Adrenocortical Function and Laboratory
Assessment
Serkan SAYINER, DVM PhD. Assist. Prof.
Near East University, Faculty of Veterinary Medicine, Department of Biochemistry
serkan.sayiner@neu.edu.tr
Adrenal Glands
Glandula suprarenalis
Adrenal Glands
▪In mammals, the adrenal glands are bilateral structures located craniomedial to the kidneys.
▪The center of the gland, the medulla, comprises coalesced chromaffin cells of neuroectodermal origin that secrete
epinephrine or norepinephrine.
▪The surrounding adrenal cortex arises from mesoderm and can be divided histologically into three zones.
• Zona glomerulosa, zona fasciculata, and zona reticularis.
Adrenal Glands
▪The secretion of the mammalian adrenal cortex comprises three main categories of hormones.
• The zona glomerulosa produces mineralocorticoids (aldosterone and deoxycorticosterone), which maintain salt balance.
• The cells of the zona fasciculata secrete glucocorticoids (cortisol and corticosterone), which are primarily involved in
carbohydrate metabolism.
• The third category of adrenocortical hormones, the androgens (e.g., androstenedione), is produced in the zona reticularis. This zone to a minor degree also secretes glucocorticoids and other hormones such as progesterone and estrogens.
Adrenal Glands
▪ Cholesterol, derived from food and from endogenous synthesis via acetyl-CoA, is the principal starting compound in
steroidogenesis.
▪ The adrenal gland is enriched in receptors that internalize low- and high-density lipoproteins.
• This uptake mechanism increases when the adrenal is stimulated and provides the major cholesterol source. Subsequent steps occur in the mitochondrion or at the endoplasmic reticulum.
• Two classes of enzymes are involved in the synthesis of steroids, the cytochrome P450 (CYP) heme-containing proteins that catalyze
mainly hydroxylation reactions and the hydroxysteroid
dehydrogenases (HSD) that are involved in oxidation and reduction reactions
Adrenal Glands
▪The steroidogenic activity of the two inner zones of the
adrenal cortex is predominantly controlled by the pituitary hormone ACTH.
• The action of ACTH on the adrenal gland is rapid; within minutes of its release, the concentration of steroids in the adrenal
venous blood increases.
▪The production of aldosterone in the zona glomerulosa is adapted to the sodium and potassium status of the
organism by a complex, multifactorial, and mainly extrapituitary control system.
Adrenal Glands
▪At normal concentrations, only about 10% of the total blood cortisol and corticosterone is in the free.
▪70% of the plasma cortisol is bound to a globulin called transcortin or corticosteroid-binding globulin (CBG).
• Transcortin has a high affinity for cortisol and corticosterone, but its binding capacity is limited. Another 20% of plasma cortisol is bound to albumin although its affinity for cortisol is much less than that of transcortin.
▪Plasma aldosterone is predominantly bound to albumin, which has a low affinity.
Adrenal Glands
▪Only unbound cortisol and its metabolites are filterable at the glomerulus. In most mammals the kidneys account for 50% to 80% of the excretion of the metabolized steroids.
The remainder is lost via the gut.
• To render them suitable for renal elimination, the steroids are inactivated and made more water soluble through enzymatic modifications. The liver is the major organ responsible for steroid inactivation and conjugation to form water-soluble compounds.
▪Most of aldosterone metabolism takes place in liver and
kidney, but the intestine and spleen might also contribute to a minor degree.
Hypoadrenocorticism
▪Adrenocortical hypofunction includes all conditions in which the secretion of adrenal steroid hormones falls below the
requirements of the animal.
• Primary hypoadrenocorticism or Addison’s disease is the result of deficiency of both glucocorticoid and mineralocorticoid secretion from the adrenal cortices (primary adrenocortical
failure). It is well known in the dog and rare in the cat.
• Secondary hypoadrenocorticism is the result of pituitary
ACTH deficiency, causing decreased glucocorticoid secretion. It is observed occasionally in dogs with pituitary insufficiency.
Hypoadrenocorticism
▪Animals receiving long-term corticosteroid treatment, despite physical and biochemical hyperadrenocorticoid changes, develop secondary adrenocortical insufficiency (iatrogenic hypoadrenocortisim) because of prolonged hypothalamo-pituitary suppression.
