03 September 2016

Lecture #2: THE PITUITARY GLAND: THE ADENOHYPOPHYSIS



Structure


1.      Also known as hypophysis and called the “master gland.”
2.      Size: 1.2 – 1.5 cm across; weight: 0.5 grams
3.      Located on the ventral surface of the brain within the skull
4.      Infundibulum – stemlike stalk that connects pituitary to the hypothalamus
5.      Made up of two separated glands, the adenohypophysis (anterior pituitary gland) and the neurohypophysis (posterior pituitary gland).

THE ADENOHYPOPHYSIS

Divided into two parts

1.      Pars anterior – forms the major portion of the adenohypophysis

2.      Pars intermedia

The tissue is composed of irregular clumps of secretory cells supported by fine connective tissue fibers and surrounded by a rich vascular network. Three types of cell can be identified base on staining reaction:

a.      Chromophobes – make up approximately one half of all cells in adenohypophysis.

b.      Acidophils – make up approximately 40% of all cells in adenohypophysis; secretes GH and PRL

c.       Basophils – form about 10% of adenohypophysis; secretes TSH, ACTH, FSH, LH and MSH.

Control of secretion in the adenohypophysis

1.      Hypothalamus secretes releasing hormones into the blood, which are then carried to the hypophyseal portal system.

2.      Hypophyseal portal system carried blood from the hypothalamus directly to the adenohypophysis where the target cells of the releasing hormone are located.

3.      Releasing hormones influence the secretion of hormones by acidophils and basophils.

4.      Through negative feedback, the hypothalamus adjusts the secretions of the adenohypophysis, which then adjusts the secretions of the target glands that in turn adjust the activity of their target tissues.

5.      In severe pain or intense emotion, the hypothalamus translates nerve impulses into hormone secretions by endocrine glands, basically creating a mind–body link.

A.    The Growth Hormone: Somatotropin

Functions of GH

1.      Promotes growth of bone, muscle and other tissues by accelerating amino acid transport into the cell.

2.      Stimulates fat metabolism by mobilizing lipids from storage in adipose cells and speeding up catabolism of the lipid after they have entered another cell.

3.      GH tends to shift cell chemistry away from glucose catabolism and toward lipid catabolism as an energy source; this leads to increase blood glucose level.

4.      GH and insulin function as antagonist and are vital to maintaining homeostasis of blood glucose levels.

5.      Found in highest concentration in all cells on adenohypophysis

Regulation of secretion of GH

1.      First, the liver is stimulated by GH to produce proteins called somatomedins (growth factors). Two somatomedins are found in human plasma: insulinlike growth factor II or IGF–II (somatomedin A) and insulinlike growth factor I or IGF–I (somatomedin C). The compound that was known as somatomedin B is actually acidic glial cell growth factor and not one of the somatomedins.

2.      Second, IGF–I binds to receptors on the cartilage and bone cells to stimulate DNA synthesis and cell growth. The regulation of GH secretion appears to be multifaceted. The hypothalamus contains growth hormone–releasing hormone (somatocrinin), a small peptide that induces the secretion of GH. Also present in the hypothalamus is growth hormone–inhibiting hormone (GHIH) or somatostatin. These two hormones act together to cause increases and decreases in the concentration of circulating GH. In addition, hypoglycemia induces the secretion of GH as well as some amino acids.

3.      Peak levels occur 1–4 hours after the onset of sleep. These nocturnal sleep bursts, which account for nearly 70% of daily GH secretion, are greater in children and tend to decrease with age. Glucose infusion will not suppress this episodic release. Emotional, physical and chemical stress, including surgery, trauma, exercise, electroshock therapy and pyrogen administration, provoke GH release and impairment of secretion leading to growth failure.

4.      Glucose administration, orally or intravenously, lowers GH in healthy subjects and provides a single physiologic maneuver useful in diagnosis of acromegaly. In contrast, hypoglycemia stimulates GH release. This effect depends on intracellular glycopenia, since the administration of 2– deoxyglucose also increases GH.

