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Hallelujah, this is what I wanted to find, an authoritative source of why the vitamin D blood test is problematic, plus some other need to know points. Read the whole thing. It explains a lot.

Dietary Supplement Fact Sheet: Vitamin D

An additional complication in assessing vitamin D status is in the actual measurement of serum 25(OH)D concentrations. Considerable variability exists among the various assays available (the two most common methods being antibody based and liquid chromatography based) and among laboratories that conduct the analyses [1,7,8]. This means that compared with the actual concentration of 25(OH)D in a sample of blood serum, a falsely low or falsely high value may be obtained depending on the assay or laboratory used [9]. A standard reference material for 25(OH)D became available in July 2009 that permits standardization of values across laboratories and may improve method-related variability [1,10].

Health Risks from Excessive Vitamin D

Vitamin D toxicity can cause non-specific symptoms such as anorexia, weight loss, polyuria, and heart arrhythmias. More seriously, it can also raise blood levels of calcium which leads to vascular and tissue calcification, with subsequent damage to the heart, blood vessels, and kidneys [1]. The use of supplements of both calcium (1,000 mg/day) and vitamin D (400 IU) by postmenopausal women was associated with a 17% increase in the risk of kidney stones over 7 years in the Women's Health Initiative [65]. A serum 25(OH)D concentration consistently >500 nmol/L (>200 ng/mL) is considered to be potentially toxic [5].
Excessive sun exposure does not result in vitamin D toxicity because the sustained heat on the skin is thought to photodegrade previtamin D3 and vitamin D3 as it is formed [6]. In addition, thermal activation of previtamin D3 in the skin gives rise to various non-vitamin D forms that limit formation of vitamin D3 itself. Some vitamin D3 is also converted to nonactive forms [1]. Intakes of vitamin D from food that are high enough to cause toxicity are very unlikely. Toxicity is much more likely to occur from high intakes of dietary supplements containing vitamin D.
Long-term intakes above the UL increase the risk of adverse health effects [1] (Table 4). Most reports suggest a toxicity threshold for vitamin D of 10,000 to 40,000 IU/day and serum 25(OH)D levels of 500–600 nmol/L (200–240 ng/mL). While symptoms of toxicity are unlikely at daily intakes below 10,000 IU/day, the FNB pointed to emerging science from national survey data, observational studies, and clinical trials suggesting that even lower vitamin D intakes and serum 25(OH)D levels might have adverse health effects over time. The FNB concluded that serum 25(OH)D levels above approximately 125–150 nmol/L (50–60 ng/mL) should be avoided, as even lower serum levels (approximately 75–120 nmol/L or 30–48 ng/mL) are associated with increases in all-cause mortality, greater risk of cancer at some sites like the pancreas, greater risk of cardiovascular events, and more falls and fractures among the elderly. The FNB committee cited research which found that vitamin D intakes of 5,000 IU/day achieved serum 25(OH)D concentrations between 100–150 nmol/L (40–60 ng/mL), but no greater. Applying an uncertainty factor of 20% to this intake value gave a UL of 4,000 IU which the FNB applied to children aged 9 and older, with corresponding lower amounts for younger children.

Interactions with Medications

Vitamin D supplements have the potential to interact with several types of medications. A few examples are provided below. Individuals taking these medications on a regular basis should discuss vitamin D intakes with their healthcare providers.
Steroids
Corticosteroid medications such as prednisone, often prescribed to reduce inflammation, can reduce calcium absorption [66,67,68] and impair vitamin D metabolism. These effects can further contribute to the loss of bone and the development of osteoporosis associated with their long-term use [67,68].
Other medications
Both the weight-loss drug orlistat (brand names Xenical® and alliTM) and the cholesterol-lowering drug cholestyramine (brand names Questran®, LoCholest®, and Prevalite®) can reduce the absorption of vitamin D and other fat-soluble vitamins [69,70]. Both phenobarbital and phenytoin (brand name Dilantin®), used to prevent and control epileptic seizures, increase the hepatic metabolism of vitamin D to inactive compounds and reduce calcium absorption [71].

FNB means Food and Nutrition Board.

>>While symptoms of toxicity are unlikely at daily intakes below 10,000 IU/day, the FNB pointed to emerging science from national survey data, observational studies, and clinical trials suggesting that even lower vitamin D intakes and serum 25(OH)D levels might have adverse health effects over time.<<

First, a note on measurement of vit D3 in the blood: Don’t confuse nanomoles/liter with nanograms/milliliter. In the U.S. we tend to use the latter measurement, ng/ml. To convert to nmol/L (used in Canada, Great Britain, etc.) you need to multiply by 2.5. Thus, a level of 50 ng/ml is equivalent to 125 nmol/L.

Second, in the quote taken above from the article, note the hedging by the FNB. They admit toxicity is unlikely when supplementing at 10,000 IU, but then cite non-experimental studies that “suggest” that such levels “might” have adverse health effects. The FNB was dragged kicking and screaming into DOUBLING their previous upper limits, not raising them 10% or 25%, or even 50%, but 100%! And they did this without allowing participation by the most active vitamin D researchers. The benefits of higher blood levels of vitamin D3 are becoming apparent even to them.

