Autocrine, Paracrine and Endocrine - simple chart

Note: it appears that the majority of Vitamin D activation is done locally, not via the endocrine system (Liver and Kidneys)

• Fully-activated Vitamin D (Calcitriol) is produced inside and outside of the kidneys – July 2020

• Brain cells can probably activate Vitamin D without liver or kidney (at least in rats) – Sept 2017