Histological assessment of goiters in elasmobranch fishes

Twelve thyroid glands from free-living (n = 3) and captive (n = 9) elasmobr anch fishes were investigated. Histological observation of the thyroids rev ealed two euthyroid glands, one case of mild chronic thyroiditis, three dif fuse hyperplastic goiters, three diffuse colloid goiters, and three multino dular colloid goiters. The term goiter best describes the thyroid enlargeme nt that results from hyperplasia and hypertrophy. Although elasmobranch goi ters are typically linked to low aquatic iodine concentrations, iodine defi ciency alone does not explain all of the observed thyroid conditions. Diffu se hyperplastic goiters typically result from either iodine deficiency or g oitrogenic release of thyroglobulin from the follicle. Colloid goiters resu lt from fluctuating iodine concentrations or goitrogenic agents that block the release of iodine from the thyroid gland. Although nitrate is a potenti al goitrogenic agent in aquaria, research indicates that nitrate interferes with the uptake and retention of iodide. This interference is not directly related to iodide storage in colloid goiters. Elasmobranchs and humans app ear to have similar patterns in goiter development and in the etiology of t he disease.