The pituitary is a small gland about 1 cm in diameter (about size of a pea). It lies just below the hypothalamus (the mid brain) connected to it by a stalk-like structure. It is popularly known as master gland because it controls the functioning of all other endocrine glands. Since most hormones secreted by pituitary stimulate other glands to produce their hormones, they are called tropic hormones.
Thus tropic hormones are such hormones which stimulate other endocrine glands to secrete their own specific hormones. For example, follicle-stimulating hormone (FSH) and leutinizing hormone (LH) are also called gonadotropic hormone because they regulate functioning of gonads (ovaries and testes) to produce sex-hormones. Except for human growth hormone, melanocyte-stimulating hormone (MSH) and prolactin (PRL), all the other hormones are tropic hormones.
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The pituitary is mainly divided into three lobes called:
i. The anterior lobe of pituitary,
ii. The posterior lobe of pituitary, and
iii. The intermediate lobe of pituitary.
1. Hormones of the Anterior Pituitary:
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The anterior pituitary releases at least six different hormones which have an effect on the body.
a. Growth Hormone (GH) or Somatotropic Hormone (STH) or Somatotropin:
i. It controls the overall development or growth of the body, muscles and bones.
ii. It increases the rate of protein synthesis.
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iii. It also stimulates fat metabolism.
b. Thyroid Stimulating Hormone (TSH):
i. It controls the growth and functioning of the thyroid gland.
ii. It stimulates the thyroid gland to produce thyroxin.
c. Adrenocorticotropic Hormone (ACTH):
i. It regulates the activity of adrenal cortex.
d. Gonad Stimulating (Gonadotropic) Hormones:
(i) Follicle stimulating hormone (FSH):
i. In males, it stimulates the process of spermatogenesis.
ii. In females, it stimulates the follicle cells in the ovaries to develop into mature eggs and also stimulates them to produce oestrogen.
(ii) Luteinizing hormone (LH):
i. In males, it stimulates the secretion of male hormone, testosterone, which in turn influences the appearance of secondary sexual characteristics.
ii. In females, it stimulates secretion of oestrogen and progesterone, which in turn influence the process of ovulation, formation and maintenance of corpus luteum and appearance of secondary sexual characteristics.
e. Prolactin hormone (PRL):
i. In females, it enhances mammary gland development and milk production.
ii. In males, it enhances the production of testosterone.
2. Hormones of the Posterior Pituitary:
The posterior pituitary stores two hormones, antidiuretic hormone (ADH) also called vasopressin, and oxytocin. Both these hormones are produced and released by hypothalamus (and not by the pituitary as given in most books). These hormones are transported to posterior pituitary and stored there.
Antidiuretic Hormone (ADH) or Vasopressin:
i. It promotes reabsorption of water from the kidney tubules. Thus, it causes the kidney to form more and more concentrated urine.
ii. It constricts blood vessels with the rise in blood pressure.
Oxytocin:
i. It stimulates vigorous contraction of the uterus during labour, leading to the child birth.
ii. It also causes the release of milk from the breast of a nursing mother
3. Hormone of Intermediate Lobe of Pituitary:
It secretes melanocyte stimulating hormone (MSH) which stimulate skin to secrete melanocyte (melanin pigment).
Summary of major hormones secreted in human body, their sources glands and principal functions.