"My primary care doctor said to stop the vitamin D because it's toxic. So I stopped it and I just take a multivitamin. He said that a multivitamin and two glasses of milk a day was all I needed."
So proclaimed Eleanor to me. This happens around once every week by doctors frightened of the vitamin D.
So I reminded Eleanor that, before starting vitamin D supplementation, her blood level of 25-OH-vitamin D3 had been 17 ng/ml--severe deficiency.
On 4000 units per day (oil-based gelcap), her blood level had been 37 ng/ml--still deficient, below the desirable range of 50-60 ng/ml. That's the dose Eleanor's doctor had declared "toxic."
When exactly does deficiency develop? There's not full agreement on this, but Dr. Michael Holick of Boston University, among the most experienced and insightful authorities on vitamin D, states that toxicity is more likely when blood levels exceed 150 ng/ml (Nutr Clin Pract. 2007 Jun;22(3):297-304).
In other words, Eleanor and her doctor should not be concerned with toxicity, but with the persistent levels of deficiency she is suffering.
Some authorities call the behavior of vitamin D "bi-phasic": Deficiency is toxic, excessive levels are toxic. We're really just trying to achieve a middle ground in vitamin D levels that are above deficiency but below toxicity.
In reality, deficiency is exceptionally common. In fact, it's the rule around here (northern U.S.), with >95% of everybody we check severely deficient in winter, mildly-moderately deficient in summer. Very few people approach normal levels year round without supplementation.
Toxicity, on the other hand, is exceedingly rare. I have seen it once in a woman who was taking a toxic dose of 50,000 units a day on the instructions of her (mis-guided) doctor. Thankfully, no ill-effects developed from this little "experiment."
So, it's not toxicity that is the overwhelmingly common worry, but deficiency, severe and sustained.