fraction is separated from unbound antigen by decantation or aspiration. The enzyme activity in the antibody bound fraction is directly proportional to the native antigen

 

and insulin secretion by blood glucose concentrations.

The MSH secretion is inhibited by glucocorticoids and

causes skin pigmentation when cortisol is absent.

Clinical tropic hormone deficiencies may be

single or multiple. If single, deficiency is most often

of gonadotropins and leads to failure of development

or involution of the sexual organs. Less often, TSH or

ACTH is diminished. Hypothalamic-releasing factors for

corticotropin (CRF), thyroid-stimulating hormone (TRF),

and luteinizing hormone (LH-RF) have been identified as

well as a probable growth hormone-releasing factor and

prolactin and MSH inhibitory factors.

Causes of Hypopituitarism Resulting in

Multiple or Single Deficiencies

Intrinsic Pituitary Disease

1. Neoplasms:

• Chromophobe adenomas

• Craniopharyngiomas

• Carcinoma

• Metastatic carcinoma.

2. Histiocytosis

3. Infections

4. Metabolic disorders, e.g. hemochromatosis

5. Vascular disorder:

• Infarction (postpartum necrosis)

• Intracerebral vascular malformation.

Extrinsic Pituitary Disorders

1. Surgical, heavy particle, or other forms of pituitary

ablation

2. Hypothalamic neoplasms or metastatic deposits.

ANTERIOR LOBE: GROWTH HORMONE (GH)

Growth hormone has no specific target tissue. All cells of

the human body are affected by this hormone. It is very

important in the growing child, but it remains essential to

many bodily functions throughout life. The GH has effects

on the growth of bone and cartilage, protein metabolism,

RNA formation, electrolyte balance, fat and glucose

FIG. 24.2: Pituitary gland metabolism.

The Endocrine System 733

Actions

The GH stimulates growth of all nonendocrine tissues in the

body and may affect secretions of the medulla and pancreas.

Normal plasma levels are less than 3 ng/mL (females,

higher than males), and after insulin hypoglycemia rise

to approximately 25 ng/mL. It increases nonesterified

fatty acids. Also, in diabetic and acromegalic patients it is

diabetogenic. The GH produces nitrogen retention in man

and monkeys. It may stimulate the growth of malignant

craniopharyngioma.

Clinical Disorders

1. Deficiency: Dwarfism

2. Excess: Gigantism (prepubertal), acromegaly (postpubertal).

METHOD OF EVALUATION:

STREPTAVIDIN-BIOTIN ELISA

Expected Ranges of Values

Because of the pulsatile and sporadic nature of growth

hormone secretion, reference intervals for basal values

are without meaning. However, normal levels rarely have

been reported above 50 ng/mL. The well rested, fasting

(12 hours) subjects should have GH values of 20 ng/mL or

less.

With this caveat in mind, 75 apparently healthy adults

were assayed the hGH immunoassay. The results are

depicted below.

Expected values for the gh iema test system (in ng/mL)

N Mean Range

Specimens 75 2.8 0-17

Provocative tests for hGH response are normally used

to access the function of the anterior pituitary. Stimulatory

procedures measure the secretion ability of the anterior

pituitary to release hGH. Children suspected of growth

retardation are common subjects for stimulatory testing.

Several dynamic tests are available to induce GH release:

exercise (3), L-dopa administration (4), insulin tolerance

test (5), and arginine infusion (6). Each laboratory should

assess the normal response, but a peak GH release in

excess of 8 ng/mL is probably normal in all cases.

Inhibitory testing measure the suppression of hGH

release from the anterior pituitary. Inhibitory tests are

useful in ascertaining growth hormone excess and the

resulting conditions of gigantism and acromegaly. The

glucose tolerance test is a dynamic test to measure growth

hormone excess. The failure of hGH levels to fall below

1 ng/mL within 60–120 minutes suggests excess hGH

secretion.

It is important to keep in mind that establishment of

a range of values, which can be expected to be found by

a given method for a population of “normal” persons, is

dependent upon a multiplicity of factors: the specificity

of the method, the population tested and the precision of

the method in the hands of the analyst. For these reasons,

each laboratory should depend upon the range of expected

values established by the manufacturer only until an inhouse range can be determined by the analysts using the

method with a population indigenous to the area in which

the laboratory is located.

