The Thyroid Gland Regulates
Metabolism
The thyroid gland wraps around the trachea just be-
low the voice box (Figure 9.7). The spongy thyroid tis-
sue is composed of many small sacs called follicles. In
the walls of the follicles, follicular cells make thyroid
hormones, thyroxine and triiodothyronine
(tri-i-o-
do-THl-ro-nen), which are stored in the cavities of the
follicles. Located between the follicles are the parafol-
licular cells, which make the hormone calcitonin.
Thyroid hormones are made from the amino acid
tyrosine and have three or four iodine atoms added to
them (thyroxine [T4] has four iodines, and triiodothy-
ronine
[T3]
has
three).
Even
though they are not lipid-soluble,
the thyroid hormones are small
enough to pass through the lip-
ids of the target cell membrane
without a surface receptor. Inside
the cell, T4 gets converted to the
active form T3. T3
binds to the
calcitonin
A hor-
mone secreted by the
parafollicular cells of
the thyroid gland that
inhibits osteoclast ac-
tivity and lowers blood
calcium levels.
Location and stru c tu re o f th e thyroid glan d • Figure 9.7
Follicular cells of the thyroid gland make thyroid hormones and store them in cavities.
Parafollicular cells lie between the follicles and make the hormone calcitonin.
Trachea
Hyoid bone
Larynx
Internal jugular vein
Left lobe of
thyroid gland
Common carotid artery
Trachea
Sternum
b. Thyroid follicles
262 CHAPTER 9
The Endocrine System
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