(FT4) in the circulation. Thyroxine-binding globulin (TBG) is by far the most important determinant of the overall binding of T4. A measure of TBG should provide a good

 

When Cushing’s syndrome is present due to

adrenocortical hyperplasia, 17-hydroxycorticosteroid

excretion levels may reach 75–100 mg/24 hours. The

response is usually absent in the presence of the usually

more autonomous adrenal carcinoma. The ACTH

stimulation of patients with the adrenogenital syndrome

produces an excessive response in 17-ketosteroid levels,

not noted in cases of idiopathic hirsutism.

In Addison’s disease, the 4 hours corticotropin test

elicits a fall of less than 50% in circulating eosinophils.

In the 8 hours IV corticotropin test and the corticotropin

gelatin solution alternate, the addisonian patient shows

little or no change in circulating eosinophils or urinary

or plasma hormone levels. In order to rule out adrenal

insufficiency unequivocally, at least 3 days of method –2

are recommended. The synthetic ACTH method is of value

in identifying a normal adrenal response. If no response is

observed, method –2 should be applied.

Patients with hypopituitarism and ACTH insufficiency

show varying responses depending upon the degree of

adrenocortical involution. Repetition of the test on 3–5

consecutive days shows a gradual rise in 17-ketosteroid

and 17-hydroxycorticosteroid output and an increasing

eosinopenia. Plasma cortisol usually shows an increase of

3–4 times in 4 hours.

All tests other than the IV administration of corticotropin are subject to occasional false-negative responses

due to extravascular inactivation of corticotropin.

Adrenocortical Inhibition Test

1. Test with dexamethasone

Tests to determine the suppressibility of ACTH by glucocorticoids may be done using dexamethasone. Dexamethasone has little effect on sodium or potassium balance

and does not interfere with corticosteroid determination.

Method: 24 hours urine specimens are collected for

analysis on 3 successive days. After the first specimen is

obtained, 0.5 mg dexamethasone is given every 6 hours

by mouth for 2 days. Excretion level is measured on each

24 hours urine specimen. In normal individuals, the

repeated administration of the 0.5 mg dose reduces by 50%

the basal excretion of 17-hydroxycorticosteroids, urinary

cortisol, and plasma cortisol per 24 hours by the end of

the second day. If suppression is not obtained, the test is

repeated with a dose of 2 mg every 6 hours for 2 days.

Interpretation: The increased corticosteroid levels, which

occur in patients with adrenocortical hyperplasia, is

usually suppressed only by the larger (2 mg) doses, whereas

the corticosteroid levels in patients with autonomous

adrenocortical neoplasms may not be suppressed even

with higher doses.

2. Rapid dexamethasone test

1 mg of dexamethasone at midnight will reduce normal

plasma levels to less than 5 ng/ml by 8.00 am. This will

effectively rule out adrenal overactivity.

3. Tests with metyrapone (an endogenous ACTH test)

Inhibition of 11 β-hydroxylase by metyrapone results

in reduced blood levels of cortisol and loss of cortisol

inhibition by ACTH. The ability of the pituitary to respond

to this stimulus by release of ACTH may be measured by

noting the increment in 17-ketosteroids, 17-hydroxycorticosteroids, or the metabolite of 11-deoxycortisol

(compound-S) produced by the adrenal and excreted in

the urine. Compound-S is formed by the adrenal cortex

after inhibition of cortisol (hydrocortisone) formation.

The rise in urinary 17-hydroxycorticosteroids is a result of

increased amounts of this metabolite.

Method

1. IV-Metyrapone, 30 mg/kg body weight in 1 liter of

normal saline is given IV for 4 hours, starting between

8.00 and 10.00 am. The same method is followed on

another day, except that 25 USP units of ACTH are

added to metyrapone infusion to compare the functional capacity of maximally stimulated adrenals

with the response evoked by the endogenous ACTH

released after administration of metyrapone alone.

2. Oral: A basal 24 hours urine 17-hydroxycorticosteroid

measurement is obtained. 0.75 g of metyrapone are

given every 4 hours for 6 doses. A second 24 hours

urine for 17-hydroxycorticosteroid levels is obtained

the day following the drug administration.

 Interpretation: Normal subjects may double their basal

24 hours 17-hydroxycorticosteroid excretion after

metyrapone, and ACTH adds nothing to this response.

Hypopituitary patients show no increase in excretion

with metyrapone, but their response to exogenous

ACTH is adequate. Addisonian patients respond to

neither stimulus. IV metyrapone causes a vigorous

response in patients with adrenal hyperplasia, but

adrenal tumors fail to respond. Chlorpromazine blocks

metyrapone responses.

