The diagram above shows you the
relative position of these glands and some of their hormone
activity. It also shows anatomically where the hormone is
produced, and the route it takes into the circulation of the rest
of the body. If you place your cursor on the name of a hormone,
you'll learn more about that hormone's place in the complex cycle
of endocrine control.
For this course, you are not
responsible for learning everything this diagram offers, but it
should help place the individual hormones and releasing factors
in anatomical context.
During pregnancy, the pituitary
gland enlarges. By weight, it increases 30% in first pregnancies,
and 50% in subsequent pregnancies. These changes are almost
entirely due to changes in the anterior lobe.
The anterior lobe of the pituitary is glandular tissue and
produces multiple hormones. The release of these hormones is
regulated by releasing and inhibiting hormones produced by the
Some of these anterior pituitary
hormones induce other glands to secrete their hormones. The rise
in blood levels of the hormones produced by the final target
glands (for example, the ovary or thyroid) inhibits the release
of anterior pituitary hormones.
Follicle-stimulating hormone (FSH)
and luteinizing hormone (LH) secretion
are inhibited during pregnancy, probably as a result of hCG
produced by the placenta, and the increased prolactin by
the anterior pituitary. They remain decreased until after
Thyroid-stimulating hormone (TSH)
is reduced in the first trimester, but usually returns to
normal for the remainder of the pregnancy. Decreased TSH
is thought to be one of the factors
associated with morning sickness, nausea and vomiting in
the first trimester.
There is a progressive
rise in adrenocorticotrophic hormone (ACTH)
plasma concentrations during pregnancy. In fact, there is
a doubling of the plasma cortisol. The myometrium and
decidua convert cortisone to cortisol, resulting in a
local cortisol concentration nine times normal.
This may contribute to the immunological protection of
Growth hormone (GH) is an
anabolic hormone which promotes protein synthesis. It
stimulates most body cells to grow in size and divide. It also facilitating
the use of fats for fuel and conserves glucose. In pregnancy, there
decrease in the number
of GH producing cells and a
corresponding decrease in GH blood
levels. The action of HPL is thought to decrease the need for and use of GH.
During pregnancy there is an increase in
the number of prolactin (PRL) secreting
cells (lactotrophs) and a significant increase in the
blood level of this hormone. Prolactin stimulates the
glandular production of colostrum. During pregnancy, the
ability of prolactin to produce milk is opposed by
progesterone. As soon as the placenta is delivered the
opposition is removed, and lactation can begin. Levels of
PRL decrease after delivery, even in the
lactating mother. Prolactin is produced in spurts, in
response to the infant's sucking.
Melanocyte-stimulating hormone (MSH)
is another anterior pituitary hormone that increases
during pregnancy. For many years, it's increase was
thought to be responsible for many of the skin changes of
pregnancy, particularly skin darkening (darkening of the
areola, chloasma, and linea nigra). However, current
belief attributes the skin changes to estrogen (and
possibly progesterone) as well as the increase in MSH.
hormone (GH) is decreased during pregnancy, but during pregnancy the hormone
Human Placental Lactogen (HPL) is thought to play a similar