Abstract

The study of metabolic physiology is necessary to understand endocrine control and is fundamental in ensuring the health and survival of any species.²⁵ St. Aubin²⁰ stated that the information on primary endocrine pathologies in marine mammals is scant compared to terrestrial animals. Thus, the activity of a specific endocrine organ, such as thyroid gland, reflected by both morphological and functional changes, can provide important information about the internal status of the subject, which guides corrective therapy.

Thyroid diseases have rarely been reported in marine mammals.^{6,7,9,13,17,20} Environmental contaminants and local environmental influences have been implicated in thyroid hormone imbalances²⁰ and development of morphological and histological abnormalities,^8,13,18 leading to diseases^7,12,13 and calf mortality^9,22-24. Ultrasound is a useful imaging tool in assessing the thyroid morphology in humans^1,10 and companion animals^3,26. Establishment of a standardized protocol for sonographic examination of the dolphin thyroid gland would be beneficial in reducing variation among operators. In addition, documentation of the essential sonographic features of normal dolphin thyroid glands would provide a basis for the diagnosis of thyroid pathology.

Baseline values for serum thyroid hormones have been established in some common captive cetacean species, but discrepancies exist.^20,21 Investigation of the association between thyroid morphological changes and thyroid hormone variations would enable more accurate characterization of hormonal changes and add to knowledge of the dolphin thyroid physiology. Various factors affect thyroid function and morphology, such as demographic parameters, physiological cycles and illnesses.^2,21,14,15 Previous studies in humans have reported significant thyroid volume variation during a normal menstrual cycle in females of reproductive age, and alteration of thyroid physiological characteristics during different reproductive phases of a women's life.^4,5,10,11 Thyroid function and morphology are also likely to be affected by the cyclic change of the hormonal environment during estrus cycle and in different reproductive events in females.^4,5,10,16,19 Recognizing the potential influence of these factors when interpreting changes in dolphin thyroid morphology would guide corrective therapy.

The primary aim of the present study was to investigate the value of sonography as a non-invasive method in assessing the anatomy and physiology of the thyroid gland in a group of captive Indo-Pacific bottlenose dolphins (Tursiops aduncus) in Hong Kong. The specific objectives of this study were: to establish an accurate and reliable thyroid ultrasound measurement method; to correlate thyroid morphology changes with serum hormone levels; and to determine any influence of the demographic parameters and physiological cycles on thyroid morphology.

This three-year study systematically monitored 18 T. aduncus, at Ocean Park, Hong Kong (7 males and 11 females). A total of 1384 thyroid examinations were performed and 241 serum thyroid hormone samples were analyzed. At the end of the study, the mean age of the population was 16.0 years (range, 3 to 37 years). Of these, 14 subjects were sexually mature and 4 were sexually immature (2 were deceased). The mean body weight of the studied population was 132.3 kg (range, 106.1 to 185.1 kg) and the mean body length of the studied population was 220.1 cm (range, 196 to 244 cm). For all subjects, sonographic examination of the thyroid gland was performed once a week. Blood samples were collected monthly whenever possible for thyroid hormones (free T3, free T4, total T3, and total T4) evaluation, using the corresponding enzyme immunoassay kits.

The accuracy of the four 2-D ultrasound methods in dolphin thyroid volume measurement was investigated, with the standard of reference determined by 3-D ultrasound. The inter- and intra-operator variability of the four 2-D ultrasound and the 3-D ultrasound thyroid volume measurement methods were also evaluated. Sonographic features of the normal dolphin thyroid gland and adjacent neck structures were documented.

Possible association of the dolphin thyroid morphology and serum thyroid hormone levels was evaluated. Various degrees of association were demonstrated between thyroid hormone levels and thyroid volume among gender, age group and sexual maturity. The most prominent association was identified when the analyses were stratified by age. Variations in dolphin thyroid morphology for somatic growth (different ages, genders, degrees of sexual maturity), reproductive development (different genders and degrees of sexual maturity) and body size, as well as potential seasonal fluctuations of thyroid morphology were also evaluated. Results indicated that thyroid morphology would vary with age, gender, sexual maturity and body size, but may not vary with season.

Further investigations were conducted to evaluate the possible variations of thyroid morphology in female dolphins during different reproductive events and estrus cycle. Reproductive event was considered to be a significant predictor for thyroid volume measurement, and significant variability of thyroid volume was found among different reproductive events in female bottlenose dolphins. Cyclic changes of thyroid volume during estrus cycle were found, with the minimum thyroid volume in the follicular phase and the maximum thyroid volume in the luteal phase.

To conclude, the present study provided additional information on the anatomy and physiology of the thyroid gland in a group of captive T. aduncus. Thyroid morphology variability regarding different demographic parameters, as well as during different physiological cycles should be taken into account when examining dolphins so as to obtain a diagnostically meaningful assessment. The technique developed can be applied to other cetacean species, including Pacific white-sided dolphins and beluga whales, which may aid in the diagnosis of pathological states of these species.

Acknowledgements

The authors are grateful to Gary Wong, Harriet Chiu and the trainers of Ocean Park's Marine Mammal Department for their invaluable contributions in dolphin training and husbandry. The completion of this project also owes much to the continual unwavering support and commitment of the veterinarians and senior management of the Park over the years.

This project was funded by The Hong Kong Polytechnic University Research Studentship (G5556 RGGH).

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