C H A P T E R 3 4
Introduction to the endocrine system
517
■■
The pituitary gland has three lobes: the anterior
lobe produces stimulating hormones in response to
hypothalamic stimulation.
■■
The posterior lobe of the pituitary stores ADH and
oxytocin, which are two hormones produced by the
hypothalamus.
■■
The intermediate lobe of the pituitary produces
endorphins and encephalins to modulate pain
perception.
ENDOCRINE REGULATION
The production and release of hormones needs to be
tightly regulated within the body. Hormones are released
in small amounts to accomplish what needs to be done
to maintain homeostasis within the body. The fine
tuning and regulation of hormone release through the
hypothalamus is often regulated by a series of negative
feedback systems. Other hormones are not controlled in
this fashion but respond to other direct stimuli.
Hypothalamic–pituitary axis
Because of its position in the brain, the hypothalamus
is stimulated by many things, such as light, emotion,
cerebral cortex activity and a variety of chemical and
hormonal stimuli. Together, the hypothalamus and the
pituitary function closely to maintain endocrine activity
along what is called the
hypothalamic–pituitary axis
(HPA)
using a series of negative feedback systems.
A
negative feedback system
works much like the law
of supply and demand in business. In business, when
there is an adequate supply of a product, production of
that product will slow down because there is an adequate
supply and no current demand for it. When the supply
is used up, demand will increase, and so production will
pick up. Production continues until the supply is adequate
and demand is reduced. When the hypothalamus senses
a need for a particular hormone, for example, thyroid
hormone, it secretes the releasing factor TRH directly
into the anterior pituitary. In response to the TRH, the
anterior pituitary secretes TSH, which in turn stimu
lates the thyroid gland to produce thyroid hormone.
When the hypothalamus senses the rising levels of
thyroid hormone, it stops secreting TRH, resulting in
decreased TSH production and subsequent reduced
thyroid hormone levels. The hypothalamus, sensing
the falling thyroid hormone levels, secretes TRH again.
The negative feedback system continues in this fashion,
maintaining the levels of thyroid hormone within a rela
tively narrow range of normal (see Figure 34.2).
It is thought that this feedback system is more
complex than once believed. The hypothalamus
probably also senses TRH and TSH levels and regulates
KEY POINTS
TRH secretion within a narrow range, even if thyroid
hormone is not produced. The anterior pituitary may
also be sensitive to TSH levels and thyroid hormone,
regulating its own production of TSH. This complex
system provides backup controls and regulation if any
part of the HPA fails. This system can also create com
plications, especially when there is a need to override
or interact with the total system, as is the case with
hormone replacement therapy or the treatment of endo
crine disorders. Supplying an exogenous hormone, for
example, may increase the hormone levels in the body,
but then may affect the HPA to stop production of releas
ing and stimulating hormones, leading to a decrease in
the body’s normal production of the hormone.
Two of the anterior pituitary hormones (i.e. growth
hormone and prolactin) do not have a target organ to
produce hormones and so cannot be regulated by the
same type of feedback mechanism. The hypothalamus
in this case responds directly to rising levels of growth
hormone and prolactin. When levels rise, the hypothal
amus releases the inhibiting factors somatostatin and
PIF directly to inhibit the pituitary’s release of growth
hormone and prolactin, respectively. The HPA func
tions through negative feedback loops or the direct use
of inhibiting factors to constantly keep these hormones
regulated.
Other forms of regulation
Hormones other than stimulating hormones are also
released in response to stimuli. For example, the
pancreas produces and releases insulin, glucagon and
somatostatin from different cells in response to varying
blood glucose levels. The parathyroid glands release
parathyroid hormone, or parathormone, in response
to local calcium levels. The juxtaglomerular cells in
the kidney release erythropoietin and renin in response
to decreased pressure or decreased oxygenation of the
blood flowing into the glomerulus. GI hormones are
released in response to local stimuli in areas of the GI
tract, such as acid, proteins or calcium. The thyroid
gland produces and secretes another hormone, called
Hypothalamus
Anterior pituitary
Thyroid gland
Thyroid hormones
TRH
TSH
Levels of TRH influence
hypothalamus activity
Levels of TSH influence
anterior pituitary and
hypothalamus activity
Levels of hormone regulate
hypothalamus and anterior
pituitary activity
FIGURE 34.2
Negative feedback system. Thyroid hormone levels
are regulated by a series of negative feedback systems influencing
thyrotropin-releasing hormone (TRH), thyrotropin (TSH) and
thyroid hormone levels.