 Direct measurements of the metabolite of compound-S

and its plasma level are also available, and the direct

measurement of their increase provides a more direct

assessment of ACTH release.

742 Concise Book of Medical Laboratory Technology: Methods and Interpretations Cortisol Abnormalities

Excess

Cushing’s syndrome.

Deficiency

Congenital adrenal hyperplasia.

At high concentrations (greater than physiologic),

glucocorticoids (such as hydrocortisone or prednisone)

are useful for the treatment of allergies and inflammation.

Each step of the inflammatory process is blocked by

glucocorticoids when given systemically (an IV injection

or orally). Topical application of glucocorticoids have

anti-inflammatory effects for the local area. The antiinflammatory activity of glucocorticoids is thought to be

due primarily to the stabilization of cell membranes and the

inhibition of phospholipases and therefore prostaglandin

synthesis. The immune response can also be suppressed

by the use of glucocorticoids. Eosinophils and lymphocytes decrease in the circulation affecting both cellular

and humoral immunity. The glucocorticoids are used for

many other conditions including asthma, renal diseases,

rheumatic disorders such as lupus and inflammatory

bowel disease.

ADRENAL MEDULLA

The adrenal medullary hormones are catecholamines—

epinephrine and norepinephrine, the parent compound

from which epinephrine is formed by addition of a methyl

group.

Epinephrine is sympathomimetic, increases cardiac

output and rate, systolic blood pressure, blood glucose,

hepatic glycogenolysis, basal metabolic rate, sweating

and causes mydriasis and skin-vessel constriction. By

contrast, norepinephrine causes bradycardia, peripheral

vasoconstriction and rise in diastolic blood pressure, and

has much less prominent metabolic effects.

Clinical Disorders

Deficiency

Hypotension. Idiopathic spontaneous hypoglycemia (failure

of epinephrine response to hypoglycemia).

Excess

Paroxysmal or persistent hypertension, headache,

sweating, tachycardia and elevated blood glucose.

THYROID

The thyroid is a large endocrine organ that functions mostly

to control metabolism. It is located in the neck between

the trachea and larynx and is bilobed with a connecting

isthmus (Fig. 24.3).

The gland is composed of many tiny follicles, that are in

effect, each a separately functioning gland with a singlelayer epithelial lining. Each follicle accumulates a storage

form of the circulating thyroid hormones, thyroglobulin.

Thyroglobulin is a large protein molecule that contains

multiple copies of the amino acid, tyrosine. The thyroid

hormones are very simple modifications of the amino

acid, tyrosine. Both T4 and T3 enter into cells and bind

to an intracellular receptors whereby they increase the

metabolic capabilities of the cell. Thyroid hormones are

necessary for normal growth and development. They

have metabolic effects on protein synthesis, lipid and

carbohydrate metabolism.

The polypeptide hormone calcitonin is also produced

by the parafolicular cells within thyroid. It functions in

calcium maintenance to decrease the levels of calcium

in the blood. When serum calcium levels are excessive,

calcitonin is released. It inhibits bone resorption (by

inhibiting osteoclast activity), allows the loss of calcium in

the urine and, therefore, decreases calcium in the blood. It

opposes the action of parathyroid hormone and has been

used clinically for the treatment of osteoporosis.

Markers of the Gland

Hormones T3, T4, FT3, FT4

Structural Tg–(thyroglobulin)

Antibodies Anti-Tg, Anti-TPO

Carrier protein TBG (thyroxine-binding globulin)

Pituitary marker TSH (thyroxine-stimulating hormone)

FIG. 24.3: Thyroid gland—anatomical position

The Endocrine System 743

Role of Carrier Protein

The hormone synthesized in the gland is transported to

various parts by carrier proteins.

TBG: Thyroxine-binding globulin protein is the major

carrier protein. It has more affinity to T4 than T3.

TBPA: Thyroxine-binding prealbumin, binds to T3 than

T4.

Thyroxine-binding Globulin (TBG) Test

Almost all the thyroid hormones in the bloodstream are

bound to proteins, and these thyroxine-binding proteins

play an important role in regulating the free thyroxine

(FT4) in the circulation. Thyroxine-binding globulin (TBG)

is by far the most important determinant of the overall

binding of T4. A measure of TBG should provide a good

approximation of the thyroxine-binding function of the

blood. This can be a valuable aid in clarification of many

clinical conditions. The TBG test is a direct measurement

of the total thyroxine-binding capacity of the specific

thyroxine-binding interalpha (alpha1-alpha2) globulin in

serum.