500
UNIT THREE
ronine (also called T
4
because it includes four atoms of
iodine), and
triiodothyronine
(tri
i-o
do-thi
ro-ne
¯n), also
called T
3
because it includes three atoms of iodine. These
hormones help regulate the metabolism of carbohydrates,
lipids, and proteins. SpeciF cally, thyroxine and triiodothy-
ronine increase the rate at which cells release energy from
carbohydrates, enhance the rate of protein synthesis, and
stimulate breakdown and mobilization of lipids. These hor-
mones are the major factors determining how many calories
the body must consume at rest to maintain life, measured
as the
basal metabolic rate
(BMR). They are essential for
normal growth and development and for maturation of the
nervous system
(f g. 13.20)
. TSH from the anterior pituitary
gland controls levels of thyroid hormones.
±ollicular cells require iodine salts (iodides) to produce
thyroxine and triiodothyronine. Such salts are normally
obtained from foods, and after they have been absorbed from
the intestine, the blood carries some of them in the form of
iodide (I
) to the thyroid gland. An efF cient active transport
protein called the
iodide pump
moves the iodides into the
follicular cells, where they are converted to iodine and con-
centrated. The iodine, with the amino acid tyrosine, is used
to synthesize these thyroid hormones.
±ollicular cells synthesize thyroglobulin, whose protein
portion includes molecules of tyrosine, many of which have
already had iodine attached by an enzymatic reaction. As the
thyroglobulin protein twists and coils into its tertiary struc-
ture, bonds form between some of the tyrosine molecules,
creating potential thyroid hormones waiting to be released.
The follicular cells take up molecules of thyroglobulin by
Thyroid Hormones
The thyroid gland produces three important hormones. The
follicular cells synthesize two of these, which have marked
effects on the metabolic rates of body cells. The extrafollicu-
lar cells produce the third type of hormone, which infl
uences
blood concentrations of calcium and phosphate ions.
The two important thyroid hormones that affect cellular
metabolic rates are
thyroxine
(thi-rok
sin), or tetraiodothy-
Larynx
Colloid
Follicular
cell
Extrafollicular
cell
Thyroid
gland
Isthmus
(a)
(b)
Follicular cells
Colloid
Extrafollicular
cells
FIGURE 13.19
A light micrograph of thyroid gland tissue (240×).
The open spaces that follicular cells surround are F
lled with colloid.
FIGURE 13.18
Thyroid gland. (
a
) The thyroid gland consists of two lobes connected anteriorly by an isthmus. (
b
) ±ollicular cells secrete thyroid
hormones.
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