Abstract

Thyroid gland disorders have been reported in many species of wildlife^5,10,15,17, but rarely in marine mammals. Environmental contaminants and local environmental influences have been implicated in thyroid hormone imbalances⁶ and development of detectable morphological and histological abnormalities^7,12,16 leading to calf mortality.^8,18-20 Cowan & Tajima⁶ described the gross and histological features of thyroid glands in stranded Atlantic bottlenose dolphins and found 31 out of 60 animals suffered thyroid gland abnormalities/pathologies. To the best of our knowledge, the formal literature is devoid of any reference to the diagnosis of thyroid gland abnormalities in living dolphins. In order to accurately diagnose and assess thyroid abnormalities in live animals, reliable methods of assessing the normal gland morphology and size must be developed to serve in correlation with biochemical and clinical data.

Ultrasound is a useful imaging tool in the assessment of thyroid morphology and physiology in humans^1,9,11 and companion animals.^2,4,15,21 The only published report of dolphin thyroid ultrasonography stated measurements were extremely variable "because of skill differences between operators".^18,19 Thus, a more standardized protocol for ultrasonographic examination of the dolphin thyroid gland would be beneficial in reducing such variation.

The aims of this study are to:

 Characterize the ultrasonographic appearance and size of the normal bottlenose dolphin thyroid gland

 Compare one 3-D and four 2-D ultrasonographic measurement methods

 Establish a standard ultrasonographic imaging protocol for thyroid assessment

In human medicine, 3-D ultrasound is recognized as the gold standard for thyroid volume measurement^13,14,22 and so was used for comparison with four 2-D ultrasound measurement methods. All examinations were performed with a Philips HD-11 ultrasound unit with a 4-8 MHz curvilinear 3-D transducer and a 2-5 MHz curvilinear 2-D transducer (Royal Philips Electronics of the Netherlands). A total of 160 individual scans were conducted in 15 T. aduncus with both 2-D and 3-D volume measurements methods (Methods A-E). All measurements of a single subject were collected at the same session.

Thyroid volumes were calculated using one 3-D and four 2-D ultrasound measurement methods. In-built software allowed accurate 3-D volumetric measurements. For 2-D measurements, the thyroid volume was calculated using the ellipsoid equation i.e., volume = π/6 x craniocaudal x mediolateral x dorsoventral dimensions (Table 1). The 3-D and 2-D thyroid volume measurements were compared by intraclass correlation coefficient (ICC) with 160 individual scans. Three operators scanned each thyroid twice in the same session, to investigate intra- and inter-observer variability. Repeatability and reproducibility of 3-D and 2-D measurements were compared using intraclass correlation coefficient (ICC) with 4 sets of complete scans. The echogenicity, homogeneity, and echotexture of the thyroid were also evaluated.

Table 1. Methods of thyroid volume measurement.

TS_MAX = the maximum transverse dimension of the thyroid gland; LS_L = the maximum longitudinal dimension of the left lobe; LS_MID = the midline of the thyroid gland; LS_R = the maximum longitudinal dimension of the right lobe

The borders of the thyroid were usually well-defined and smooth, however ill-defined borders were observed in two subjects. The echopattern was generally homogeneous and relative echogenicity varied between animals. Preliminary results (Table 2) show consistent and repeatable measurement of the thyroid gland is possible.

Table 2. Mean thyroid volume, measurement accuracy, mean measurement error and range of measurement error of Methods A-D compared with the 3-D ultrasound measurement method and reliability of Methods A-D and the 3-D ultrasound measurement method of three operators.

The preliminary results of this study indicate that both 2-D and 3-D ultrasound can be used to evaluate morphology and size of the dolphin thyroid gland. Access to 3-D ultrasound equipment is limited at this time, however, results show that 2-D methods also achieve high measurement accuracy. Of these, method D resulted in the smallest standard deviation and the highest reproducibility.

Investigation is ongoing to assess the reliability of presented methods.

Acknowledgements

The authors are grateful to Ocean Park's Marine Mammal Department for dolphin training, husbandry and guidance. Thanks also to Dr. Kristi West and Prof. Daniel Cowan for advice and information. This project was funded by The Hong Kong Polytechnic University Research Studentship (G5556 RGGH).

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