Thyroid gland malfunctions are likely to have detrimental effects on general health, reproduction, and calf survivorship in Tursiops truncatus. For example, congenital goiter has recently been recognized by pathologists as a significant problem in T. truncatus and is believed to be a cause of calf mortality.1 To eventually treat thyroid diseases in dolphins, we must develop techniques for diagnosing abnormalities. Our study aimed to establish reliable landmarks for ultrasound imaging of the thyroid gland in T. truncatus. Ultrasound techniques were developed and validated at Dolphin Quest Oahu. The two study dolphins were first trained for voluntary examination of the thyroid gland by desensitization of ventral probe contact between the pectoral fins and in the neck region. Ultrasound systems used to find and measure the thyroid gland have included the Sonovet 2000, Tringa 50S, and Aloka 500. Probe types have included convex, curvilinear, and sector probes ranging between 3.5 and 5 MHz.
Ultrasound examinations focused on determining anatomical landmarks that could be used repeatedly for locating the thyroid gland. The ultrasound probe is initially placed at the thoracic inlet in a transverse orientation slightly cranial to the pectoral insertion. Key landmarks are the sternum and brachiocephalic vein, which runs in a transverse direction cranial to the sternum. The brachiocephalic vein appears as a hypoechoic vascular structure and is located at a depth of approximately 9 to 10 cm from the skin surface in adult T. truncatus. Once the location of the vein is established, probe orientation is changed from transverse to longitudinal, and the vein relocated in a cross-sectional view. The thyroid gland lies slightly cranial to the brachiocephalic vein and is identified in a longitudinal plane as an oval to round structure. Attention to focal depth and optimization of transducer gain settings are necessary to resolve the gland from the surrounding sternohyoid muscle group. A defined isthmus is not always present, but maximum width and length of both left and right aspects of the gland can be imaged by scrolling transversely in the longitudinal orientation. Width measurements for bottlenose dolphins have been found to range between 1.5 and 1.9 cm, and length, between 3.2 and 4.1 cm. Techniques were validated by ultrasound-guided dye injection into the right thyroid lobe of a post-mortem specimen. The dyed, right thyroid lobe was clear on subsequent dissection and confirmed that ultrasound can be used to locate and measure the thyroid gland in T. truncatus.
1. Garner MM, C. Shwetz, JC Ramer, JM Rasmussen, K Petrini , DF Cowan, JT Raymond, GD Bossart, GA Levine. 2002. Congenital Diffuse Hyperplastic Goiter Associated With Perinatal Mortality In Eleven Captive-Born Bottlenose Dolphins (Tursiops truncatus). Journal of Zoo and Wildlife Medicine 33 (4): 350-355.