Catharine J. Wheaton ¹ Kathleen Sullivan ¹ Enass Bassiouny ² Charlene Burns ¹ Matthew Smukall ³ Jill Hendon ⁴ Natalie D. Mylniczenko ^1*

• ¹ Disney’s Animals Science and Environment, Lake Buena Vista, United States
• ² Veterinary Diagnostic Laboratory, College of Veterinary Medicine, Michigan State University, East Lansing, Illinois, United States
• ³ Bimini Biological Field Station, South Bimini, Bahamas
• ⁴ Gulf Coast Research Laboratory, University of Southern Mississippi, Ocean Springs, Mississippi, United States

Thyroid disease is an important condition to understand in elasmobranchs, with goiters being predominant. To identify dysfunction, measuring serum thyroid hormone levels is a standard of practice for diagnosing disease in most species. Although these levels have been reported in the literature, the testing methodology is varied and values are not clinically useful for most aquarium species. In a group of aquarium-housed elasmobranchs, thyroid hormone levels had been persistently low or not detectable in otherwise healthy animals as well as animals with thyroid disease. The concern for reliability of these results to diagnose thyroid disease, prompted a shift to serum iodine levels as a proxy to determine thyroid health. This study assesses thyroid hormone and iodine levels as compared to thyroid disease stage in elasmobranchs with and without diet supplements, to determine the efficacy of using these serum values to guide clinical decisions. Serum thyroid hormone results obtained were lower than the readable range of the standard curve in both sharks and rays; thus reported values are usually extrapolated. Including additional RIA standards down to the limit of sensitivity improved detection, however increasing the sample volume tested was determined to be the most important factor for obtaining measurable results in low-value thyroid hormone samples. Here we report serum iodine levels in three groups of southern stingrays (Hypanus americanus). Other elasmobranch species maintained in aquaria with and without thyroid disease were used for biological comparisons. Nongoiter, diseased animals reliably had elevated levels (over baseline) of thyroid hormones and iodine; in goiter cases, hormones were not useful. Additionally, it was found that cobalt levels were also elevated in some disease states and correlated positively with serum iodine levels. Current available thyroid testing may not provide clinically useful values unless methodology is adjusted, or disease is severe. Serum iodine may be a useful marker to investigate thyroid health. Further, while thyroid disease may be identifiable with thyroid hormones, it is not straightforward or substantial enough alone for diagnosis.