When stimulated by the feeding of ethylenethiourea for a period of 6 months, dog thyroid glands increased to approximately 30 times their normal size. Not only follicular cells but also C cells had hyperplastic and hypertrophic features. Compared with normal controls there was in the C cells a marked decrease in secretory granules immunoreactive for calcitonin. Furthermore, vesicular inclusions of various sizes and dilated nuclear envelopes, which showed a positive immunoreaction for calcitonin, were observed in the C cells. These findings indicate that the antithyroid drug interferes with synthesis of calcitonin by C cells, probably by inhibiting the conversion of a larger precursor to calcitonin.
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