Neither vitamin D3 cholecalciferol nor 25-hydroxyvitamin D (calcifediol, AKA, confusingly, "calcidiol") act as hormones. A hormone is a substance which by its level in the bloodstream (and perhaps the cerebrospinal fluid) affects the behaviour of cells, potentially anywhere in the body. This is endocrine signaling.
1,25-dihyrdoxyvitamin D calcitriol activates the vitamin D receptor much more than the other two compounds. It acts as a hormone when it is produced in the kidneys and goes into circulation, to control cells which are involved in several aspects of calcium-phosphate-bone metabolism. This is a genuine hormonal function, with a very low circulating level (usually below 1 ng/mL) - and it is the one hormonal function of the three vitamin D compounds.
Vitamin D3 (from UV-B skin exposure or supplements - there is very little in food or multivitamins) is hydroxylated over a period of days to a week in the liver and goes into long-term circulation as 25-hydroxyvitamin D. A good level of this is 50 ng/mL 125 nmol/L or more, because this is what the immune system needs to function properly.
Please see the Quraishi et al. 2014 https://jamanetwork.com/journals/jamasurgery/fullarticle/1782085 graph at the start of this page, where you will find link to and discussion of the vitamin D research articles which are most important to the proper functioning immune system: https://vitamindstopscovid.info/00-evi/ . The section there: 02-compounds - explains the three compounds properly, and shows why the immune system's use of 25-hydroxyvitamin D, to be converted (by individual cells, in particular cell-type specific circumstances) _intracellularly_ to 1,25-dihydroxyvitamin D, has nothing to do with hormonal signaling. This intracrine and paracrine signaling is within individual cells, or to nearby cells. Vitamin D (here using the term to refer collectively to the three compounds, though it is only 25-hydoxyvitamin D and 1,25-dihydroxyvitamin D which are involved) based intracrine (AKA, not quite correctly "autocrine") and paracrine signaling play a crucial role in the ability of immune cells to respond to their individual circumstances. The immune system is unaffected by the relatively stable, very low, hormonal level of circulating 1,25-dihydroxyvitamin D.
The most important thing to measure is 25-hydroxyvitamin D, since immune cells need 50 ng/mL or more to function properly, and while the kidneys can often get by with half this. The level of circulating, hormonal, 1,25-dihydroxyvitamin D is important for calcium-phosphate-bone metabolism, but this does not affect the immune system. If there are concerns about calcium-phosphate-bone metabolism, circulating 1,25-dihydroxyvitamin D should be measured, but as far as I know (I am an electronic technician and computer programmer, not a doctor) its level varies a lot between individuals and so on its own, the level does not tell us much about what might be wrong with calcium-phosphate-bone metabolism. Calcium and parathyroid hormone levels provide more more information.
Please read the research articles at: https://vitamindstopscovid.info/00-evi/ including on bodyweight ratios of vitamin D3 to supplement with per day, and the use of calcifediol to boost 25-hydroxyvitamin D levels safely over 50 ng/mL in clinical emergencies such as sepsis, COVID-19, Kawasaki disease and MIS-C. Also, though I have not cited it yet this page, please see Prof. Sunil Wimalawansa's recent article in Nutrients:
Neither vitamin D3 cholecalciferol nor 25-hydroxyvitamin D (calcifediol, AKA, confusingly, "calcidiol") act as hormones. A hormone is a substance which by its level in the bloodstream (and perhaps the cerebrospinal fluid) affects the behaviour of cells, potentially anywhere in the body. This is endocrine signaling.
1,25-dihyrdoxyvitamin D calcitriol activates the vitamin D receptor much more than the other two compounds. It acts as a hormone when it is produced in the kidneys and goes into circulation, to control cells which are involved in several aspects of calcium-phosphate-bone metabolism. This is a genuine hormonal function, with a very low circulating level (usually below 1 ng/mL) - and it is the one hormonal function of the three vitamin D compounds.
Vitamin D3 (from UV-B skin exposure or supplements - there is very little in food or multivitamins) is hydroxylated over a period of days to a week in the liver and goes into long-term circulation as 25-hydroxyvitamin D. A good level of this is 50 ng/mL 125 nmol/L or more, because this is what the immune system needs to function properly.
Please see the Quraishi et al. 2014 https://jamanetwork.com/journals/jamasurgery/fullarticle/1782085 graph at the start of this page, where you will find link to and discussion of the vitamin D research articles which are most important to the proper functioning immune system: https://vitamindstopscovid.info/00-evi/ . The section there: 02-compounds - explains the three compounds properly, and shows why the immune system's use of 25-hydroxyvitamin D, to be converted (by individual cells, in particular cell-type specific circumstances) _intracellularly_ to 1,25-dihydroxyvitamin D, has nothing to do with hormonal signaling. This intracrine and paracrine signaling is within individual cells, or to nearby cells. Vitamin D (here using the term to refer collectively to the three compounds, though it is only 25-hydoxyvitamin D and 1,25-dihydroxyvitamin D which are involved) based intracrine (AKA, not quite correctly "autocrine") and paracrine signaling play a crucial role in the ability of immune cells to respond to their individual circumstances. The immune system is unaffected by the relatively stable, very low, hormonal level of circulating 1,25-dihydroxyvitamin D.
The most important thing to measure is 25-hydroxyvitamin D, since immune cells need 50 ng/mL or more to function properly, and while the kidneys can often get by with half this. The level of circulating, hormonal, 1,25-dihydroxyvitamin D is important for calcium-phosphate-bone metabolism, but this does not affect the immune system. If there are concerns about calcium-phosphate-bone metabolism, circulating 1,25-dihydroxyvitamin D should be measured, but as far as I know (I am an electronic technician and computer programmer, not a doctor) its level varies a lot between individuals and so on its own, the level does not tell us much about what might be wrong with calcium-phosphate-bone metabolism. Calcium and parathyroid hormone levels provide more more information.
Please read the research articles at: https://vitamindstopscovid.info/00-evi/ including on bodyweight ratios of vitamin D3 to supplement with per day, and the use of calcifediol to boost 25-hydroxyvitamin D levels safely over 50 ng/mL in clinical emergencies such as sepsis, COVID-19, Kawasaki disease and MIS-C. Also, though I have not cited it yet this page, please see Prof. Sunil Wimalawansa's recent article in Nutrients:
"Rapidly Increasing Serum 25(OH)D Boosts the Immune System, against Infections—Sepsis and COVID-19" 2022-07-21 https://www.mdpi.com/2072-6643/14/14/2997 .