Encyclopedia of Health and Medicine

166 The Endocrine System
thyrotropin-releasing hormone (TRH) A peptide
HORMONE the HYPOTHALAMUS produces in response
to decreased levels of the thyroid hormones in the
BLOOD circulation. TRH initiates the hormonal cascade
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, stimulating
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 circulation
signal the hypothalamus to “turn off” TRH
secretion.
For further discussion of TRH within the context
of the endocrine system’s structure and function
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 HORMONE the THYROID
GLAND synthesizes 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 molecules
(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 LIVER synthesizes. 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 regulate
cellular METABOLISM (the conversion of energy
within cells).
The HYPOTHALAMUS regulates the thyroid hormone
cascade, which it initiates by producing THY-
ROTROPIN-RELEASING HORMONE (TRH). TRH stimulates
the anterior lobe of the PITUITARY GLAND to 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 HYPOTHYROIDISM and slowed metabolism.
An overactive thyroid gland produces too
much T 3 and T 4 , resulting in HYPERTHYROIDISM and
an accelerated metabolic rate. T 4 is the most common
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 also THYROIDITIS; THYROID STORM.
triiodothyronine (T 3 ) A peptide HORMONE the
THYROID GLAND synthesizes 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 . Triiodothyronine
is designated T 3 because its chemical
structure contains three iodine molecules in
addition to two tyrosine molecules. About 20 percent
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 HYPOTHALAMUS regulates the thyroid hormone
cascade, which it initiates by producing THY-
ROTROPIN-RELEASING HORMONE (TRH). TRH stimulates
the anterior lobe of the PITUITARY GLAND to 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 HYPOTHYROIDISM and slowed metabolism.
An overactive thyroid gland produces too
much T 3 and T 4 , resulting in HYPERTHYROIDISM and
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.