Human Physiology, 14th edition (2016)

332 Chapter 11

gonadotropins (FSH and LH) and acts on the kidneys to

help regulate water and electrolyte balance.

The population of cells that produces growth hormone

constitutes about 40% to 50% of the endocrine cells of the

anterior pituitary. Smaller populations of specialized cells pro-

duce prolactin, TSH, and ACTH. A separate population of spe-

cialized cells, constituting about 10% of the anterior pituitary,

produces both gonadotropins—FSH and LH. The specialized

anterior pituitary cells are regulated by specific hormones of

the hypothalamus, as will be described in a later section.

The pars intermedia of the adenohypophysis ceases to exist

as a separate lobe in the adult human pituitary, but it is present

in the human fetus and in adults of other animals. Until recently,

it was thought to secrete melanocyte-stimulating hormone

(MSH), as it does in fish, amphibians, and reptiles, where it

causes darkening of the skin. In humans, however, plasma

of the anterior pituitary, “trophic” (conventionally shortened to
tropic, meaning “attracted to”) is incorporated into them. This is
why the shortened forms of the names for the anterior pituitary
hormones end in the suffix - tropin. The hormones of the anterior
pituitary, listed here, are summarized in table 11.6.

1. Growth hormone (GH, or somatotropin). GH promotes
   the movement of amino acids into cells and the incorpo-
   ration of these amino acids into proteins, thus promoting
   overall tissue and organ growth. Some of growth hor-
   mone’s actions, including growth of cartilage and bones
   and protein synthesis in muscles, result from a group of
   molecules (the somatomedins) produced by the liver under
   growth hormone stimulation (chapter 19, section 19.5).


2. Thyroid-stimulating hormone (TSH, or thyrotropin). TSH
   stimulates the thyroid gland to produce and secrete thyrox-
   ine (tetraiodothyronine, or T 4 ) and triiodothyronine (T 3 ).


3. Adrenocorticotropic hormone (ACTH, or corticotropin).
   ACTH stimulates the adrenal cortex to secrete the glucocor-
   ticoids, such as cortisol (hydrocortisone).


4. Follicle-stimulating hormone (FSH, or folliculotropin).
   FSH stimulates the growth of ovarian follicles in females
   and the production of sperm cells in the testes of males.


5. Luteinizing hormone (LH, or luteotropin). This hormone
   and FSH are collectively called gonadotropic hormones.
   In females, LH stimulates ovulation and the conversion of
   the ovulated ovarian follicle into an endocrine structure
   called a corpus luteum. In males, LH is sometimes called
   interstitial cell stimulating hormone, or ICSH; it stimulates
   the secretion of male sex hormones (mainly testosterone)
   from the interstitial cells (Leydig cells) in the testes.


6. Prolactin (PRL). This hormone is secreted in both males
   and females. Its best known function is the stimulation of
   milk production by the mammary glands of women after
   the birth of a baby. Prolactin plays a supporting role in
   the regulation of the male reproductive system by the

CLINICAL APPLICATION

Dwarfism is short stature produced by inadequate secre-

tion of growth hormone (GH) during childhood, and is often

accompanied by a deficiency in the other anterior pituitary

hormones. It can be treated with injections of recombinant

growth hormone. Gigantism is caused by oversecretion of GH

during childhood, usually the result of an adenoma (a benign

tumor) of the anterior pituitary. Oversecretion of GH during

adulthood cannot cause further growth in height because the

epiphyseal plates of cartilage that produce this growth have

closed (changed to bone). Instead, it produces the disfigure-

ment characteristic of acromegaly (chapter 19; see fig. 19.17).

Oversecretion of GH for both gigantism and acromegaly can

be treated with somatostatin, a hypothalamic hormone that

inhibits GH secretion (as discussed shortly), or by surgery.

Hormone Target Tissue Principal Actions Regulation of Secretion

ACTH (adrenocorticotropic

hormone)

Adrenal cortex Stimulates secretion of glucocorticoids Stimulated by CRH (corticotropin-releasing

hormone); inhibited by glucocorticoids

TSH (thyroid-stimulating

hormone)

Thyroid gland Stimulates secretion of thyroid hormones Stimulated by TRH (thyrotropin-releasing

hormone); inhibited by thyroid hormones

GH (growth hormone) Most tissue Promotes protein synthesis and growth;

lipolysis and increased blood glucose

Inhibited by somatostatin; stimulated by

growth hormone-releasing hormone

FSH (follicle-stimulating

hormone)

Gonads Promotes gamete production and

stimulates estrogen production in

females

Stimulated by GnRH (gonadotropin-

releasing hormone); inhibited by sex

steroids and inhibin

PRL (prolactin) Mammary glands

and other sex

accessory organs

Promotes milk production in lactating

females; additional actions in other

organs

Inhibited by PIH (prolactin-inhibiting

hormone)

LH (luteinizing hormone) Gonads Stimulates sex hormone secretion; ovulation

and corpus luteum formation in females;

stimulates testosterone secretion in males

Stimulated by GnRH; inhibited by sex

steroids

Table 11.6 | Anterior Pituitary Hormones