USA Today has a news article about a study by an Oregon endocrinologist who bought "55 bottles of vitamin D supplements from 12 different manufacturers." The study found the actual amounts of Vitamin D in the pills "ranged from 9% to 140% of what was on the label."
The closest match to the advertised dose was one brand with the seal of the U.S. Pharmacopeial Convention (USP), a voluntary, non-profit testing group, the article reports. The article doesn't name the brand, which isn't particularly helpful to readers.
The article reports that the doctor who did the research "says her study is evidence that more regulation is needed to get companies in line."
I'd argue that it's evidence of no such thing; instead, it's evidence of how voluntary, non-profit regulation can be a useful substitute for top-down, government regulation. The USP mark worked about as well as one would want an FDA regulatory process to work. It doesn't help the customers who buy the vitamins without the USP seal, which I suppose is an argument for FDA regulation along the lines the doctor suggests. But presumably USP and the brand that has the seal will spend some marketing dollars making the results of this study known. The USA Today article is already doing that, too. So it's possible the free market and voluntary regulation will do the job here without requiring more government regulation.