Growth Hormone (hGH)

Chemiluminescence Immunoassay

(Courtesy: Lilac Medicare)

Determination of Growth Hormone Concentration in

Human Serum by a Microplate Immunoenzymometric

Assay

Summary and Explanation of the Test

Growth hormone (hGH, somatotropin), secreted from

the anterior pituitary, is a polypeptide with two intrachain disulfide bridges, which circulates free or bound to

number of different GH-binding proteins. Several forms

of growth hormone have been identified with the major

being of molecular weight 22,000 daltons containing

191 amino acid residues. A 20,000-dalton variant, which

posseses all known biological functions of GH, has

also been demonstrated to be important. The primary

biological actions of the hormone are in direct growth

promoting. GH exerts its effect directly on target organs

such as bones and muscles and indirectly through the

release of somatomedins, a family of insulin-like growth

factor (IGF) hormones, produced in the liver. In particular,

somatotropin C (IGF-1) is essential for bone growth during

childhood.

The clinical usefulness of the measurement of growth

hormone (GH) in children has been well established in

ascertaining linear bone growth along the epiphyseal

plate. Abnormal elevated levels lead to gigantism while

complete absence slows the rate of growth to one-third to

one-half of normal. In adults, the epiphyseal growth plates

have fused; GH excess gradually produces acromegaly, a

coarse thickening of the bones of the skull, hands and feet.

In this method, GH calibrator, patient specimen

or control is first added to a streptavidin coated well.

Biotinylated monoclonal and enzyme labeled antibodies

(directed against distinct and different epitopes of GH)

734 Concise Book of Medical Laboratory Technology: Methods and Interpretations are added and the reactants mixed. Reaction between the

various GH antibodies and native GH forms a sandwich

complex that binds with the streptavidin coated to the

well.

After the completion of the required incubation period,

the enzyme-growth hormone antibody bound conjugate

is separated from the unbound enzyme-growth hormone

conjugate by aspiration or decantation. The activity of the

enzyme present on the surface of the well is quantitated by

reaction with a suitable substrate to produce light.

The employment of several serum references of known

growth hormone levels permits the construction of a

dose response curve of activity and concentration. From

comparison to the dose response curve, an unknown

specimen’s activity can be correlated with growth hormone

concentration.

Principle

Immunoenzymometric Assay

The essential reagents required for an immunoenzymometric assay include high affinity and specificity

antibodies (enzyme and immobilized), with different

and distinct epitope recognition, in excess, and native

antigen. In this procedure, the immobilization takes

place during the assay at the surface of a microplate well

through the interaction of streptavidin coated on the well

and exogenously added biotinylated monoclonal anti-GH

antibody.

Upon mixing monoclonal biotinylated antibody,

the enzyme-labeled antibody and a serum containing

the native antigen, reaction results between the native

antigen and the antibodies, without competition or steric

hindrance, to form a soluble sandwich complex. The

interaction is illustrated by the following equation:

EnzAb(x-GH) + AgGH + BtnAb(m)

 Ka ↑↓ Ka

EnzAb(x-GH)-AgGH-BtnAb(m)

BtnAb(m) = Biotinylated Monoclonal Antibody (Excess

Quantity)

AgGH = Native Antigen (Variable Quantity)

EnzAb(p)= Enzyme labeled Antibody (Excess Quantity)

EnzAb(x-GH)-AgGH-BtnAb(m)= Sandwich Complex

Ka = Rate Constant of Association

Ka = Rate Constant of Dissociation

Simultaneously, the complex is deposited to the well

through the high affinity reaction of streptavidin and

biotinylated antibody. This interaction is illustrated below:

EnzAb(x-GH)-AgGH-BtnAb(m) + StreptCW ⇒ immobilized

complex

StreptCW = Streptavidin immobilized on well

Immobilized complex = Sandwich complex bound to the

well.

After equilibrium is attained, the antibody bound

fraction is separated from unbound antigen by decantation

or aspiration. The enzyme activity in the antibody bound

fraction is directly proportional to the native antigen

concentration. By utilizing several different serum

references of known antigen values, a dose response curve

is generated from which the antigen concentration of an

unknown is ascertained.

Clinical Condition

Deficiency

In children, careful attention to growth records and X-ray

of the skull and hands (especially of wrists) for bone age,