5.      A protein meal or intravenous administration of amino acids causes GH release. Paradoxically, states of protein–calorie malnutrition also increase GH, possible as a result of decreased IGF–I production and lack of inhibitory feedback.

6.      Fatty acids suppress GH responses to certain stimuli, including arginine and hypoglycemia. Fasting stimulates GH secretion, possible as a means of mobilizing fat as an energy source and presents protein loss.

7.      Neurotransmitters:
Increased                   Decreased

α–adrenergic agonist                       clonidine                   phentolamine

β–adrenergic agonist                       propranolol               isoproterenol

Serotonin                                           precursors                 antagonist

Dopamine                                         agonist                       antagonist
                                                            levodopa                   phenothiazines
                                                            apomorphine
                                                            bromocriptine

 Clinical significance of GH

1.      Hypersecretion

a.      Acromegaly and Gigantism: Signs and symptoms

(1)   Soft tissue proliferation with enlargement of the hands and feet accompanied by increased sweating, heat intolerance, oiliness of the skin, fatigue and weight loss.

(2)   Increased in ring, gloves and shoe size

(3)   Acne, sebaceous cysts and fibromata mollusca, skin tags and papillomas are common, as in acanthosis nigricans of the axillae and neck and hypertrichosis in women

(4)   Photophobia, paresthesias, degenerative arthritis, cardiomegaly, and renal calculi also occur.

(5)   Hyperinsulinemia, glucose intolerance, irregular or absent menses, decreased libido, hypothyroidism, galactorrhea, gynecomastia, hypoadrenalism

Laboratory findings

(1)   Postprandial plasma glucose may be elevated

(2)   Serum insulin is increased

(3)   Elevated serum  phosphorous due to increased renal tubular resorption of phosphate

(4)   Hypercalciuria

2.      Hyposecretion

a.      Laron’s dwarfism: Signs and symptoms

(1)   Very short, poorly growing children with delayed skeletal maturation, normal GH and IGF–I values, and no signs of organic disease.

Laboratory findings:

(1)   Characterized by high plasma GH and low plasma IGF–I concentrations. Growth rate does not increase and IGF–I values do not rise when exogenous hGH is administered. However, IGF–I administration raises growth rate and suppresses GH concentrations. The basic defect is an inability to produce IGF–I in response to growth hormone because of impaired or absent GH receptors. GHBP is absent in the serum. It is inherited as an autosomal recessive disorder.

b.     Pygmies

(1)   They have a normal plasma GH, low IGF–I and normal IGF–II concentrations. They would not respond to exogenous GH with improved growth rate or a rise in IGF–I, which is of greater importance in stimulating growth than IGF–II.

Laboratory diagnosis

1.      Levodopa Test

a.      Method               

The patient should be fasting and at bed rest after midnight. Levodopa (500 mg) is given by mouth.

b.     Sample collection

Blood samples for plasma GH determinations are obtained at 0, 30 and 60 minutes.

c.       Contraindication

Nausea and vomiting may occur 45–60 minutes after levodopa is given. This test is safer than the insulin hypoglycemia test in older patients.

d.     Interpretation

A normal response is maximal level of GH greater than 6 ng/ml (279 pmol/L); however the peak response is usually more than 20 ng/ml (930 pmol/L)

2.      Arginine infusion Test

a.      Method

The patient should be fasting after midnight. Give arginine hydrocholoride, 0.5 g/kg intravenously, up to a maximum of 30 grams over 30 minutes. Pre–treatment with estrogen in post–menopausal women and in men can also be done.

b.     Sample collection

Blood for plasma GH determination is collected at 0, 30, 60, 90 and 120 minutes. Arginine infusion also stimulates insulin and glucagon.

c.       Contraindication

Nausea and vomiting may occur. This test is contraindicated in patients with severe liver disease, renal disease or acidosis.

d.     Interpretation

The response is greater in women than in men. The lower limit of normal for the peak GH response is 6 ng/ml (279 pmol/L) in non–estrogen treated patients and 10 ng/ml (465 pmol/L) in estrogen – treated patients and pre –menopausal women.