▪Atrophy of the two inner zones of the adrenal cortex results from the loss of endogenous ACTH stimulation. This is
observed in the species in which corticosteroid therapy is common practice such as the dog and the horse.
Hyperadrenocorticism
▪In principle, the adrenal cortex of animals might, as in humans, give rise to three distinct clinical syndromes of hyperfunction:
• Mineralocorticoid excess or hyperaldosteronism (Conn’s syndrome),
• Glucocorticoid excess or hypercortisolism (Cushing’s syndrome), and
• Androgen excess (adrenogenital syndrome).
▪Hypercortisolism is common in the dog though rare in the cat and the horse.
Hyperadrenocorticism
▪ In about 15% of hyperadrenocorticoid dogs, the disease is due to a primary adrenocortical tumor (ACTH independent or adrenal dependent). In the other 85% of cases, the
(ACTH dependent or pituitary dependent) hypercortisolism is associated with a pituitary lesion, producing excess
ACTH.
▪ In cats, hypercortisolism (Cushing’s syndrome) is usually concurrent with diabetes mellitus and may be either primary adrenocortical or pituitary dependent.
▪ Primary hyperaldosteronism in cats may be due to an adrenocortical tumor. The nontumorous form of primary
hyperaldosteronism in cats is associated with progressive renal disease.
Routine Laboratory Tests
▪In both hypoadrenocorticism and hyperadrenocorticism,
there are a number of abnormal laboratory findings that are more common than in other diseases.
• In primary hypoadrenocorticism, the decreased aldosterone production results in hyponatremia and hyperkalemia.
• In hyperadrenocorticism, the classic hematological abnormality is eosinopenia, which may be associated with lymphopenia and occasionally with leukocytosis and erythrocytosis.
• The elevated ALP is the most common laboratory
abnormality in dogs with corticosteroid excess (either exogenous or endogenous).
Tests of Basal Adrenocortical Function
▪ Plasma corticoid concentrations: are subject to considerable variation owing to the pulsatile nature of the secretion,
physiological variation, and alterations in transport proteins.
Therefore, single determinations are regarded of little diagnostic value in assessing hypo- or hyperadrenocorticism.
▪ Urinary Corticoids: For the dog, corticoids (largely cortisol) are measured by radioimmunoassay and related to the creatinine
concentration; urinary cortisol/creatinine ratio (UCCR).
UCCR was found to have a higher diagnostic accuracy in the diagnosis of hyperadrenocorticism than the commonly used dexamethasone screening test.
Tests of Basal Adrenocortical Function
▪17-hydroxyprogesterone (17-OHP): Certain functional
adrenocortical tumours do not have cortisol as their principle secretory product but instead the tumour has developed in such a way that it is adrenal steroid precursors that are
released into the circulation.
• Many dogs with this type of functional adrenal tumour will have clinical signs suggestive of hyperadrenocorticism. A common cortisol response to ACTH in these cases is a “flat-line, mid- range” pattern. Measuring 17OHP before and after ACTH
stimulation (same sample as you would take for cortisol) can be very helpful in identifying these tumours.
Tests of Basal Adrenocortical Function
▪Plasma ACTH: Resting plasma ACTH concentrations are not useful in the diagnosis of hyperadrenocorticism. Its
value lies mainly in the differential diagnosis of hyperadrenocorticism. In animals with primary
adrenocortical failure, plasma ACTH concentrations are usually extremely high. Plasma ACTH measurements are useful when in doubt about a primary or secondary
adrenocortical insufficiency. Instead, it is more practical to perform ACTH stimulation test.
Dynamic Tests
▪Various maneuvers have been introduced to test the
physiology of the hypothalamo-pituitary-adrenocortical axis.
▪As in other areas of endocrinology, stimulation tests are used when hypofunction is suspected, and suppression tests are used when hyperfunction is suspected.
▪It is therefore important to rule out nonpathological factors that can alter hormone concentrations. Apart from stress, the nutritional state of the animal may also play a role.
ACTH Stimulation Test (Dog)
▪The ACTH stimulation test is performed when there is suspicion of decreased adrenocortical reserve capacity
1. primary adrenocortical insufficiency (Addison’s disease) and,
2. (iatrogenic) secondary adrenocortical insufficiency.
▪Blood for cortisol measurements is collected immediately before and 90 min after intravenous administration of 0.25 mg synthetic ACTH.
