CHAPTER 23 Answers to Commonly Asked Endocrine Diagnostic Questions
GENERAL QUESTIONS
SHOULD I SUBMIT SERUM OR PLASMA SAMPLES FOR HORMONE ANALYSIS?
Testing laboratories vary in their recommendations for the type of sample for each assay. In our laboratory serum is preferred for thyroid hormone testing (total thyroxine [T4] and free T4). Samples for free T4 measurement should be sent with refrigeration (i.e., frozen gel packs) if it is likely that transit to the lab will require more than 3 days. No cooling is needed for samples for total T4. For cortisol measurement (e.g., dexamethasone suppression or adrenocorticotropic hormone [ACTH] stimulation testing), either serum or EDTA-plasma can be submitted. If serum is used, it is best to refrigerate samples during shipment. If EDTA-plasma is collected, samples do not require refrigeration if they are shipped to arrive within 2 days. We recommend serum for samples submitted for insulin measurement, whereas EDTA-plasma should be collected for endogenous ACTH determination.
QUESTIONS ABOUT ADRENAL FUNCTION TESTING
WHICH IS THE BEST TEST TO DIAGNOSE HYPERADRENOCORTICISM (CUSHING’S DISEASE) IN CATS?
We prefer the dexamethasone suppression test (over the ACTH stimulation test), using a dose of 0.1 mg/kg intravenously (IV) of dexamethasone. Note that this dose is equivalent to that used for high-dose dexamethasone testing in dogs. To perform the test, collect a presample of cortisol and then administer dexamethasone IV. Collect post-dexamethasone cortisol samples at 4 and 8 hours postinjection. Cats with spontaneous hyperadrenocorticism usually have post-dexamethasone cortisol concentrations (at 4 and/or at 8 hours) above 35 nmol/L (1.3 µg/dL).
Once the disease has been diagnosed differentiation of the pituitary-dependent form from a cortisol-secreting adrenocortical tumor is recommended. Tests that may be used for this purpose include high-dose dexamethasone suppression, following the same protocol given above except using 1 mg/kg of the drug. A post-dexamethasone cortisol concentration below 35 nmol/L and/or less than 50 per cent compared with the prevalue at 4 or 8 hours is consistent with pituitary-dependent disease. The failure to observe adequate suppression of cortisol concentrations can not be taken to confirm an adrenocortical tumor, and additional tests should be used in this instance. Other tests for differentiation include measurement of endogenous ACTH concentrations (high to high-normal in cats with pituitary-dependent disease, and low or not detectable in cats with functional adrenocortical tumors) and diagnostic imaging (ultrasound, radiography, computed tomography, and/or magnetic resonance imaging).
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