CHAPTER 23 Answers to Commonly Asked Endocrine Diagnostic Questions Robert J. Kemppainen CHAPTER OUTLINE General Questions 251 Should I Submit Serum or Plasma Samples for Hormone Analysis? 251 Questions about Adrenal Function Testing 251 Which Is the Best Test to Diagnose Hyperadrenocorticism (Cushing’s Disease) in Cats? 251 When Is an ACTH Stimulation Test of Use in Cats? How Do I Perform the Test? 252 Is There Value in Measuring Urine Cortisol:Creatinine Ratio in Cats? 252 Questions about Thyroid Function Testing 252 Does the Finding of a Normal Total T4 Level in a Hyperthyroid-Suspect Cat Rule Out the Disease? 252 What Is the Value of Measuring Free T4 Levels in Cats? 252 Does the Finding of an Elevated Total T4 in a Cat Mean That the Patient Is Hyperthyroid? 253 What Is the Best Way to Monitor Methimazole Therapy? 253 Questions about Acromegaly 253 I Suspect That the Insulin Resistance in the Diabetic Cat I Am Treating May Be Caused by Acromegaly. How Do I Test for This Problem? 253 The following questions represent frequent enquiries received by our endocrine service regarding endocrine testing in cats. The questions center mainly on testing for adrenal and thyroid problems, because our laboratory provides assays evaluating function of these tissues. Questions regarding testing for reproductive problems are not covered. The questions and answers that follow may be considered, to a degree, to be lab-specific, because reference ranges and some interpretative advice vary among testing laboratories. Readers should understand that the endocrine testing field is evolving constantly, and options as to selection of the best test and how to interpret its results change over time. Therefore these answers are subject to change and it is always best to contact the laboratory to get the latest information. 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). Only gold members can continue reading. 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CHAPTER 23 Answers to Commonly Asked Endocrine Diagnostic Questions Robert J. Kemppainen CHAPTER OUTLINE General Questions 251 Should I Submit Serum or Plasma Samples for Hormone Analysis? 251 Questions about Adrenal Function Testing 251 Which Is the Best Test to Diagnose Hyperadrenocorticism (Cushing’s Disease) in Cats? 251 When Is an ACTH Stimulation Test of Use in Cats? How Do I Perform the Test? 252 Is There Value in Measuring Urine Cortisol:Creatinine Ratio in Cats? 252 Questions about Thyroid Function Testing 252 Does the Finding of a Normal Total T4 Level in a Hyperthyroid-Suspect Cat Rule Out the Disease? 252 What Is the Value of Measuring Free T4 Levels in Cats? 252 Does the Finding of an Elevated Total T4 in a Cat Mean That the Patient Is Hyperthyroid? 253 What Is the Best Way to Monitor Methimazole Therapy? 253 Questions about Acromegaly 253 I Suspect That the Insulin Resistance in the Diabetic Cat I Am Treating May Be Caused by Acromegaly. How Do I Test for This Problem? 253 The following questions represent frequent enquiries received by our endocrine service regarding endocrine testing in cats. The questions center mainly on testing for adrenal and thyroid problems, because our laboratory provides assays evaluating function of these tissues. Questions regarding testing for reproductive problems are not covered. The questions and answers that follow may be considered, to a degree, to be lab-specific, because reference ranges and some interpretative advice vary among testing laboratories. Readers should understand that the endocrine testing field is evolving constantly, and options as to selection of the best test and how to interpret its results change over time. Therefore these answers are subject to change and it is always best to contact the laboratory to get the latest information. 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). Only gold members can continue reading. Log In or Register to continue Share this:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window) Related Related posts: Methicillin-Resistant Staphylococci Genetic Screening of Familial Hypertrophic Cardiomyopathy Exocrine Pancreatic Insufficiency Cardiac Neoplasia Stay updated, free articles. Join our Telegram channel Join Tags: Consultations in Feline Internal Medicine Volume 6 Aug 6, 2016 | Posted by admin in INTERNAL MEDICINE | Comments Off on Answers to Commonly Asked Endocrine Diagnostic Questions Full access? Get Clinical Tree
