ABSTRACT
This chapter argues that vitamin D regulates (VDRs) the thyroid-stimulating hormone signaling pathway in a rat thyroid cell line (FRTL-5). However, VDRs and effects of vitamin D have been demonstrated in several other cells and tissues primarily involved in the regulation of Ca metabolism. In an attempt to reduce the hypercalcemia and to augment the effect on growth inhibition and differentiation several vitamin D analogs have been synthesized. Vitamin D analogs have also been tried in the treatment of breast carcinomas and as immunosuppressive agents. Most effects of thyrotropin (TSH) on thyroid cells may be mimicked by cell permeable cAMP analogs. In the rat thyroid FRTL-5 cell line calcitriol, the biologically most active of the naturally occurring vitamin D metabolites, attenuates both TSH-stimulated cAMP production and the effects of cAMP. Effects of calcitriol on these proteins indicate how the vitamin D endocrine system may regulate cAMP signaling in both classical and nonclassical target tissues.
