C cells evolve at the same rhythm as follicular cells when thyroidal status changes in rats

C cells are primarily known for producing calcitonin, a hypocalcemic and hypophosphatemic hormone. Nevertheless, besides their role in calcium homeostasis, C cells may be involved in the intrathyroidal regulation of follicular cells, suggesting a possible interrelationship between the two endocrine populations. If this premise is true, massive changes induced by different agents in the activity of follicular cells may also affect calcitonin-producing cells. To investigate the behaviour of C cells in those circumstances, we have experimentally induced two opposite functional thyroid states. We hyperstimulated the follicular cells using a goitrogen (propylthiouracil), and we suppressed thyroid hormone synthesis by oral administration of thyroxine. In both scenarios, we measured T-4, TSH, calcitonin, and calcium serum levels. We also completely sectioned the thyroid gland, specifically immunostained the C cells, and rigorously quantified this endocrine population. In hypothyroid rats, not only follicular cells but also C cells displayed hyperplastic and hypertrophic changes as well as increased calcitonin levels. When exogenous thyroxine was administered to the rats, the opposite effect was noted as a decrease in the number and size of C cells, as well as decreased calcitonin levels. Additionally, we noted that the two cell types maintain the same numerical relation (10 +/- 2.5 follicular cells per C cell), independent of the functional activity of the thyroid gland. Considering that TSH serum levels are increased in hypothyroid rats and decreased in thyroxine-treated rats, we discuss the potential involvement of thyrotropin in the observed results.