Growth Hormone -
Growth Hormone Releasing Hormone [GHRH] is produced in the hypothalamus. It stimulates the Anterior pituitary gland to produce and release GROWTH HORMONE [GH] into the bloodstream. It then acts on virtually every tissue of the body to control several physical functions and processes. In response to GH, Insulin-like growth hormones is produced in the liver to stimulate metabolic and growth actions. GH also promotes protein synthesis and encourages the use of
fats for fuel to conserve glucose.
Prolactin- Stimulates milk production.
Dopamine [PIH] is produced by the Hypothalamus and restricts the production of prolactin. The more dopamine, the less prolactin. Prolactin also enhances the release of dopamine PIH, creating a negative feedback loop. Unlike other anterior pituitary hormones, PRL release is controlled by PIH. Decreased PIH secretions leases to a surge in PRL release. Estrogen stimulates prolactin release.
Thyroid Stimulating
hormone -
Thyrotropin releasing hormone TRH is produced in the hypothalamus and stimulates the anterior pituitary gland to make and release Thyroid stimulating hormone TSH into the bloodstream. When TSH is released it bind to receptors on thyroid cells causing the thyroid to produce thyroxin T4 and triiodothyronine T3. If levels of T4 and T3 are too high, the production of TSH will stop. T3 and T4 which are Thyroid hormones TH enters target cells, binds to intracellular receptors within the
nucleus and initiates transcription of mRNA for protein synthesis. TH increases basal metabolic rate and heat production. It also regulates tissue growth and development and is critical for normal skeletal and nervous system development and maturation.
Adrenocorticotropic hormone - made by the corticotrophin cells in the anterior pituitary and secreted into the bloodstream in several intermittent pulses throughout the day.
-Secretion of adrenocorticotropic hormone ACTH is controlled
by three inter-communicating regions of the body, the hypothalamus, the pituitary gland and the adrenal glands. This is called the hypothalamic-pituitary-adrenal axis. When adrenocorticotropic hormone ACHT levels in the blood are low, a group of cells in the hypothalamus release a hormone called corticotrophin-releasing hormone CRH which stimulates the pituitary gland to secrete adrenocorticotropic hormone ACTH into the bloodstream. High levels of adrenocorticotropic hormone ACHT are detected by
the adrenal glands which stimulate the secretion of CORTISOL, causing blood levels of cortisol to rise. As the cortisol levels rise, they start to slow down the release of corticotrophin-releasing hormone CRH from the hypothalamus and adrenocorticotropic hormone ACTH from the pituitary gland. As a result, the adrenocorticotropic hormone ACTH levels start to fall. This is called a negative feedback loop.
-Stress, both physical and psychological, also stimulates adrenocorticotropic hormone
production and hence increases cortisol levels.
-Cortisol help the body to resist stressors.
-ACTH levels are generally high in the morning and falls throughout the day.
Follicle-stimulating hormone - Follicle stimulating hormone FSH is essential to pubertal development and function of the ovaries and testes. It is secreted from the anterior pituitary gland into the bloodstream. In women, FSH stimulates the growth of ovarian follicles in the ovary before the release of an egg at
ovulation and promotes estrogen production. In men, FSH acts on the Sertoli cells of the testes to promote sperm production.
-follicle stimulating hormone is regulated by the levels of a number of circulating hormones released by the ovaries and testes. This system is called the hypothalamic-pituitary-gonadal axis.
-In women, when hormone levels fall towards the end of the menstrual cycle, this is sensed by nerve cells in the hypothalamus. These cells produce more gonadotrophin-releasing
hormone GnRH which in turn stimulates the pituitary gland to produce more FSH and LH and release these into the bloodstream. The rise in FSH stimulates the growth of the follicle in the ovary. With this growth, the cells of the follicles produce increasing amount of oestradiol and inhibin. In turn, the production of these hormones is sensed by the hypothalamus [or pituitary gland in the case of inhibin] and less gonadotrophin-releasing hormone GnRH and FSH will be released. However, as the
follicle matures, there is a surge in oestradiol [estrogen] production which switches the feedback mechanisms in the brain from negative to positive feedback, causing more gonadotrophin-releasing hormone GnRH, FSH and LH to be released. This peak in hormone levels eventually leads to ovulation.
-Thus, during each menstrual cycle there is a rise in FSH secretion in the first half of the cycle that stimulates follicular growth in the ovary. If this inter-cycle rise of FSH does not occur, final
maturation of a single follicle and release of the egg [ovulation] does not take place.
-Progesterone produced by the corpus luteum can also feed back onto the hypothalamus and inhibit follicle stimulating hormone production.
-In men, the production of follicle stimulating hormone is regulated by the circulating levels of testosterone and inhibin, both produced by the testes. Testosterone levels are sensed by nerve cells in the hypothalamus so that gonadotrophin-releasing hormone
secretion is either increased or decreased. The levels of inhibin are sensed by cells in the anterior pituitary gland. If levels of testosterone and/or inhibin rise, follicle stimulating hormone production is reduced and vice versa if they fall, in a typical negative feedback loop.
Luteinizing hormone -
LH is produced and released by cells in the anterior pituitary gland; it is crucial in regulating the function of the testes and the ovaries. LH has two critical functions: the control
of sex steroid production [testosterone and estrogen] and the support of germ cell production [gametogenesis]. LH acts on specialized cells which surround the male and female germ cells [sperm and oocyte respectively] to provide an environment to support their maturation and development. The local production of sex steroids [testosterone and oestrogen] is crucial for this process.
-In men, LH stimulates Leydig cells in the testes to produce testosterone. In addition to acting locally to
support sperm production, testosterone also exerts effects all around the body to generate male characteristics [eg, increased muscle mass, enlargement of the larynx to generate a deep voice and the growth of facial and body hair].
-In women, LH carries out different roles in the two halves of the menstrual cycle. In weeks one to two of the cycle, LH is required to stimulate the ovarian follicles in the ovary to produce the female sex hormone, Estrogen. Around day 14 of the cycle, a surge in
LH levels causes the ovarian follicle to tear and release a mature oocyte [egg] from the ovary, a process called ovulation. For the remainder of the cycle [weeks three to four], the remnants of the ovarian follicle form a corpus luteum. LH stimulates the corpus luteum to produce progesterone which is required to support the early stages of pregnancy, if fertilization occurs.
-The secretion of LH from the anterior pituitary gland is regulated through a system called the
hypothalamic-pituitary-gonadal axis. Gonadotrophin-releasing hormone GnRH is released from the hypothalamus and binds to receptors in the anterior pituitary gland to stimulate both the synthesis and release of LH [and FSH]. The released LH is carried in the bloodstream where it binds to receptors in the testes and ovaries. Using this mechanism, LH can control the functions of the testes and ovaries themselves.
-To control levels of LH, the release of other hormones from the gonads suppress
the secretion of gonadotrophin-releasing hormone GnRH and, in turn, LH from the hypothalamus and anterior pituitary gland [negative feedback]. In men, testosterone exerts this negative feedback and in women oestrogen and progesterone exert the same effect except at the midpoint in the menstrual cycle. At this point, oestrogen secretions from the ovary have reached relatively high levels and this stimulates a surge of LH from the pituitary gland which triggers ovulation.