There’s little doubt that getting vitamin D3 from sun or from tanning is preferred due to the automatic mechanism the body has for preventing excess accumulation. But few people do so, and supplementation then remains the only option. Besides, the older one gets, the less is absorbed through the skin, in many cases.

Now, I wouldn’t take 10,000 IU unless I knew I was starting at an exceptionally low level, and I would test regularly to make sure I wasn’t getting into a level of 100 ng/ml (250 nmol/L), but 5,000 IU/day was acknowledged even by the FNB to be required to get to the 48-60 ng/ml level, the level that the active researchers consider to be close to that required to get the optimum benefits. And people weighing 200-250 pounds are likely to need even higher levels, because weight matters too.

For black FReepers, please note that supplemental vitamin D is not without risk as I have excerpted below as well as the fact that's been known for decades that blacks suffer from osteoporosis and its related fractures much less than whites.

Skepticism Mounts on Need for Vitamin D Supplementation

One of these concerns is that not all of the extra calcium absorption promoted by boosting vitamin D is going into bone to prevent fractures. Some of it may actually be taken up by atherosclerotic plaque, increasing the risk of cardiovascular events, Dr. Lenore M. Buckley cautioned at a symposium sponsored by the American College of Rheumatology.
--snip--
Discussing findings from a recent cross-sectional study involving 340 African American patients with type 2 diabetes, Dr. Buckley said that serum 25-hydroxyvitamin D levels were positively associated with increased calcified atherosclerotic plaque in the aorta and carotid arteries (J. Clin. Endocrinol. Metab. 2010 Jan. 8, 2010; [doi:10.1210/jc.2009-1797]
“The effects of supplementing vitamin D to raise the serum 25-hydroxyvitamin D level on atherosclerosis in African Americans are unknown. Prospective trials are needed,” the investigators said.
--snip--
There is intriguing evidence to indicate the optimal level of vitamin D to promote bone health, muscle strength, immunity, and other key functions may vary by race. Data from the National Health and Nutrition Examination Survey show that very few white children aged 1-12 years are vitamin D deficient using the classic threshold of 15 ng/mL. In contrast, about 10% of non-Hispanic black 1- to 6-year-olds are vitamin D deficient, as are close to 30% of those in the 7- to 12-years-old age bracket (Pediatrics 2009; 124: e362-70 [doi:10.1542/peds.2009-0051]).
Many observers see this racial disparity as a public health problem reflecting unequal access to services. But there is a conundrum here: If vitamin D deficiency is rampant in black children, why do they have greater bone strength and muscle mass, on average, than white children?
Such observations make “one wonder whether the definition of normal levels should vary by race,” according to the rheumatologist.
Support for this theory comes from studies showing that pushing serum vitamin D levels to 30 ng/mL or higher in whites reduces their parathyroid hormone levels, while pushing levels above 20 ng/ml in African Americans—young or old—doesn't further decrease parathyroid hormone or increase bone density.
Asked by audience members what she does about vitamin D in her own practice, Dr. Buckley said she generally tries to get patients into the 20-29 ng/mL range, while in African Americans and patients with known cardiovascular disease she aims for 15 ng/mL or slightly more—“and I worry that might be too high sometimes.”

The sheet you gave is from the GOVERNMENT!!! The people who think our diets should be FILLED with grains! You can't use that as fact. 600 IU of D as a supplement?? Why, you'd get colds and flus just like anyone else. The use of supplements of both calcium (1,000 mg/day) and vitamin D (400 IU) by postmenopausal women was associated with a 17% increase in the risk of kidney stones over 7 years in the Women's Health Initiative [65].

Duh, this is from the CALCIUM supplementation. Calcium in supplement form causes kidney stones. They didn't even tease out another supplement, so D goes down with the ship? Plus, they didn't specify D3, which is the only kind to supplement.

The entire paragraph on toxicity levels for Vitamin D is as misleading as anything coming out of the Obama White House. YES, those high serum blood levels may be toxic - over 100, they mention 400, etc. That's INSANE. The sheet acts as if you can get that level of D in your bloodstream from 5000 or 10,000 IU??? NOT BLOODY LIKELY! You need to get yourself tested, and increase your supplementation until your tests show a bloodstream level of 60 for a younger, healthy person and about 70-75 for anyone at risk - older, not so well, higher risk of cancer and chronic illnesses.

Most of the new research shows that we thrive with a higher level of Vitamin D (it is more of a hormone than a vitamin at this point by its actions in the body).

We started supplementing, kids to adults, last fall, with blood tests to achieve the optimum levels, and for the first time in forever, NO ONE GOT A SINGLE COLD OR FLU the entire winter. Not one cough or sniffle. One boy got a stomach flu and no one else caught it. That was since last early fall. We were exposed to every flu and bug known to mankind, all our kids' friends got them and we spent time close to them all the days before they became ill.

We started to buy Vit D supplements for all the elderly in our families and they are doing better too, though they tend to not even take their vities daily. One couple in their 90s for the first time only had ONE cold (one of the couple) and that was a miracle for them, back east with the cold winters.