Colloid Goiter in an Eastern Diamondback Rattlesnake (Crotalus adamanteus)

Abstract

Goitrous enlargement of the thyroid glands is relatively rare in reptiles but has been observed occasionally in giant terrestrial tortoises (Geochelone elephantopus and Testudo gzgantea).4 These tortoises are prone to develop hyperplastic goiter in captivity when fed diets high in goitrogenic vegetables. They seem to possess a particularly high metabolic requirement for iodine, which is met in their native habitat by ingesting plants that sequester halogen^.^ Goiter is uncommon in snakes. Out of 48 1 snake necropsy accessions at the National Zoological Park, Washington, D.C., since 1975, only two had a diagnosis of goiter (D. K. Nichols, personal communication). The present case report describes colloid goiter in a captive eastern diamondback rattlesnake (Crotalus adamanteus). An adult female eastern diamondback rattlesnake that had been in the Zoo Atlanta collection since 1976 was found dead in its cage. No premonitory signs had been reported. At necropsy, a 2.5x 1-cm multilobulated thoracic mass was found cranial to the heart (Fig. 1). No other gross abnormalities were noted. Multiple tissues were collected, fixed in 10% neutral buffered formalin, and submitted to the Department of Pathology, College of Veterinary Medicine, University of Georgia. Following gross examination, the submitted tissues were trimmed, embedded in paraffin, and sectioned at 3 Mm, and sections were stained with hematoxylin and eosin (HE). Macroscopically, the mass was presumed to be thyroid tissue because of its appearance and location. In snakes, the thyroid is unpaired and located ventral to the trachea and cranial to the heart.5 Microscopically, the mass was confirmed to be thyroid gland, and all follicles were distended markedly by abundant amounts of colloid (Fig. 2) . The majority of the epithelial cells were flattened and attenuated (Fig. 3); however, some cells were more cuboidal to columnar in shape and contained intracytoplasmic colloid (Fig. 4). Mild infiltrates of lymphocytes and plasma cells were within the perivascular connective tissue. The liver had mild diffuse hepatocellular lipidosis. The kidneys had mild to moderate hemosiderin accumulation within renal tubular epithelial cells. A very mild lymphocytic infiltrate was scattered diffusely throughout the tracheal submucosa. The specific cause of the snake's death could not be determined histologically. In both young adult and adult mammals, colloid goiter is a result of diffuse hyperplastic goiter involution after sufficient iodine has been added to the diet and/or goitrogenic compounds have been removed from the diet.' The hyperplastic cells continue to produce colloid; however, the colloid accumulates because of reduction of colloid endocytosis by the follicular epithelial cells due to decreased circulating levels of thyrotropin secondary to the return of thyroxine and triiodothyronine levels to norma1.l Older animals tend to have diminished requirements for thyroxine and diminished pituitary thyrotropin secretion, conditions that also lead to follicular distention due to diminished colloid endocytosis. I