thyrotropin-releasing hormone (TRH) A peptide
HORMONEthe HYPOTHALAMUSproduces in response
to decreased levels of the thyroid hormones in the
BLOODcirculation. TRH initiates the hormonal cas-
cade that regulates the synthesis and release of
thyroid hormones from the THYROID GLAND. TRH
stimulate the anterior lobe of the PITUITARY GLAND
to release THYROID-STIMULATING HORMONE(TSH). TSH
in turn binds with TSH receptors on the surface of
the follicular cells in the THYROID GLAND, stimulat-
ing them to produce TRIIODOTHYRONINE (T 3 ) and
THYROXINE (T 4 ), the primary thyroid hormones.
Increased levels of T 3 and T 4 in the blood circula-
tion signal the hypothalamus to “turn off” TRH
secretion.
For further discussion of TRH within the con-
text of the endocrine system’s structure and func-
tion please see the overview section “The
Endocrine System.”
See also ANTIDIURETIC HORMONE (ADH); CORTI-
COTROPIN-RELEASING HORMONE(CRH); GONADOTROPIN-
RELEASING HORMONE (GNRH); GROWTH HORMONE–
RELEASING HORMONE(GHRH).
thyroxine (T 4 ) A peptide HORMONEthe THYROID
GLANDsynthesizes from iodine and the amino acid
tyrosine, both of which enter the body from dietary
sources. The follicular cells in the thyroid gland
synthesize T 4. Thyroxine is designated T 4 because
its chemical structure contains four iodine mole-
cules (as well as two tyrosine molecules). About 80
percent of the thyroid gland’s hormone production
is T 4. T 4 travels through the blood circulation bound
to the protein carrier thyroxine-binding globulin
(TBG), which the LIVERsynthesizes. All cells in the
body have receptors for T 4 , which passes across the
cell membrane (cell wall) to bind with receptors in
the cell cytoplasm. Upon binding T 4 appears to drop
an iodine molecule to become the more potent TRI-
IODOTHYRONINE(T 3 ). In combination, T 3 and T 4 regu-
late cellular METABOLISM(the conversion of energy
within cells).
The HYPOTHALAMUS regulates the thyroid hor-
mone cascade, which it initiates by producing THY-
ROTROPIN-RELEASING HORMONE(TRH). TRH stimulates
the anterior lobe of the PITUITARY GLANDto secrete
THYROID-STIMULATING HORMONE(TSH). TSH, in turn,
stimulates the thyroid gland to synthesize and
release T 4 (as well as T 3 ). An underactive thyroid
gland produces inadequate amounts of T 3 and T 4 ,
resulting in HYPOTHYROIDISMand slowed metabo-
lism. An overactive thyroid gland produces too
much T 3 and T 4 , resulting in HYPERTHYROIDISMand
an accelerated metabolic rate. T 4 is the most com-
mon ingredient in thyroid hormone supplements.
For further discussion of T 4 within the context
of the endocrine system’s structure and function
please see the overview section “The Endocrine
System.”
See alsoTHYROIDITIS; THYROID STORM.triiodothyronine (T 3 ) A peptide HORMONE the
THYROID GLANDsynthesizes from iodine and the
amino acid tyrosine, both of which the body
acquires through dietary sources. The follicular
cells in the thyroid gland synthesize T 3. Tri-
iodothyronine is designated T 3 because its chemi-
cal structure contains three iodine molecules in
addition to two tyrosine molecules. About 20 per-
cent of the thyroid gland’s hormone production is
T 3. T 3 is about 10 times more potent than THYROX-
INE(T 4 ), the other major thyroid hormone. All
cells in the body have receptors for T 3 , which
passes across the cell membrane (cell wall) to bind
with those receptors within the cell cytoplasm. T 3
then directly influences the cell’s DNA, guiding its
production of proteins. In combination, T 3 and T 4
regulate cellular METABOLISM.
The HYPOTHALAMUSregulates the thyroid hor-
mone cascade, which it initiates by producing THY-
ROTROPIN-RELEASING HORMONE(TRH). TRH stimulates
the anterior lobe of the PITUITARY GLANDto secrete
THYROID-STIMULATING HORMONE(TSH). TSH, in turn,
stimulates the thyroid gland to synthesize and
release T 3 (as well as T 4 ). An underactive thyroid
gland produces inadequate amounts of T 3 and T 4 ,
resulting in HYPOTHYROIDISMand slowed metabo-
lism. An overactive thyroid gland produces too
much T 3 and T 4 , resulting in HYPERTHYROIDISMand
an accelerated metabolic rate. T 3 is sometimes an
ingredient in thyroid hormone supplements.
For further discussion of T 3 within the context
of the endocrine system’s structure and function
please see the overview section “The Endocrine
System.”
See also THYROIDITIS; THYROID STORM.166 The Endocrine System