The ABCs of Vitamin D

Vitamin D is produced from 7-dehydrocholesterol in the skin when the skin is UV-irradiated by sunlight. In humans, vitamin D3 or cholecalciferol is specifically produced. Plants produce predominately vitamin D2 or ergocalciferol. While human bodies can utilize vitamin D2, they preferentially use D3 and the rest of this post will be talking about D3.

Vitamin D is actually more of a steroid hormone than a vitamin. Unlike vitamins, Vitamin D is produced in the body, and like hormones, it is produced in skin cells and acts on cells at sites distant from its site of production. The primary functions of vitamin D include actions to increase blood calcium levels. Low blood calcium levels cause a release of parathyroid hormone (PTH), which in turn activates vitamin D. Vitamin D then actively increases calcium absorption from the intestine and helps mobilize calcium from the bone. Vitamin D has been studied and associated with health benefits ranging from decreasing the risks of getting various types of cancer to lowering the risks of heart attacks and type 1 diabetes, causing it’s measurement to become an almost routine part of most physicals and resulting in a large testing volume in the lab.

In the body, Vitamin D exists in multiple forms. The vitamin D produced in the skin is hydroxylated in the liver to give 25-hydroxy-vitamin D (25-OH-D), the main circulating form of Vitamin D. This form is not biologically active. When the correct physiological signals are received however (low calcium and high PTH), another hydroxyl group is added to 25-OH-D in the kidneys, to form the biologically active form, 1,25-dihydroxy-vitamin D (1,25-diOH-D). 1,25-diOH-D is present in very low concentrations.

25-OH-D is the form measured when assessing a person’s overall vitamin D status. It is in the greatest concentration in the body and has a half-life of 2 to 3 weeks. It can be measured by a variety of immunoassays as well as by tandem mass spectrometry. Unfortunately not all assays measure the same forms of 25-OH-D, and thus values can differ significantly depending on the assay used to measure them. This is a major problem because vitamin D has health-based reference intervals, not population based. This means that studies have determined that 25-OH-D concentrations below 30 µg/dL suggest vitamin D deficiency. So all assays use this 30 µg/dL cut-off, even though all assays don’t measure the same amount of vitamin D in samples. Cholesterol is another example of an analyte with health-based reference intervals. We say a person’s cholesterol should not exceed 200 mg/dL, rather than establishing the population-based reference intervals for cholesterol for our population.

1,25-diOH-D is much more difficult to measure because it occurs in much lower concentrations, with a half-life in the body of 4 to 6 hours. It is generally only measured when renal function is impaired, or to check for diseases involving vitamin D metabolism. It is often ordered in error when the healthcare provider actually wants to know the patients overall vitamin D status, the 25-OH-D concentration. Assays for measuring 1,25-diOH-D include radioimmunoassays and extraction followed by liquid chromatography-tandem mass spectrometry. This testing is usually performed in reference labs.

-Patti Jones PhD, DABCC, FACB, is the Clinical Director of the Chemistry and Metabolic Disease Laboratories at Children’s Medical Center in Dallas, TX and a Professor of Pathology at University of Texas Southwestern Medical Center in Dallas.

6 thoughts on “The ABCs of Vitamin D”

  1. Hi Dr Jones,
    Thanks for this short ,crisp and lucid write up in Vit D. It is indeed useful.
    I was just wondering that which cut off for Vit D should we use? 30 or 20. To go with the Endocrine Society Guidelines or the Institute of Medicine (IOM) report? Interestingly both are from the US.
    Sutirtha Chakraborty

  2. Hi Dr Chakraborty,

    That’s a great question, and one that I think everyone who measures Vit D struggles with. To a certain extent, it probably depends on your assay, since some assays give higher values than others. For me, when we validated the assay here we ran 60 random patients along the way. 43 of them had values of 30 or below. Not wanting to believe that 72% of my patient population was vitamin D deficient in Texas in the summer, I chose to use 20 for my cut-off rather than 30.

  3. Dear Dr Jones,
    We did a study (unpublished) of apparently healthy subjects( n=1550, adults, screened through out the year( 2014), so no fixed season, chemiluminescence immunoassay method used). Guess what the findings were…………..
    86.9 percent of males and 90.5 % females had Vit D less than 20.
    Is this possible??? or we are measuring incorrectly? Or Vit D is just a hoax?

    (Also “unfortunately” Vit D deficient )

  4. Dear Dr Chakraborty,

    I would say it’s a combination of things, one of them being that your assay probably runs lower than the assays used to set the 20 ng/mL cut-off. This is a known problem with vitamin D assays. Even though everyone is working to standardize these assays, on a relatively recent Proficiency Testing survey in the US, one sample gave means that ranged from 23 – 85 ng/mL, depending on the assay used! Another sample gave a range of means from 2 – 18 ng/mL.
    What is a hoax is not so much vitamin D itself, as it is using a single cut-off for deficiency for all assays.
    I believe the deficiecny cut-offs were determined by measuring vitamin D and PTH simultaneously in samples. The idea here is basically that when your vitamin D level is too low, your PTH will rise. The cut-off for deficient is then determined by the point where PTH rises. You may be able to set your own cut-off values, using this type of approach.


  5. Hi Dr. Jones, thank you for your blog, I love reading and learning from them. Could you please write on diabetes according lab perspective and the current elaborate more on the them.
    Thank you again!

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