PTH and secondary hyperparathyroidism: In the gene that codes for PTH production in parathyroid cells, there is a vitamin D response element that, when it detects a deficiency (<20 ng/mL), stimulates the transcription of that gene, thereby increasing the production of this hormone, leading to secondary hyperparathyroidism, an early alteration of CKD with a significant impact on bone and cardiovascular tissue [23].