R egulation o f thyroid h o rm o n e secretio n
by th e h yp o th alam u s and p itu itary glan d • Figure 9.8
The hormones of the hypothalamus, pituitary gland, and thyroid make up a negative
feedback loop that controls thyroid hormone levels in the blood.
I Hypothalamus senses lowthyroid hormone
levels in the blood and releases TRH.
Hypothalamus
TRH acts on the anterior
pituitary to secrete TSH.
I TSH acts on the follicular
cells of the thyroid.
Thyroid
jgjp
follicle
*
What are the effects on the body when the thyroid
gland secretes too little thyroid hormone secretion?
Elevated levels of thyroid hormones in the
blood inhibit further secretions of TRH and
TSH from the hypothalamus and pituitary.
Follicular cells release thyroid
hormones into the blood.
thyroid hormone receptor, which is located on nuclear
DNA. Once T3
is bound to the receptor, new proteins get
made. T3
has the following effects on target
cells, which are most cells in the body:
It stimulates the breakdown of glucose and
fatty acids for ATP production (increasing the
basal metabolism rate).
During ATP production, cells use
more
oxygen and release more heat.
Protein synthesis increases.
thyrotropin-
releasing hormone
(TRH)
A hormone
secreted by the hypo-
thalamus that stimu-
lates the anterior pitu-
itary gland to secrete
thyroid-stimulating
hormone (TSH)
Thyroid hormones increase the excretion of cholester-
ol, thereby reducing its level in the blood. Together with
insulin and hGH, thyroid hormones stimulate
body growth.
The
hypothalamus
and
anterior
pitu-
itary gland regulate the secretions of thyroid
hormones by using a negative-feedback loop
involving
thyrotropin-releasing
hormone
(TRH)
, thyroid-stimulating hormone
(TSH),
and thyroxine (Figure 9.8).
Parafollicular cells of the thyroid secrete
calcitonin, which inhibits a type of bone cell
Endocrine Glands Regulate Key Body Functions
263
PROCESS DIAGRAM
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