@sophie93, you may find this article informative, https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2912737/#:~:text=It%20has%20been%20suggested%20that,%2C%20chronic%20fatigue%2C%20age%2Drelated. It lays out in pretty good detail why so many people are vitamin D deficient, but also why most places don't routinely test and the difficulty of prescribing the correct supplement dose for each individual.

@sophie93, the article mentions that routine "vitamin D testing may benefit those at risk for severe deficiency (Table 2) or those with laboratory or radiographic findings commonly associated with vitamin D deficiency (Table 3)." If you are comfortable sharing, do you have some known issues with deficiency and other issues that may cause?