Endocrine Testing Protocols: Hypothalamic Pituitary Adrenal Axis - Endotext - NCBI Bookshelf
The ratio between adrenocorticotropic hormone levels and cortisol levels Clinical manifestations must be associated with biochemical tests. The ACTH stimulation test measures how well the adrenal glands Along with the blood tests, you may also have a urine cortisol test or urine. Tests that are used in general practice include the urine cortisol:creatinine ratio ( UC:CR), the ACTH stimulation test, the low-dose dexamethasone suppression.
Dexamethasone is a corticosteroid that exerts negative feedback on the hypothalamus and pituitary gland just as cortisol does. In a healthy animal, dexamethasone administration causes a decrease in CRH and ACTH and thus suppresses cortisol production for up to 48 hours. Typical cortisol levels for a healthy dog are 0. Imaging studies can also be used to differentiate PDH from adrenal tumor.
Understanding Common Endocrine Tests
In PDH, the adrenal glands, as seen by ultrasonography, magnetic resonance imaging, or computed tomography, are usually bilaterally enlarged. Magnetic resonance imaging or computed tomography can also identify a pituitary tumor. When an adrenal tumor is present, one adrenal gland is usually enlarged whereas the other, normal gland is atrophied from decreased ACTH stimulation. T3 is the more biologically active of the two, but more T4 is produced.
Only the free, unbound hormone is biologically active. Free T4 enters cells, where it is transformed into T3.
ACTH stimulation test
Hypothyroidism Hypothyroidism in dogs may be primary or secondary. In primary hypothyroidism, the thyroid gland is destroyed, either by replacement of thyroid tissue with adipose tissue idiopathic thyroid atrophyor by what is suspected to be an immune-mediated process targeting the gland lymphocytic thyroiditis. In secondary hypothyroidism, the pituitary gland fails to release adequate amounts of TSH. Diagnosis of hypothyroidism in dogs can be challenging because dogs with hypothyroidism and those that are healthy can have overlapping test results.
Clinical signs of hypothyroidism include lethargy, weight gain, heat-seeking behavior, and hair loss. Thyroid tests that are available through reference laboratories include measurement of total and free T4, total and free T3, TSH, and autoantibodies against thyroglobulin a precursor of T4 and T3. Total T4 tests measure bound and unbound T4, whereas free T4 tests measure only the unbound form. Total T4 is usually measured by radioimmunoassay.
Again, normal ranges vary depending on the reference laboratory, but there is an overlap in the range between healthy dogs e. If the total T4 is 1. Free T4 is measured either by radioimmunoassay or by equilibrium dialysis; only equilibrium dialysis has been shown to be sensitive. None of these has been shown to be useful in diagnosing canine hypothyroidism. In theory, TSH should be elevated in a dog with hypothyroidism because of decreased negative feedback to the pituitary gland.
If the dog is clinically healthy, the presence of autoantibodies does not necessarily mean that hypothyroidism will develop. As in animals with adrenal tumors, the excessive T4 and T3 exert negative feedback on the hypothalamus and pituitary glands, causing a decrease in TRH and TSH, but the hyperplastic gland or thyroid tumor functions independently of hormone control.
Hyperthyroidism is usually diagnosed on the basis of elevated total T4 or free T4 levels. If the T4 is normal but hyperthyroidism is still suspected, a T3 suppression test can be conducted. Levels of T4 are compared before and after administration of T3, which suppresses TSH and therefore suppresses T4 in healthy cats.
In cats with hyperthyroidism, the thyroid continues to produce T4 independently; therefore, no suppression is seen. Diabetes Mellitus Diabetes mellitus is most often characterized by deficient insulin secretion by the pancreatic islet or b cells. All cells, except erythrocytes and neurons, require insulin for glucose uptake; without insulin, they starve for energy while glucose builds to high levels in the bloodstream.
Clinical signs of uncomplicated diabetes mellitus include polyuria, polydipsia, polyphagia, and weight loss. The initial diagnosis of diabetes mellitus is fairly straightforward, but endocrine tests are frequently used in long-term monitoring. Diabetes is diagnosed by finding persistently elevated fasting blood glucose levels and glucose in the urine.
