Summary of the vitamin D endocrine system. Target organs for the steroid hormone 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃] are defined by the presence of the vitamin D receptor (VDR); 37 tissues are known to possess the VDR (8). The paracrine production of 1, 25(OH)₂D₃ is known to occur in at least 10 tissues (8). New evidence shows that 5 additional physiologic systems (immune, pancreas, heart-cardiovascular, muscle, and brain systems) and the traditional intestinal-bone calcium system are responsible for producing biological vitamin D–related responses that are important to good health. 

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