3.      Glucose–Growth Hormone Suppression Test

a.      Method

The patient should be fasting after midnight. Give glucose, 75–100 grams orally.

b.     Sample collection

GH and glucose should be determined at 0, 20 and 60 minutes after glucose administration

c.       Contraindication

Patients may complain of nausea after the large glucose load

d.     Interpretation

GH levels are supposed to be less than 2 ng/ml (93 pmol/L) in healthy subjects. Failure of adequate suppression or a paradoxical rise may be seen in acromegaly, starvation, protein–calorie malnutrition and anorexia nervosa

4.      GRH Test

a.      Method

GRH (1ug/kg) is given intravenously as bolus injection.

b.     Sample collection

Blood samples for GH are drawn at 0, 30 and 60 minutes.

c.       Contraindication

Mild flushing and a metallic taste or smell occurs in a few patients.

d.     Interpretation

The range of normal responses is wide. Most patients have a peak GH response of greater than 10 ng/ml (465 pmol/L) at 30–60 minutes.


B.     The Prolactin (PRL): Lactogenic Hormone

Functions of PRL

1.      During pregnancy, PRL promotes development of the breasts, anticipating milk secretion; after the baby is born, PRL stimulates the mother’s mammary glands to produce milk.

2.      PRL plays a supportive role (with luteinizing hormone) in maintaining the corpus luteum of the ovary during the final phase of the menstrual cycle; sometimes called luteotropic hormone.


Regulation of secretion of PRL

Secretion of PRL appears to be under the control of prolactin–inhibiting factor (PIF), known as dopamine and comes from the hypothalamus. If dopamine levels decline, PRL is secreted. If dopamine levels increases, PRL secretion is inhibited. No prolactin–releasing factor has ever been identified, but thyrotropin–releasing hormone (TRH) has been shown to induce increases in PRL levels. The actual role of TRH in regulating PRL secretion has not been established.

PRL secretion is episodic. An increase is observed 60–90 minutes after sleep but in contrast to GH, is not associated with a specific sleep phase. Peak levels are usually attained between 4 and 7 AM. This sleep–associated augmentation of PRL release is not part of a circadian rhythm like that of ACTH; it is related strictly to the sleeping period regardless of when it occurs during the day.

Other stimuli like stresses, including surgery, exercise, hypoglycemia and acute myocardial infarction, cause significant elevation of PRL levels. Nipple stimulation in non–pregnant women also increases PRL. This neurogenic reflex may also occur from chest wall injury such as mechanical trauma, burns, surgery and herpes zoster of thoracic dermatomes. This reflex discharge of PRL is abolished by denaturation of the nipple or by spinal cord or brain stem lesions.

Pharmacologic agents affecting PRL secretion

                  Increase                                                                     Decrease

Dopamine antagonist: phenothiazines,                      Dopamine agonist: levodopa,
Haloperidol, metaclopramite, reserpine,                  apomorphine, bromocriptine,
methyl–dopa, amoxipine, opiates                              pergolide

Opiods                                                                             GABA

Neuramine oxidase inhibitors            

Cimetidine, verapramil, licorice


Clinical significance of PRL

1.      Hypersecretion

a.      Prolactinomas: Clinical Features

(1)   Galactorrhea – maybe induced by a wide variety of stimuli ranging from local irritation or stimulation of the chest  wall to ingestion of drugs that interfere with hypothalamic release of dopamine or its binding to the pituitary lactotrophs. Careful breast examination is required in most patients to demonstrate it.

(2)   In women – amenorrhea, oligomenorrhea with anovulation, decreased vaginal lubrication, osteopenia, weight gain, fluid retention and irritability, elevated levels of dehydroepiandrosterone (DHEA) sulfate, anxiety and depression

(3)   In men – hypogonadism, decreased libido, headache, visual impairment or hypopituitarism, decreased testosterone levels.

