Dolphin thyroid: Some anatomical and physiological findings

Abstract

1. Plasma thyroxine (T4), protein bound iodine (PBI), inorganic iodine, total iodine, thyroxine binding globulin (TGB), unsaturated TBG (resin T3 uptake test) and free thyroxine were measured in a series of tests on two species of porpoise (or dolphins) and two species of small whales. This was the first such work done on cetaceans. The values presented for the bottlenose dolphin, Tursiops truncatus, were derived from 86 plasma specimens from 31 different animals (Table 2) and should be clinically useful for the future diagnosis of thyroid disease in that species. 2. A total of twenty-four plasma specimens from the Pacific white-striped dolphin, Lagenorhynchus obliquidens (Fig. 3), the killer whale Orcinus area (Fig. 4) and the pilot whale Globicephala scammoni (Fig. 5) gave values that must be regarded as preliminary until a larger number of animals can be sampled. 3. All of these marine cetaceans have larger thyroids and smaller body surface areas than terresterial mammals of comparable weight. 4. In bottlenose dolphins and Pacific white-striped dolphins the total thyroid weight is about 2.25 times the total adrenal gland weight. The adrenals and the thyroid are both about one-third larger (on a gland weight to body weight basis) in Pacific white-striped dolphins. 5. Differences in water temperature of 6° C to 9° C did not appear to affect the plasma thyroid indices of bottlenose dolphins or killer whales (Fig. 4). A 72-hour fast caused an increase in plasma T4 levels of each of 3 bottlenose dolphins with an average rise of 1.2 μg/100 ml. 6. The high level of plasma T4 and PBI appear to correlate well with the relatively high metabolic rate observed in T. truncatus. However, the species that appears to have the highest metabolic rate, the Pacifie white-striped dolphin, Lagenorhynchus obliquidens (Fig. 3), had the lowest plasma level of T4, PBI, total iodine, TGB capacity and cholesterol (Table 2). This data points out the need for a comprehensive study of thyroid function in wild and captive white-striped dolphins and other species that are difficult to maintain in captivity.