Hormones of the Pituitary

The pituitary gland is pea-sized structure located at the base of the brain. In humans, it consists of two lobes:

• the Anterior Lobe and
• the Posterior Lobe

The Anterior Lobe

The anterior lobe contains six types of secretory cells, all but one of which (#2 above) are specialized to secrete only one of the anterior lobe hormones. All of them secrete their hormone in response to hormones reaching them from the hypthalalmus of the brain.

Thyroid Stimulating Hormone (TSH)

TSH (also known as thyrotropin) is a glycotrotein consisting of:

• a beta chain of 112 amino acids and
• an alpha chain of 89 amino acids. The alpha chain is identical to that found in two other pituitary hormones, FSH and LH. Thus it is its beta chain that gives TSH its unique properties.

The secretion of TSH is

• stimulated by the arrival of thyrotropin releasing hormone (TRH) from the hypothalamus.
• inhibited by the arrival of somatostratin from the hypothalamus.

As its name suggests, TSH stimulates the thyroid gland to secrete its hormones

• thyroxine (T₄)
• triiodothyronine ( T₃)

It does this by binding to transmembrane G-protein-coupled receptors (GPCRs) on the surface of the cells of the thyroid.

Some people develop antibodies against their own TSH receptors. When these bind the receptors, they "fool" the cell into making more T₃ and T₄ causing hyperthyroidism. The condition is called thyrotoxicosis or Graves disease.

Hormone deficiencies

A deficiency of TSH causes hypothyroidism: inadequate levels of T₄ and T₃. Recombinant human TSH has recently become available to treat patients with TSH deficiency.

Some people inherit mutant TSH receptors. This, too, results in hypothyroidism.

Follicle-Stimulating Hormone (FSH)

FSH is a heterodimer of

• the same alpha chain found in TSH (and LH)
• a beta chain of 115 amino acids, which gives it its unique properties.

Synthesis and release of FSH is triggered by the arrival from the hypothalamus of gonadotropin-releasing hormone (GnRH). The effect of FSH depends on one's sex

FSH in females

In sexually-mature females, FSH (assisted by LH) acts on the follicle to stimulate it to release estrogens.

FSH in males

In sexually-mature males, FSH acts on spermatogenia stimulating (with the aid of testosterone) the production of sperm.

Luteinizing Hormone (LH)

LH is synthesized within the same pituitary cells as FSH and under the same stimulus (GnRH). It is a heterodimeric glycoprotein consisting of

• the same 89-amino acid alpha subunit found in FSH and TSH
• a beta chain of 115 amino acids that is responsible for its properties.

The effects of LH also depend on sex.

LH in females

In sexually-mature females, LH

• stimulates the follicle to secrete estrogen in the first half of the menstrual cycle
• a surge of LH triggers the completion of meiosis I of the egg and its release (ovulation) in the middle of the cycle
• stimulates the now-empty follicle to develop into the corpus luteum,which secretes progesterone during the latter half of the menstrual cycle.

LH in males

LH acts on the interstitial cells of the testes stimulating them to synthesize and secrete the male sex hormone, testosterone.

LH in males is also known as interstitial cell stimulating hormone (ICSH).

Prolactin (PRL)

Prolactin is a protein of 198 amino acids. During pregnancy it helps in the preparation of the breasts for future milk production.

After birth, prolactin promotes the synthesis of milk.

Prolactin secretion is

• stimulated by TRH
• repressed by estrogens and dopamime.

In pregnant mice, prolactin stimulates the growth of new neurons in the olfactory center of the brain.

Growth Hormone (GH)

Human growth hormone (also called somatotropin) is a protein of 191 amino acids. The GH-secreting cells are stimulated to synthesize and release GH by the intermittent arrival of growth hormone releasing hormone (GHRH) from the hypothalamus. GH promotes body growth by:

• binding to receptors on the surface of liver cells
• this stimulates them to release insulin-like growth factor-1 (IGF-1; also known as somatomedin)
• IGF-1 acts directly on the ends of the long bones promoting their growth

Pituitary abnormalities

• In childhood,
  • hyposecretion of GH produces the stunted growth of a dwarf
  • hypersecretion leads to gigantism
• In adults, a hypersecretion of GH leads to acromegaly.

Hormone-replacement therapy

GH from domestic mammals like cows and pigs does not work in humans. So for many years, the only source of GH for therapy was that extracted from the glands of human cadavers. But this supply was shut off when several patients died from a rare neurological disease attributed to contaminated glands. Now, thanks to recombinant DNA technology, recombinant human GH is available. While a great benefit to patients suffering form GH deficiency, there has also been pressure to use it to stimulate growth in youngsters who have no deficiency but whose parents want them to grow up tall.

ACTH - the adrenocorticotropic hormone

ACTH is a peptide of 39 amino acids. It is cut from a larger precursor proopiomelanocortin (POMC).

ACTH acts on the cells of the adrenal cortex, stimulating them to produce

• glucocorticoids, like cortisol
• mineralocorticoids, like aldosterone
• androgens (male sex hormones, like testosterone
• in the fetus, ACTH stimulates the adrenal cortex to synthesize a precursor of estrogen called dehydroepiandrosterone sulfate (DHEA-S) which helps prepare the mother for giving birth.

Production of ACTH depends on the intermittent arrival of corticotropin-releasing hormone (CRH) from the hypothalamus.

Hypersecretion of ACTH is a frequent cause of Cushings disease.

Alpha Melanocyte-Stimulating Hormone (MSHa)

MSHa is also a cleavage product of proopiomelanocortin (POMC). In fact, MSHa is identical to the first 13 amino acids at the amino terminal of ACTH.

The Posterior Lobe

The posterior lobe of the pituitary releases two hormones, both synthesized in the hypothalamus, into the circulation.

• Antidiuretic Hormone (ADH).
  ADH is a peptide of 9 amino acids. It is also known as arginine vasopressin.

ADH acts on the collecting ducts of the kidney to facilitate the reabsorption of water into the blood. This it acts to reduce the volume of urine formed (giving it its name of antidiuretic hormone).

 

• A deficiency of ADH or
• inheritance of mutant genes for its receptor

leads to excessive loss of urine, a condition known as diabetes insipidus. The most severely-afflicted patients may urinate as much as 30 liters (almost 8 gallons!) of urine each day. The disease is accompanied by terrible thirst, and patients must continually drink water to avoid dangerous dehydration.

• Oxytocin
  Oxytocin is a peptide of 9 amino acids. Its principal actions are:
  • stimulating contractions of the uterus at the time of birth
  • stimulating release of milk when the baby begins to suckle

Oxytocin is often given to prospective mothers to hasten birth