(4)   Tumor progression

b.     Microadenomas – intrasellar adenomas less than 1 cm in diameter that present with manifestations of hormonal excess without sellar enlargement or extrasellar extension. Panhypopituitarism does not occur and such tumors are very successfully treated.

c.       Macroadenomas – those larger than 1 cm in diameter and cause generalized sellar enlargement. Tumors 1–2 cm in diameter confined to sella turcica can usually be successfully treated; however, larger tumors and especially those with suprasellar, sphenoid sinus or later extensions – are much more difficult to manage. Panhypopituitarism and visual loss, increase in frequency with tumor size and suprasellar size are also evident.

           
C.    The Adrenocorticotropic Hormone (ACTH)

Functions of ACTH

1.      Stimulates the secretion of glucocorticoids, minerolocorticoids and androgenic steroids from the adrenal cortex.

2.      Increase RNA, DNA and protein synthesis.

3.      Its biologically active fragment beta–lipotropin (B–LPH) and beta–endorphin acts as endogenous opiates suggesting a roloe in pain appreciation.

4.      Stimulates cyclic AMP production and subsequent lipolysis from adipose tissue.

5.      It is associated with Melanocyte Stimulating Hormone (MSH), thereby has a role on hyperpigmentation.


Regulation of synthesis

ACTH is synthesized from a large precursor molecule called proopiomelanocortin. The physiologic secretion of ACTH is mediated through neural influences by means of a complex of hormones, the most important of which is corticotropin–releasing hormone (CRH).

1.      The circadian rhythm is superimposed on episodic secretion; it is the result of central nervous system events that regulate both the number and magnitude of CRH and ACTH secretory episodes. Control secretion is low in the late evening and continues to decline in the first several hours of sleep, at which time plasma cortisol levels may be undetectable. During the third and fifth hours of sleep, there is an increase in secretion; but the major secretory episodes begin in the sixth to eight hours of sleep and then begin to decline as wakefulness occurs. About half of the total daily cortisol output is secreted during this period. Cortisol secretion then gradually declines during the day with fewer secretory episodes of decreased magnitude; however, there is increased cortisol secretion in response to eating and exercise.

Although this general pattern is consistent, there is considerable intra and interindividual variability, and the circulation rhythm maybe altered by changes in sleep pattern, light dark exposure and feeding times. Cyproheptadine inhibits the circadian rhythm, possible by its antiserotonergic effects, whereas other drugs usually have no effects. The rhythm is also changed by:

a.      Physical stress such as major illnesses, surgery, trauma or starvation.
b.      Psychologic stress, including severe anxiety, endogenous depression and manic phase of manic depressive psychosis.
c.       Central nervous system and pituitary disorders
d.     Cushing’s syndrome
e.      Liver disease and other conditions that affect cortisol metabolism
f.        Chronic renal failure
g.      Alcoholism
h.      Pyrogen and vasopressin administration

2.      Fast feedback inhibition of ACTH secretion is rate–dependent – i.e., it depends on the rate of increase of the glucocorticoid but not the dose administered. This phase is rapid and basal and stimulated ACTH secretion both diminish within minutes after the plasma glucocorticoid level increases. This fast feedback phase is transient and last less than 10 minutes, suggesting that this effect is not mediated via cystolic glucocorticoid receptors but rather via actions on the cell membrane.

3.      Delayed feedback inhibition after the initial rate–dependent effects on glucocorticoid further suppress CRH and ACTH secretion by mechanisms that are both time and dose–dependent. Thus with continued glucocorticoid, ACTH level continue to decrease and become unresponsive to stimulation. The ultimate effect of prolonged glucocorticoid administration is suppression of CRH and ACTH release and atrophy of the zona fasciculate and reticularis as a consequence of ACTH deficiency. The suppressed hypothalamic– pituitary–adrenal axis fails to respond to stress and stimulation. Delayed feedback appears to act via the classic glucocorticoid receptor, thus reducing synthesis of the messenger RNA for pro–opiomelanocortin, the precursor of ACTH.


