FIGURE 2.

Functional model of vitamin D deficiency. (1) Sufficient vitamin D levels and relatively low tissue requirements facilitate normal organ function and maintain general health. (2) PTH stimulation ensures adequate generation of 1,25(OH)₂D during times of moderate circulating vitamin D deficiency and moderate tissue requirements. Organ functions and general health are initially preserved as tissue requirements are met by an activated PTH axis. (3) Severe vitamin D deficiency and maximal tissue requirement in the setting of an insufficient PTH axis results in loss of homeostasis, which results in morbidity and eventually, mortality. Adapted with permission from [82].