ACTH Stimulation Test (Dog)
▪In healthy dogs, the cortisol concentrations rise to 270 to 690nmol/L.
▪In Addison’s disease, the control value is usually low and does not increase following ACTH administration.
▪In animals with secondary adrenocortical insufficiency, the basal cortisol values may be low as well, and, depending on the severity (duration) of the ACTH deficiency, the cortisol
rise is subnormal or absent.
▪The standard ACTH stimulation test is the test of choice in monitoring anti-adrenal therapy.
ACTH Stimulation Test (Cat)
▪Hyperadrenocorticism is a rare condition in the cat, much more so than in the dog. In its absence, recommendations have been derived from studies in healthy cats and some extrapolated from the dog.
▪Blood for cortisol measurements is collected immediately
before, and 1 and 3 hour after intravenous administration of 0.125 mg synthetic ACTH (0.25mg may be used in cats over 5kg).
▪Normal cats will increase up to about 400 nmol/L.
Low-Dose Dexamethasone Suppression Test
▪The low-dose dexamethasone suppression test is used when hyperadrenocorticism (Cushing’s syndrome) is
suspected.
▪In the morning, 0.01 mg dexamethasone/kg body weight is administered intravenously.
▪Blood for cortisol measurements is collected immediately before and at 3 and 8 h after dexamethasone
administration.
Low-Dose Dexamethasone Suppression Test
▪A plasma cortisol concentration exceeding 40nmol/liter at 8 h after dexamethasone administration can be regarded as diagnostic for hyperadrenocorticism.
• The measurements at 0 and 3 h are not needed for the
diagnosis of hyperadrenocorticism but may be informative in the differential diagnosis.
• Quite commonly the high value at 8 h is preceded by a lower value at 3 h. Thus, at 8 h the pituitary-adrenocortical system
escapes from the suppression by dexamethasone. If the value at 3 or 8 h is at least 50% lower than the basal value, the
disease may be regarded as pituitary dependent.
High-Dose Dexamethasone Suppression Test
▪Differentiation between pituitary-dependent
hyperadrenocorticism and hypercorticism arising from adrenocortical tumors is possible with the high-dose dexamethasone suppression test.
▪Blood for cortisol determination is collected immediately before and 3 to 4 h after administration of 0.1-1.0 mg
dexamethasone/kg body weight i.v.
▪If the plasma cortisol declines by more than 50%, the
diagnosis of pituitary-dependent hyperadrenocorticism is justified.
Feline Combined High-Dose Dexamethasone/ACTH Test
▪ Because more than one diagnostic endpoint is assessed, this test may be more accurate than an ACTH stimulation test alone.
• Collect basal blood sample. Inject 0.1mg/kg Dexamethasone i/v.
• Collect a second blood sample at 2 hours.
• Immediately inject 0.125 mg of synthetic ACTH (Synacthen*) i/v.
• Collect a third blood sample at 3 hours (1 hour after ACTH).
▪ Normal cats show at least 50% suppression to a value
<40nmol/L after dexamethasone and normal cortisol response after ACTH stimulation (up to 400 nmol/L). Cats with HAC show little suppression after dexamethasone and an exaggerated
response after ACTH.
Urinary Cortisol/Creatinine Ratios
(with High-Dose Dexamethasone Test)
▪Determination of corticoid/creatinine ratios can be performed when hyperadrenocorticism is suspected.
▪The owner is asked to collect morning urine samples at set times (e.g., 7 A .M.) on 3 consecutive days. On the
preceding evenings, the animal should have its last walk at identical times (e.g., 11 P.M.). After collection of the second urine sample, dexamethasone is administered orally. At 8-h intervals, the owner administers 0.1 mg dexamethasone/kg body weight.
Urinary Corticoid/Creatinine Ratios
(with High-Dose Dexamethasone Test)
▪UCCR exceeding 10 x 10-6 can be regarded as compatible with hyperadrenocorticism.
▪When the ratio of the third urine sample is 50% lower than the mean of the first two ratios, the diagnosis of a pituitary- dependent hyperadrenocorticism is justified.
▪A lesser decrease may be due to either adrenocortical tumors or dexamethasone-resistant pituitary-dependent hyperadrenocorticism.
Your Questions?
Send to serkan.sayiner@neu.edu.tr
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