Clinical significance of ACTH

1.      Hypersecretion

a.      Cushing’s syndrome

(1)   ACTH–dependent

(a)   Cushing’s disease                      68%
(b)   Ectopic ACTH syndrome         15%

(2)   ACTH–independent

(a)   Adrenal adenoma                      9%
(b)   Adrenal carcinoma                    8%

Signs and symptoms:

(a)   Obesity

Weight gain is the first symptom usually affecting mainly the face, neck, trunk, abdomen with relative sparing of the extremities. Accumulation of fat in the face leads to typical “moon faces” which is present in 75% of cases and is accompanied by facial plethora in most patients. Fat accumulation around the neck is prominent in the supraclavicular and dorsocervical fat pads; the latter is responsible for the “buffalo hamp.”

(b)  Skin changes

Atrophy of the epidermidis and its underlying connective tissue leads to thinning (a transparent appearance of the skin) and facial plethora. Easy bruisability following minimal trauma is present in about 40%. Striae occur in 50–70% which is typically red to purple, depressed below the skin surface secondary to loss of underlying connective tissue, and wider than the pinkish white striae that may occur with pregnancy or rapid weight gain. These striae are most commonly abdominal but may also occur over the breast, hips, buttocks thighs and axillae.

Mucocutaneous fungal infections are frequent including tinea versicolor, involvement of the nails (onchomycosis) and oral candidiasis. Minor wounds and abrasions may heal slowly and surgical incisions sometimes undergo dehiscence.

(c)    Hirsutism

Present in 80% of female patients owing to hypersecretion of adrenal androgens. Facial hirsutism is most common but increased hair growth may also occur over the abdomen, breasts, chest and upper thighs. Acne and seborrhea usually accompany hirsutism. Virilism is unusual except in cases of adrenal carcinoma, in which it occurs in about 20%.

(d)  Hypertension

The diastolic blood pressure is greater than 100 mmHg and is responsible for mortality and morbidity of the syndrome.

(e)   Gonadal dysfunction

This is very common as a result of elevated androgens (in females) and cortisol (in males and to a lesser extent in females). Amenorrhea occurs in 75% of premenopausal women and is usually accompanied by infertility. Decreased libido is frequent in males and some have decreased body hair and soft testes.

(f)    Psychologic disturbances

Mild symptoms consist of emotional lability and increased irritability. Anxiety, depression, poor concentration and poor memory may also be present. Euphoria is frequent and occasional patients manifest overtly manic behavior. Sleep disorders are present in most patients, with either insomnia or early morning awakening.

Severe psychologic disorders occur in a few patients and include severe depression, psychosis with delusions or hallucinations and paranoia. Some patients have committed suicide.

(g)   Muscle weakness

This occurs in about 60% of cases; it is more often proximal and is usually most prominent in the lower extremities.

(h)  Osteoporosis

Osteoporosis is present in most patients; back pain is an initial complaint in 58% of cases. Pathologic fractures in severe cases involving the ribs and vertebral bodies. Compression fractures of the spine are demonstrable radiographically in 16 – 22%.
(i)     Renal calculi, thirst and polyuria


Laboratory findings in Cushing’s syndrome

(a)   High normal hemoglobin, hematocrit and red cell count are usual; polycythemia is rare.

(b)   Total white blood cell counts are usual normal, however, both the percentage of lymphocytes and total lymphocytes count may be subnormal. Eosinophils are also depressed, a total eosinophil count less than 100/ul is present in most patients.

(c)    Normal serum electrolytes, however, hypokalemic alkalosis occurs when there is marked steroid hypersecretion with ectopic ACTH syndrome or adrenocortical carcinoma.

(d)  Serum calcium is normal; serum phosphorous is low normal or slightly depressed. Hypercalciuria is present in 40% cases.

