Hyperplastic thyroid lesions in fish.

The teleost thyroid lacks a capsule, can be found ectopically in certain tissues throughout the animal, and is frequently predisposed to extensive proliferation. Several genetically susceptible small fish species as well as several larger salmonid and cyprinid species undergo a marked proliferation of the thyroid due to external conditions, such as poor water quality and abnormal nutrition, often making it difficult for one to distinguish these lesions as either cancer or goiter. The history of this controversy and present data from my study in which a marked growth of the thyroid occurred in guppies (Poecilia reticulatus) held at 27 degrees C but not in those held at 17 degrees C are reviewed. The significance of this predisposition in small fish to carcinogenicity testing is discussed.