Mitotic rate of the rat's thyroid gland during hypertrophy induced by an antithyroid agent (carbimazole)

Female rats (200 g body weight) were either untreated or given carbimazole (0.1 g/100 ml) in their drinking water for up to 12 weeks. The mitotic rate of the thyroid follicular cells was estimated using vincristine sulphate. Measurement of whole-body oxygen consumption showed that the rats were clearly hypothyroid after 1 week of treatment. The mitotic rate rose rapidly from the control value of 3.65% metaphases/day to about 25%/day after 4 days and was maintained at this level for at least 5 weeks of treatment. The changes in mitotic rate were apparently synchronous with the histological changes. The results of earlier studies are reviewed and discussed in relation to recent concepts of cell populations.