(e)   Glycosuria is present in patients with fasting or post prandial hyperglycemia. Most patients have secondary hyperinsulinemia and abnormal glucose tolerance.


2.      Hyposecretion

a.      Addison’s disease: Primary Adrenocortical Deficiency

Signs and symptoms:

(1)   Hyperpigmentation of the skin and mucous membrane and is increased in sun–exposed areas and accentuated over pressure areas such as the knuckles, toes, elbows and knees. It is accompanied by increased numbers of black or dark brown freckles.

(2)   General weakness, fatigue and malaise, anorexia and weight loss are invariable features of the disorder.

(3)   Increase in gastrointestinal symptom maybe misdiagnosed with primary intraabdominal process.

(4)   Hypotension is present in about 90% of patients and is accompanied by orthostatic symptoms and occasionally syncope. Salt cravings occur in about 20% of patients.

(5)   Severe hypoglycemia may occur in children. This finding is unusual in adults but may be provoked by fasting, fever, infection or nausea and vomiting especially in acute adrenal crisis.

(6)   Amenorrhea, loss of axillary and pubic hair as a result of decreased secretion of adrenal androgens.

Types of Primary Adrenocortical Deficiency

(1)   Autoimmune Adrenocortical Insufficiency

Characterized by lymphocytic infiltration of the adrenal cortex histologically. The adrenals are small and atrophic and the capsule is thickened. The adrenal medulla is preserved, though cortical cells are largely absent, show degenerative changes and are surrounded by a fibrous stroma and lymphocytic infiltrates.

Alopexia, malabsorption syndrome, chronic hepatitis, vitiligo and pernicious anemia are also common.

Two polyglandular syndromes accompanying Autoimmune Adrenocortical Insufficiency

(a)   Adrenal insufficiency, hyperparathyroidism and chronic mucocutaneous candidiasis

(b)   Adrenal insufficiency, Hashimoto’s thyroiditis and insulin – dependent diabetes mellitus

(c)    Ovarium failure is common in both syndrome

(2)   Adrenocortical insufficiency due to invasive and hemorrhagic disorders

(a)   Adrenal tuberculosis and other destructive cause – due to hematogenous infection of the cortex and usually occurs as a complication of systemic tuberculous infection (lung, gastrointestinal tract or kidney). The adrenal glands are replaced by caseous necrosis; both cortical and medullary tissue is destroyed. Calcification of the adrenals is frequent and is radiologically demonstrable.

(b)   Bilateral adrenal hemorrhage – in children, cause by fulminant meningococcemia and Pseudomonas septicemia.

– in adults, cause by anticoagulant therapy given for other major illnesses, septicemia, coagulation disorder, adrenal vein thrombosis, adrenal metastases, trauma, abdominal surgery and obstetric gestational and postpartum complication.

b.     Secondary Adrenocortical Deficiency

Manifested by weakness, lethargy, easy fatigability, anorexia, nausea and occasionally vomiting. Myalgia and arthralgia also occur. Hypoglycemia is occasionally the presenting features. Acute decompensation with severe hypotension or shock unresponsive to vasopressors may occur.


Laboratory diagnosis of ACTH

1.      ACTH stimulation test

a.      Procedure

Two 24 hour urine collections are obtained, prior to and the day after. The rest for baseline 17–KS and 17–OHCS or ketogenic steroid is determined.

25 I.U. of ACTH is added to 500 ml of normal saline and is administered to a patient continuously for exactly 8 hours. The timing of the infusion is critical.

Urine is collected during the administration and is continued for a total of 24 hours. It is assayed for 17–OHCS and 17–KS.

b.     Result

Normal person – urinary steroid level is raised three to five times above the baseline

No increase is observed in

(1)   Addison’s disease
(2)   Primary adrenocortical failure

Very little increase above the already high value in:

(1)   Cushing’s  syndrome
(2)   Adrenocortical tumors

Little if any response in

(1)   Congenital adrenal hyperplasia


2.      Metopirone test (metyrapone)

a.      Method

Overnight test: Metyrapone is given orally between 11 and 12 PM with a snack to minimize gastrointestinal discomfort. The dose is 2 grams for patients weighing less than 70 kg; 2.5 grams for patients weighing 79 – 90 kg and 3 grams for patients weighing over 90 kg.

