolactin (PRL)

GROWTH HORMONE AXIS
‣ The hypothalamus secretes Growth Hormone Releasing Hormone (GHRH)
‣ GHRH stimulates somatotroph cells of the anterior pituitary to release Growth Hormone (GH),
also known as somatotropin
‣ To exert several metabolic effects in many tissues of the body GH has
‣ direct actions on tissues by binding to cells
‣ indirect effects whereby it stimulates the liver to produce Insulin-like Growth
Factor-1 (IGF-1) to mediate actions
‣ The direct actions of GH include:
‣ Increased lipolysis
‣ Reduced glucose oxidation
‣ Reduced protein oxidation

‣ The effects of GH via IGF-1 can be thought of as “anabolic” (compound building) like
insulin and include:
‣ Increases protein synthesis
‣ Increases carbohydrate oxidation
‣ The overall effects, arising from an interplay between the two substances, are
important for the following:
‣ Skeletal growth
‣ Muscle strength
‣ Bone density
‣ Cardiac function

REGULATION
‣ Several factors including stress, exercise, nutrition and
sleep modulate the production of growth hormone
‣ GHRH promotes GH release
‣ Somatostatin, produced by the hypothalamus, the intestines
and delta cells of the endocrine pancreas, inhibits GH
‣ There is also inhibition of GH production by IGF-1, which
prevents somatotroph releasing GH and promotes
somatostatin release from the hypothalamus

HYPOTHALAMIC-PITUITARY-THYROID AXIS
‣ The hypothalamus produces Thyrotrophin Releasing Hormone (TRH)
‣ TRH stimulates thryotropic cells in the anterior pituitary to produce Thyroid Stimulating
Hormone (TSH)
‣ TSH is released in low amplitude pulses, following a circadian rhythm
‣ In this case, there are higher levels at night and lower levels in the morning
‣ TSH binds to receptors on follicular cells of the thyroid gland, stimulating the production of
thyroid hormones: Tri-iodothyronine (T3) and Tetra-iodothyronine (T4), also known as Thyroxine
‣ Control of this system is via negative feedback
‣ High levels of T3 and T4 inhibit
‣ TRH production by the hypothalamus
‣ TSH production by the anterior pituitary gland

HYPOTHALAMIC-PITUITARY-THYROID AXIS
‣ The actions of T3 and T4 are widespread
‣ Some important functions include:
‣ Metabolic – increasing basal metabolic rate and promoting
catabolism e.g. lipolysis, glycogenolysis, glycolysis and proteolysis
‣ Nervous system – important for speed of reflexes and mental
activity
‣ Cardiovascular system – increases synthesis of cardiac muscle
protein, increases cardiac output
‣ Bone – increases bone mineralization

HYPOTHALAMIC-PITUITARY-GONADAL AXIS
‣ The Hypothalamic pituitary gonadal axis is fundamental to the control of the
reproductive system
‣ Gonadotropin Releasing Hormone (GnRH) is released in a pulsatile fashion
from the hypothalamus
‣ This stimulates gonadotroph cells of the anterior pituitary to produce two
hormones equally:
‣ Luteinising Hormone (LH)
‣ Follicle Stimulating Hormone (FSH)
‣ These then act on the gonads (testes/ovaries), and have different functions in
males and females

IN MALES
‣ LH stimulates the testes to produce testosterone, the main male
sex steroid hormone
‣ Testosterone has systemic effects to produce male secondary
sex characteristics:
‣ Growth of pubic hair, axillary hair and facial hair
‣ Growth of external genitalia
‣ Deepening of voice
‣ Muscle growth

IN MALES
‣ In addition, testosterone helps to maintain libido (sexual drive) and promotes
anabolic reactions
‣ Within the testes, testosterone acts to aid the formation of
sperm (spermatogenesis)
‣ Testosterone has a negative feedback effect on the hypothalamus to inhibit
GnRH production
‣ FSH drives sperm production at the testes (spermatogenesis), as well as
stimulating the testes to produce the hormone inhibin
‣ Inhibin has a selective negative feedback effect
‣ inhibits FSH production, but does not inhibit LH production

