Abstract
Summary The thyroid gland and adenohypophysis were studied after twice weekly oral administration of zinc ethylenebis(dithiocarbamate) (zineb) by gavage to rats for 4·5 months in doses equivalent to 1/10 or 1/100 LD 50 . Compared with the control group, the thyroids of the rats treated with the higher dosage level of zineb were enlarged and composed of numerous microfollicles lined with columnar cells. Succinic dehydrogenase and cytochrome oxidase activities were raised in the columnar cells, while the PAS-positive granules were reduced. Furthermore, the colloid within the thyroid follicles stained less intensely with the PAS stain. The changes in the ultrastructure of the thyroid were consistent with long-term thyrotropic-hormone (TTH) stimulation. An increased number of basophilic cells, containing PAS-positive granules, was observed in the adenohypophysis, again in the rats on the higher level of treatment. In the same group, increases up to 80% were detected in the uptake of 131 I by the thyroid, 5 days after the last exposure to zineb. A high TTH level was recorded in the blood plasma of both zineb-treated groups.
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