Monitoring tests include serum fructosamine and the glucose curve. Fructosaminerefers to serum proteins, primarily albumin, that are irreversibly bound to glucose. Because the fructosamine is related to the average glucose level, it is not affected by acute increases in glucose, such as those caused by the stress of venipuncture.
To conduct this test, the pet is hospitalized for the day. The owner feeds the pet its normal breakfast and administers its morning insulin. The initial blood glucose level is measured at the time of insulin injection or within 1 hour. Blood glucose levels are then measured every 1 to 2 hours throughout the day, and the results are plotted on a graph.
Conclusion Many tests may be required to diagnose an endocrine disorder, and further tests are required to monitor treatment. Technicians should be familiar with the rationale for conducting various tests. Lathan P, Tyler J: Pathogenesis and clinical features. Compend Contin Educ Pract Vet 27 2: Canine and Feline Endocrinology and Reproduction, ed 3. Screening tests to diagnose hyperadrenocorticism in cats and dogs.
Compend Contin Educ Pract Vet 22 1: Differentiating tests to evaluate hyperadrenocorticism in dogs and cats. Compend Contin Educ Pract Vet 22 2: Drugs that induce hepatic cytochrome P enzymes, such as barbiturates, phenytoin, rifampin and aminoglutethimide, increase the metabolism of dexamethasone and other steroids.
Measurement of serum dexamethasone a few hours after the last dose will help determine if there is abnormal metabolism. All these caveats are in addition to the other problems associated with measurement of cortisol as noted above, including the variable diurnal variation as well as interference with concurrent administration of glucocorticoids, estrogen or other medication that increase cortisol binding globulin.
A popular screening test for confirming hypercortisolism is the overnight 1 mg dexamethasone. A single dose of 1 mg is administered or 0. The dexamethasone dose is given prior to the diurnal rise in endogenous ACTH release and therefore suppresses the early AM cortisol. An intravenous line is placed 30 minutes before the test for rapid phlebotomy and to eliminate a temporary rise in cortisol associated with a needle stick.
Blood is drawn at ' and 0' for cortisol and ACTH 2 ml in a lavender top tube on ice. Blood is obtained at 15, 30, 60, 90,and minutes for cortisol and ACTH 2 ml in a lavender top tube on ice. The test can be performed at any time of the day, although the initial studies describing the test have been done in the morning.
The patient may experience slight nausea, metallic taste, urgency to urinate, a change in blood pressure either increase or decreasea change in heart rate, headaches, abdominal discomfort, facial flushing, and lightheadedness. These side effects are mild and last for only few minutes. It should be noted that the criterion for Cushing's disease is based on the presence of hypercortisolism.
The CRH test will not adequately differentiate subjects with pseudoCushings and those with true pituitary dependent Cushing's disease. Several investigators have found that modifications of the CRH stimulation test can increase further the sensitivity and specificity in the diagnosis of the etiology of Cushing's disease. While the simultaneous use of vasopressin can augment the response to CRH, dexamethasone can be used to suppress all but pathologic responses to CRH stimulation .
On the morning of the 3rd day an additional dose of dexamethasone is given at 6 AM. The patient arrives at the testing center by 8 AM and an intravenous line is placed 30 minutes before the test for rapid phlebotomy and to eliminate a temporary rise in cortisol associated with a needle stick.
Blood is obtained at 15, 30 60, 90and minutes for cortisol and ACTH 2 ml in a lavender top tube on ice. The test can be performed at any time of the day, although it is usually done in the morning. Side effects that the patient may experience are: A normal response would be a plasma cortisol concentration less than 1. Values of cortisol greater than 1. While this is a general recommendation, each laboratory should confirm based on the sensitivity of the respective cortisol assay.
Furthermore, it is important to confirm the serum level of dexamethasone at the time of the blood draw to assure patient compliance with the dexamethasone regimen. Once the diagnosis of ACTH dependent Cushing's syndrome has been made based on endocrinologic testing, the next step in the evaluation of such patients should be an MRI of the pituitary to confirm the presence of a pituitary mass.
