Influences of biological variables and geographic location on circulating concentrations of thyroid hormones in wild bottlenose dolphins (Tursiops truncatus)

Thyroid hormones (TH) are key regulators of metabolism and development, yet our understanding of the variability in serum TH concentrations in free-ranging marine mammals is limited. Thus, we examined the interrelationships between TH and age, sex, reproductive status, geographic location, and ocean temperatures in wild bottlenose dolphins (Tursiops truncatus). Circulating concentrations of TH (total thyroxine (tT4), free T4 (fT4), and total triiodothyronine (tT3)) were determined in a total of 195 dolphins; 80 from the coastal waters of Charleston, South Carolina (CHS) and 115 from the Indian River Lagoon, Florida (IRL). Age had the most influence on circulating TH concentrations in dolphins at both sites with decreasing concentrations (p < 0.0001) observed with increasing age for all TH. No significant differences were found between males and non-reproductive females. Geographic location significantly influenced tT4 and tT3 concentrations; CHS dolphins had higher concentrations than IRL animals. These TH differences between CHS and IRL dolphins may be attributed to the colder year-round water temperature that CHS dolphins inhabit compared to IRL dolphins and could constitute an adaptive response to their colder environment. Results from this study highlight the importance of establishing reference values for dolphins in different geographic locations to support valid comparisons. This initial assessment provides a foundation of how biological and environmental variables could affect circulating TH in dolphins, which will help to elucidate the impacts of disease, pollution, and climate change on the thyroid hormone system of aquatic mammals.

► Age, reproductive status and location influenced thyroid hormones (TH) in dolphins.
► TH concentrations significantly decreased in male and female dolphins with increasing age.
► Lower TH occurred in pregnant and lactating females vs. juvenile and non-pregnant females.
► Dolphins in SC had significantly higher tT4 and tT3 concentrations than those in FL.
► Study highlights importance of establishing reference TH in wild dolphin populations.