Exogenous Calcitonin Suppresses Growth Fraction of Thyroid C Cells

Calcitonin (CT) is a peptide hormone synthesized and secreted by the parafollicular (C) cells of the thyroid. Little is known about the mechanisms controlling proliferation of C cells by other humoral factors including CT, and there is no report that CT suppresses C cell proliferation. The effects of short-term administration of CT on C cell growth fraction were analyzed using BrdU and CT double immunohistochemical method in the rat thyroid. Continuous administration of 0.4 IU/kg and 40 IU/kg synthetic salmon CT for 14 days induced dose-dependent suppression of BrdU labeling number of C cells without decrease of C cell to the total thyroid follicular cell ratio. These results suggest that exogenous CT may induce negative feedback to C cell proliferation in the thyroid.