My Vitamin D was (and is still) low. I'm also currently at 40, but this is after taking at least 5,000 IU a day and suntanning all summer... seems like it's going up but at a snail's pace. I was thinking about increasing the dosage to 10,000 IU a day, but I'm afraid it might cause hypercalcemia since I'm also afraid to supplement with Vitamin K lol.
How much Vitamin D are you taking (if you're taking any)? Is yours also taking a long time to go up?
Edit: Mine is at 40 ng/mL which is in range but still kind of low.