I went from 40 nmol/l to 170 nmol/l in three months with 5000 IU/day. I was advised to stop taking vitamin D3 supplements by my doctor in spring/summer time, but I noticed that I am feeling much worse after stopping for a weeks now. I have heard of this before; i.e., someone even developed a minor depression after not taking vitamin D3 for just two weeks.
Is there anything known about some kind of withdrawal symptoms of vitamin D3? I didn't check my blood again, but it seems highly unlikely that my levels are below what it should be. It appears that there is some kind of brain system, which complains when levels of vitamin D3 are lower than they were before (regardless of the value).