IN FEMALES
‣ LH stimulates the ovaries to produce androgens. FSH stimulates:
‣ Follicle growth
‣ Conversion of androgens to oestrogen
‣ Release of the hormone Inhibin from the ovaries
‣ Estrogen at moderate concentrations has a negative feedback
effect on LH and FSH secretion
‣ Estrogen alone (in the absence of progesterone) at high
concentrations promotes LH and FSH secretion

IN FEMALES
‣ Inhibin, as in males, has a selective negative feedback
effect on FSH only and not LH
‣ Progesterone is a sex steroid released by the corpus
luteum, which is the remain of the follicle after ovulation
‣ Progesterone increases the inhibitory effect of moderate
oestrogen concentration
‣ Progesterone prevents the positive feedback of a high
oestrogen concentration

PROLACTIN AXIS
‣ Prolactin (PRL) is produced by the lactotroph cells of the anterior pituitary
gland
‣ Its pulsatile secretion follows a circadian rhythm with a nocturnal peak
during sleep and a second (but lesser) peak in the evening
‣ The main action of prolactin is to initiate and maintain lactation (milk
production/secretion) of breast tissue
‣ Prolactin is under inhibition by dopamine (DA), a neurotransmitter
released from the hypothalamus
‣ Note that the prolactin axis is the only axis where a hypothalamic hormone
inhibits the release of the pituitary hormone instead of stimulating it

PROLACTIN AXIS
‣ Thyrotropin Releasing Hormone (TRH), also from the
hypothalamus, stimulates the release of prolactin but this is only
a minor positive feedback effect
‣ Estrogens also have a positive feedback effect on Prolactin,
stimulating its release
‣ Prolactin has a negative feedback effect on GnRH at the
hypothalamus, inhibiting its release
‣ Prolactin blocks the action of LH on the gonads (ovaries and
testes)

REVIEW QUESTIONS
‣ Which of the following is NOT a hormone released from
the Anterior Pituitary gland?
‣ Anti-diuretic hormone (ADH)
‣ Adrenocorticotrophic hormone (ACTH)
‣ Prolactin
‣ Growth hormone

REVIEW QUESTIONS
‣ Anti-diuretic is one of the two hormones released from the
posterior pituitary, which is histologically and
embryologically completely different to its anterior twin.
The other hormone is oxytocin.

REVIEW QUESTIONS
‣ Which of the following would result in a DECREASE in Growth
Hormone release from the anterior pituitary?
‣ Increased Growth Hormone Releasing Factor release from
hypothalamus
‣ Decreased Somatostatin release from hypothalamus and GI
system
‣ Increased Somatostatin release from hypothalamus and GI
system
‣ Complete surgical removal of tumour producing ectopic GH

REVIEW QUESTIONS
‣ Somatostatin inhibits the release of Growth Hormone

REVIEW QUESTIONS
‣ If T3 and T4 blood concentrations are high, and TRH
concentrations are low, where is the problem likely to be?
‣ Hypothalamus
‣ Anterior Pituitary
‣ Thyroid Gland
‣ More information is needed

REVIEW QUESTIONS
‣ The problem may be in the thyroid, or in the ant. pituitary.
We require the TSH concentration. If low, the problem is
the thyroid. If high, it may be the pituitary.

REVIEW QUESTIONS
‣ Women who have undergone a hysterectomy cannot
produce progesterone, because it is synthesised by the…?
‣ Uterus
‣ Ova
‣ Corpus luteum
‣ Oviduct

REVIEW QUESTIONS
‣ Progesterone is released by the corpus luteum - the
remains of the follicle after the ovum has ovulated.

REVIEW QUESTIONS
‣ What is unique about the prolactin axis compared to the
other pituitary axes?
‣ It is only ever found in females
‣ It is the only axis that is circadian in rhythm
‣ Its releasing hormone inhibits the pituitary hormone
‣ There are no known cancers affecting the axis

REVIEW QUESTIONS
‣ Dopamine, released from the hypothalamus, inhibits
prolactin.

References
These slide reflect a summary of the contents of
TeachMePhysiology.com and are to be used for
educational purposes only in compliance with the
terms of use policy.
Specific portions referenced in this summary are as
follows:
‣ https://teachmephysiology.com/endocrine-system/pituitary-gland/
anterior-pituitary-gland/
Additional sources are referenced on the slide
containing that specific content.

4. pituitary gland

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4. pituitary gland