Variation of Thyroid Volume during the Normal Estrus Cycle in the Indo-Pacific Bottlenose Dolphin, Tursiops aduncus
Interpretation of thyroid morphology in marine mammals must take into account the dynamic changes that occur in association with significant life-history events.9 Thyroid function and morphology are likely to be affected by the cyclic change of the hormonal environment during the estrus cycle and in different reproductive events in females.2,5,6,8 Recognizing changes in the thyroid during different reproductive events in female dolphins will help in diagnosis of pathology and monitoring of the thyroid during treatment. This study was undertaken to investigate variations in thyroid morphology of female Indo-Pacific bottlenose dolphins (Tursiops aduncus) during the estrus cycle, using ultrasonography. Several males were also examined for comparison.
Sixteen animals (6 males, 10 females) were included in the study. Mean age was 15.1 years (range, 2-35 years). Ultrasonographic examination was performed weekly for two years, using established methods.1,7 Eight normal estrus cycles were observed. When folliculogenesis was observed, ovarian scans were performed more frequently to document follicle growth and thereby predict ovulation. The thyroid findings in an estrus cycle for each female were divided into 3 phases: follicular, ovulatory and luteal.
A significant difference in thyroid volume was recorded during the normal estrus cycle (p < 0.05), with the mean thyroid volume increasing from follicular (14.70 cm3 ± 6.05 cm3) to ovulatory (14.84 cm3 ± 5.72 cm3) and then to the luteal phase (16.50 cm3 ± 6.90 cm3). The mean thyroid volume in the luteal phase was significantly greater than in the follicular (p < 0.05) or ovulatory phase (p < 0.05). The difference in mean thyroid volume between the follicular and the ovulatory phases was insignificant (p > 0.05). In male dolphins, the mean thyroid volume was 11.27 cm3 ± 2.41 cm3, and there was no significant variation of mean thyroid volume during the 8 weeks of the study (p > 0.05). The overall mean thyroid volume (all phases) in female dolphins was significantly higher than the thyroid volume in male dolphins (p < 0.05).
This study showed substantial variability of thyroid volume throughout the estrus cycle in female bottlenose dolphins. The thyroid volume variation may reflect the amount of hormone released into the circulation.4 The gender difference suggests that a "cyclic" effect on thyroid metabolism in female dolphins may exist, and is influenced by the reproductive hormones involved during the normal estrus cycle. Studies have suggested that estrogen exerts a stimulating effect on thyroid function.3 Since estrogen levels appeared to vary erratically in the female dolphin estrus cycle, other reproductive factors may also be associated with the changes, but these are yet to be clarified. Thyroid volume variability during the estrus cycle should be taken into account when examining female dolphins to obtain a diagnostically meaningful assessment.
The authors thank Ocean Park's Marine Mammal Department for dolphin training, husbandry and guidance and Ocean Park Corporation for support of this project. This project was funded by The Hong Kong Polytechnic University Research Studentship (G5556 RGGH).
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