Three day test: Twenty four hour urine collections are made for 3 consecutive days and metyrapone, 750 mg is given every 4 hours for six doses on the second day.

b.     Sample collection

Overnight test: Blood for plasma 11–deoxycortisol and cortisol determination is obtained at 8 AM in the morning after the metyrapone is given.

Three day test: The three consecutive 24–hour urine samples are analyzed for 17–hydroxycorticosteroids and creatinine determinations.

c.       Possible side effects, contraindication:

Gastrointestinal upset may occur. Adrenal insufficiency may occur. Metyrapone should not be used in sick patients or those in whom primary adrenal insufficiency is suspected.

d.     Interpretation

Overnight test: Serum 11–deoxycortisol should increase to >7 ug/dL (0.19 umol/L). Cortisol should be <10 ug/dl (0.28 umol/L) in order to ensure adequate inhibition of 11β–hydroxylation.

Three day test: Urine 17–hydroxycorticosteroid should be double on day 2 or 3.


3.      Dexamethasone suppression test

This is used to differentiate Cushing’s syndrome caused by adrenal tumor from adrenal hyperplasia. ACTH is suppressed with a synthetic glucocorticoid, dexamethasone.

A drug is administered in doses of 0.5 mg at 6 hours intervals on two successive days and 2 mg at 6 hours intervals in the following days. Prior to the test, two 24 hour baseline control urine specimen are collected and a specimen on each second day of suppression therapy (0.5 mg and 2 mg, respectively) for measurement of 17–OHCS or 17–ketogenic steroids levels.

Normal subjects show a decrease of about one–half in urinary corticosteroid levels after administration of 0.5 mg dexamethasone.

In bilateral adrenal hyperplasia, suppression will be observed only after repetition of the procedure with the larger dose. Normal subjects show suppression to less than 3 mg/day or 2 mg dose but patients with Cushing’s syndrome do not suppress.


4.      CRH Test

a.      Method

CRH (1 ug/kg) is given intramuscularly as a bolus injection.

b.     Sample collection

Blood samples for ACTH and cortisol are taken at 0,15, 30 and 60 minutes.

c.       Contraindication

Flushing often occurs. Transient tachycardia and hypotension have also been reported.

d.     Interpretation

The ACTH response is dependent on the assay utilized and occurs 15 minutes after CRH is administered. The peak cortisol response occurs at 30 – 60 minutes and is usually greater than 10 ug/dl (276 nmol/L)

D.    The Gonadotropins

1.      Follicle Stimulating Hormone (FSH): Functions:

a.      In females, stimulates primary graafian follicles to grow toward maturity and stimulates follicle cells to secrete estrogen

b.      In males, stimulates development of the seminiferous tubules of the testes and maintains spermatogenesis.

2.      Luteinizing Hormone (LH): Functions:

a.      In females, stimulates the formation and activity of the corpus luteum of the ovary. It also stimulates corpus luteum to secrete estrogen and progesterone.

b.      In males, stimulates interstitial cells in the testes to develop and secrete testosterone.

Regulation of secretion of LH and FSH

1.      Episodic secretion

2.      Positive feedback

During the menstrual cycle, estrogens provide a positive influence on GnRH effects on LH and FSH secretion, and the rise in estrogen during the follicular phase is the stimulus for the LH and FSH ovulatory surge. This phenomenon suggests that the secretion of estrogen is to some extent influenced by an intrinsic ovarian cycle. Progesterone amplifies the duration of the LH and FSH surge and augments the effect of estrogen. After this midcycle surge, the developed egg leaves the ovary. Ovulation occurs approximately 10–12 hours after the LH peak and 24–36 hours after the estradiol peak. The remaining follicular cells in the ovary are converted, under the influence of LH, to a progesterone–secreting structure, the corpus luteum. After about 12 days, the corpus luteum involutes, resulting in decreased estrogen and progesterone levels and then uterine bleeding.

