1,25-Dihydroxycholecalciferol attenuates thyrotropin stimulated iodide accumulation in rat thyroid follicular FRTL-5 cells by reducing iodide porter number.

We have recently shown that rat thyroid follicular FRTL-5 cells have functional receptors for 1,25-dihydroxycholecalciferol (1,25- (OH)2D3) and that 1,25-(OH)2D3 attenuates the thyrotropin (TSH) induced iodide uptake. Here we show that the dibutyrylcyclic AMP induced iodide uptake was significantly reduced by 1,25-(OH)2D3, indicating that 1,25-(OH)2D3 affects the cAMP signal pathway beyond cAMP generation. The Vmax of the iodide porter was significantly reduced in 1,25-(OH)2D3 treated cells as compared to cells treated with TSH alone, indicating that 1,25-(OH)2D3 reduces the effective number of iodide porters in FRTL-5 cells.