606
UNIT 3
Regulation and Integration of the Body
16
Check Your Understanding
7.
What is the key difference between the way the
hypothalamus communicates with the anterior pituitary and
the way it communicates with the posterior pituitary?
8.
Anita drank too much alcohol one night and suffered from a
headache and nausea the next morning. What caused these
“hangover” effects?
9.
List the four anterior pituitary hormones that are tropic
hormones and name their target glands.
For answers, see Appendix H.
The Thyroid Gland
Describe the effects of the two groups of hormones
produced by the thyroid gland.
Follow the process of thyroxine formation and release.
Location and Structure
Te butterfly-shaped
thyroid gland
is located in the anterior
neck, on the trachea just inferior to the larynx (Figure 16.1
and
Figure 16.9a
). A median tissue mass called the
isthmus
(is
9
mus) connects its two lateral
lobes
. Te thyroid gland is the
largest pure endocrine gland in the body. Its prodigious blood
supply (from the
superior
and
inferior thyroid arteries
) makes
thyroid surgery a painstaking (and bloody) endeavor.
Unlike other anterior pituitary hormones, PRL release is
controlled primarily by an inhibitory hormone,
prolactin-
inhibiting hormone (PIH)
, now known to be
dopamine
,
which prevents prolactin secretion. Decreased PIH secretion
leads to a surge in PRL release. Tere are a number of
prolactin-
releasing factors
, including ±RH, but their exact roles are not
well understood.
In females, prolactin levels rise and fall in rhythm with es-
trogen blood levels. Estrogen stimulates prolactin release,
both directly and indirectly. A brief rise in prolactin levels just
before the menstrual period partially accounts for the breast
swelling and tenderness some women experience at that time,
but because this PRL stimulation is so brief, the breasts do not
produce milk. In pregnant women, PRL blood levels rise dra-
matically toward the end of pregnancy, and milk production
becomes possible. A²er birth, the infant’s suckling stimulates
release of prolactin-releasing factors in the mother, encouraging
continued milk production.
Homeostatic Imbalance
16.3
Hypersecretion of prolactin is more common than hyposecre-
tion (which is not a problem in anyone except women who
choose to nurse). In fact, hyperprolactinemia is the most fre-
quent abnormality of anterior pituitary tumors. Clinical signs
include inappropriate lactation, lack of menses, infertility in fe-
males, and impotence in males.
(a) Gross anatomy of the thyroid gland, anterior view
Colloid-filled
follicles
(b) Photomicrograph of thyroid gland follicles (145
×
)
Follicular cells
Parafollicular cells
Hyoid bone
Thyroid cartilage
Common carotid
artery
Epiglottis
Superior thyroid
artery
Isthmus of
thyroid gland
Left subclavian
artery
Left lateral
lobe of thyroid
gland
Inferior thyroid
artery
Trachea
Aorta
Figure 16.9
The thyroid gland.
previous page 640 Human Anatomy and Physiology (9th ed ) 2012 read online next page 642 Human Anatomy and Physiology (9th ed ) 2012 read online Home Toggle text on/off