Part of the Michigan State University College of Veterinary Medicine, the MSU VDL serves the veterinary diagnostic needs for clients across Michigan and beyond. Perhaps the best-known service area of the MSU VDL, the Endocrinology Section performs more tests than any other area of the laboratory. Clients often ask about what’s new in the Endocrinology Section. In recent months the MSU VDL has seen an increase in vitamin D testing and has gained a new academic specialist. In addition, Dr. Brian Petroff, endocrinology section chief, offers a practice pearl.
Endocrinology Hot Topics: Vitamin D Toxicosis in Companion Animals
At the 2020 Michigan Veterinary Conference held in January, Dr. Kent Refsal, professor in the MSU VDL Endocrinology Section, presented on a topic that has been much discussed over the last year—vitamin D toxicosis in companion animals. Multiple recalls of canine diets in 2018-2019 were widely publicized. At the MSU VDL, testing to assess vitamin D exposure in animals increased 55% during fiscal year 2019 (July 1, 2018 – June 30, 2019). Dr. Refsal discussed vitamin D metabolism and vitamin D toxicity, both acute and cumulative.
The most common causes of acute toxicity are the result of exposure to vitamin D-containing rodenticides or ingestion of a prescription cream containing calcipotriene, an analog of calcitriol used for management of psoriasis. Acute vitamin D toxicity is suspected with rapid onset of lethargy, vomiting, polydipsia, and polyuria accompanied by increased serum phosphorus, calcium, creatinine, and azotemia. Specific confirmation of vitamin D3 or D2 toxicity would be increased concentrations of 25-hydroxyvitamin D (25OHD) in serum.
Cumulative toxicity occurs with accumulation of small daily doses of vitamin D over a prolonged period as was seen in the cases of pet food diet recalls (2006, 2010, 2015, and 2018-2019). Clinical signs are generally less severe than in acute toxicity and are dependent on the amount of increased vitamin D in the diet. Onset of clinical signs is gradual, and owners typically notice increased thirst and urination and a decrease in appetite. Laboratory tests revealing hypercalcemia ± hyperphosphatemia and mild azotemia should prompt investigation into possible causes of hypercalcemia. The assay of 25OHD may not be considered until other differentials such as malignancy or primary hyperparathyroidism are excluded. With its prolonged half-life, circulating concentrations of 25OHD may remain above the reference range for weeks or months after resolution of hypercalcemia.
Practice pearl: Cholesterol and Canine Hypothyroidism
A complete diagnostic thyroid profile is important in working up potential cases of canine hypothyroidism. The documentation of simultaneous low thyroid hormones (T3 and T4) and elevated thyroid stimulating hormone (TSH) allows a confident diagnosis in the dog with clinical signs of the disease. Thyroglobulin autoantibodies, if elevated, provide additional evidence of thyroidal autoimmune disease —the main cause of canine hypothyroidism.
A common challenge, however, is interpreting testing results when thyroid hormones are low but TSH is normal. This can occur in dogs when thyroid hormone production is affected by non-thyroidal illness (sometimes called euthyroid sick syndrome) or from thyroidal side effects of medications. However, about 1 in 5 dogs with true primary hypothyroidism have normal TSH concentrations. In such cases with low T4 or T3 and normal TSH, knowing the fasting cholesterol concentration is helpful. The majority of hypothyroid dogs will have simultaneous elevations of cholesterol. In fact, the measurement of cholesterol was an early diagnostic test for hypothyroidism. So, when dealing with a dog with low thyroid hormone(s) but normal TSH, while careful consideration of the clinical presentation is most important, primary hypothyroidism is less likely if cholesterol is normal.
Welcoming a New Academic Specialist to the Endocrinology Section
Dr. Vicki Miksicek joined the Michigan State University Veterinary Diagnostic Laboratory as an Academic Specialist in Endocrinology effective January 6, 2020.
Dr. Miksicek received her DVM from Michigan State University in 1992 after completing a BS in Zoology at MSU. She has a breadth of experience gained as a private practitioner in several mid-Michigan companion animal practices including Animal Care Hospital in Rockford, Bancroft Veterinary Clinic in Bancroft, and Tri-County Mobile Veterinary Clinic in Haslett.
Within the Endocrinology section of the VDL, Dr. Miksicek will work alongside the other endocrinologists to provide support for diagnostic testing, including interpretation of test results and phone consultation services.
“After years in private practice, I am excited to use my veterinary education and experience in a unique way,” says Miksicek. “Assisting other clinicians with the evaluation of their endocrine cases, while working with such a knowledgeable and respected group of people is very rewarding. In my first month I’ve seen cases from all over the world and from many exotic species. It’s a great opportunity to experience and learn about comparative endocrinology in action.”
For More Information
Additional information about adrenal function testing, assessment of reproduction, calcium disorders, equine endocrine testing, thyroid function testing in cats and dogs, thyroid medication and monitoring, testing protocol and sample handling is available on the MSU VDL website at animalhealth.msu.edu. Visit the Endocrinology Section to take a deeper dive. For complete information about Endocrinology tests, specimen requirements, or submittal procedures, please visit the MSU VDL’s catalog of available tests. Don’t hesitate to call the laboratory at 517.353.1683 if you have any questions.
An Important Reminder:
Our current method for measurement of cortisol in peripheral blood (Immulite 2000 XPi) requires separated serum collected in a plain clot tube (no separator gel).