3.      Negative feedback

In women, primary gonadal failure or menopause results in elevations of LH and FSH, which can be suppressed with long term, high dose estrogen therapy. However, a shorter duration of low dose estrogen may enhance the LH response to GnRH.

In men, primary gonadal failure with low circulating testosterone levels is also associated with elevated gonadotropins. However, testosterone is not the sole inhibitor of gonadotropins in men, since selective destruction of the tubules (e.g., by cyclophosphamide therapy) results in azoospermia and elevation of only FSH.

Inhibin, a polypeptide secreted by the Sertoli cells of the seminiferous tubules, is the major factor that inhibits FSH secretion by negative feedback. Androgens stimulate inhibin production, thus peptide may help to locally regulate spermatogenesis.


Laboratory evaluation of LH and FSH        

1.      GnRH Test

Method

The patient should be at rest but need not be fasting. Give GnRH (gonadorelin), 100 ug intravenously, over 15 seconds.

Sample collection

Blood samples for LH and FSH determinations are taken at 0, 30 and 60 minutes. Since the FSH response is somewhat delayed, a 90 minutes specimen may be necessary.

Possible side effects / Contraindication

Side effects are rare and no contraindication have been found

Interpretation

This response is dependent on sex and time of the menstrual cycle. An increase of LH of 1.3 – 2.6 ug/L (11.7 – 23.4 IU/L) is considered to be normal; FSH usually responds more slowly and less markedly and may not increase in healthy subjects.

2.      Clomiphene Test

Method

Clomiphene is administered orally. For women, give 100 mg daily for 5 days (being on day 5 of the cycle if the patient is menstruating); for men, give 100 mg daily for 7–10 days.

Sample collection

Blood for LH and FSH determinations is drawn before and after clomiphene is given.

Possible side effects / Contraindications

This drug may, of course, stimulate ovulation, and women should be advised accordingly.

Interpretation

In women, LH and FSH levels peak on the fifth day to a level above the normal range. After the fifth day, LH and FSH levels decline.

In men, LH should double after 1 week; FSH will also increase, but to a lesser extent.

E.     The Thyroid Stimulating Hormone (FSH)

Functions of TSH or thyrotropin

1.      Increases the size of the thyroid follicular cells.
2.      Increases he uptake of iodides by the thyroid cells for the ECF.
3.      Increases the release of the thyroxine from the thyroid colloid follicles. TSH stimulates growth of the thyroid and biosynthesis of thyroxine.

Regulation of secretion of TSH

1.      Regulation of TSH is also under control of thyrotropin–releasing hormone (TRH) released by hypothalamus

2.      TSH secreted by the pituitary binds to receptors on the basal side of the follicular cell resulting in rapid stimulation of both endocytosis (transport of thyroglobulin from the lumen into circulation on the basal membrane side) and exocytosis (transport of thyroglobulin into the follicular lumen), making extensive movement of the apical membrane necessary.

3.      The membrane formation and redistribution are activated by adenylate cyclase and consequential increase of cAMP is evident.

4.      Initial stimulation of TSH (0–5 minutes) results in a decrease in exocytic vesicles and a corresponding increase in the apical membrane surface area returns to prestimulation levels, and there is a marked increase in the surface area of the endocytic structures.

5.      Within 20–30 minutes, the endocytic structures separate into pseudopods and colloid droplets.

6.      The colloid droplets fuse with lysosomes, digesting thyroglobulin and present thyroid hormone to the basal membrane surface.


7.      This cascade of events accounts for the rapid increase of thyroid hormone observed in response to TSH stimulation. Slightly longer TSH stimulation enhances exocytic processes, resulting in formation of prothyroid hormone storage (thyroglobulin) in the follicular lesions. 




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