Regulation and Integration of the Body
Check Your Understanding
What is the key difference between the way the
hypothalamus communicates with the anterior pituitary and
the way it communicates with the posterior pituitary?
Anita drank too much alcohol one night and suffered from a
headache and nausea the next morning. What caused these
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
is located in the anterior
neck, on the trachea just inferior to the larynx (Figure 16.1
). A median tissue mass called the
mus) connects its two lateral
. Te thyroid gland is the
largest pure endocrine gland in the body. Its prodigious blood
supply (from the
inferior thyroid arteries
thyroid surgery a painstaking (and bloody) endeavor.
Unlike other anterior pituitary hormones, PRL release is
controlled primarily by an inhibitory hormone,
inhibiting hormone (PIH)
, now known to be
which prevents prolactin secretion. Decreased PIH secretion
leads to a surge in PRL release. Tere are a number of
, including ±RH, but their exact roles are not
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.
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
(b) Photomicrograph of thyroid gland follicles (145
lobe of thyroid
The thyroid gland.