Thyroid hormones and serum iodine in clinically normal versus chronically malnourished Florida manatees Trichechus manatus latirostris

An unusual mortality event (UME) was declared from December 2020 to April 2022 after increased deaths of emaciated Florida (FL) manatees Trichechus manatus latirostris occurred in the Indian River Lagoon (FL, USA) following seagrass reduction. While thyroid hormones are known to fluctuate with fasting, stress, and disease in pinnipeds and cetaceans, investigations in sirenians are limited. Serum from clinically normal free-ranging FL manatees (n = 35) and individuals affected by the UME (n = 20) was analyzed via radioimmunoassay and chemiluminescent immunoassays for total triiodothyronine (TT3), total thyroxine (TT4), and free thyroxine (fT4) quantification after assay validation. Total iodine (TI) and inorganic iodine (INORG-I) were measured using inductively coupled plasma mass spectrometry; organic iodine (ORG-I) was calculated from these values. Reference intervals for TT3 and TT4 were generated for clinically normal manatees. There were no differences in TT3, TT4, TI, or INORG-I between groups. Body condition index weight difference expressed as a percentage of the population mean (BCI%) and fT4 were significantly lower in UME manatees, most consistent with chronic starvation. However, BCI% and fT4 were not significantly different between UME manatees who survived rehabilitation versus those who died, suggesting these are unreliable prognostic markers. Serum ORG-I was significantly higher in UME manatees. High thyroid tissue iodine in necropsied UME manatees (n = 6), associated with follicular cysts in a subset (n = 3), suggests a potential impact on thyroid gland metabolism. These data expand the current understanding of pathophysiological aspects of the UME due to chronic malnutrition and may help guide supportive care for FL manatees undergoing rehabilitation.