However, subjecting a patient to surgical pituitary exploration in the absence of a demonstrable mass is likely to result in an unsuccessful surgery. Although this test is less reliable in lateralizing the ACTH source i.
Simultaneous measurement of prolactin in the central samples can normalize the data if there is any difference in the location of the catheters . This test is done in conjunction with a skilled interventional neuroradiologist. It is important that the endocrinologist is personally present in the room during the procedure so that assurance can be made that the proper blood tests were drawn at the specified times.
The patient is brought to the angiogram suite without sedation. A large bore IV line is placed in an antecubital fossa to be certain there is access to peripheral blood sampling and CRH injection. Catheters 5 French are placed in the femoral veins and threaded under fluoroscopic guidance to the inferior petrosal sinus. Injection of IV contrast confirms proper placement of the catheters.
Patients are on constant, pulse, blood pressure and oxygenation monitors during the course of the procedure. Test tubes are prechilled in ice and labeled so that during the rapid sampling period, blood can be placed in the tubes without delay. It is recommended to routinely obtain 4 baseline measurements at, -5 and at 0 minutes.
This allows for practice allowing proper coordination between the radiologists drawing blood from the IPSS and the individual drawing blood from the brachial vein. Appropriate amounts of blood should be removed to discard the dead space of the catheter this varies depending on the size of the catheter used.
Blood is then sampled from both central and peripheral lines at 2', 5' 10' and 15'. After the 15' time point and right before the IPSS catheters are removed, repeat fluoroscopic localization of the catheters should be performed to confirm that there was no displacement during the sampling. However, sampling on peripheral blood may continue as described in the CRH test discussed above. Patients greater than pounds in weight may not be able to be supported by the standard fluoroscopic table.
Furthermore such large patients may have an abdominal pannus that precludes reasonable access to the femoral veins. In such instances the IPSS can be performed via catheters placed in the antecubital vein with the patient immobilized in the sitting position. Strokes have been reported in the literature as a potential complication.
To minimize this possibility it is recommend that the catheters remain in the petrosal sinus for no more than 30 minutes. Plasma ACTH values are normalized to the prolactin value in order to correct for possible different localization of the catheters, or movement of the catheters during the study.
The differential diagnosis in these cases includes unilateral cortisol secreting adenoma or carcinoma with contralateral non-functioning cortical adenoma, bilateral cortisol secreting adenomas, macronodular adrenal hyperplasia and primary pigmented nodular adrenocortical disease. Adrenal vein sampling measuring cortisol can be very helpful in this scenario and give valuable information to elucidate the proper diagnosis and guide therapy.
This test is done in conjunction with a skilled interventional radiologist under sedation.
ACTH/Cortisol Ratio May Be Reliable Test to Diagnose Cushing's Disease
The procedure is usually performed early morning after an overnight fast on the second day of either a low dose 0. This eliminates the probability of endogenous ACTH secreting causing interference with the interpretation of autonomous adrenal gland cortisol secretion. The adrenal veins can be catheterized by the percutaneous femoral vein approach, the position of the catheter tip should be verified by venogram.
Concentrations of cortisol and epinephrine should be measured in blood obtained from both adrenal veins and the external iliac vein for the detection of peripheral venous concentrations Potential complications include thrombosis with subsequent infarction or hemorrhage adrenal insufficiency and hypertensive crisis, however these are rare The epinephrine concentrations are usually much higher on the right adrenal vein compared to the left, this is presumably due to the anatomy differences and the catheter proximity to the right adrenal medulla.
For this reason, although plasma epinephrine is measured to confirm success of adrenal vein catheterization, it cannot be used to correct for blood sample dilution between the 2 adrenal veins. There have been few case reports in which aldosterone has been used for side-to-side dilution differences, however whether it can be used for this purpose remains unclear 34, An adrenal-to-peripheral venous cortisol gradient greater than 6.
Lateralization can be determined by measuring the side-to-side cortisol gradient high-side to low-side.