**Abstract** :  
  
Vitamin D is a hormone, synthesized by the skin and metabolized by the kidney
to an active hormone, calcitriol. An excess of vitamin D may lead to
hypercalcemia. Vitamin D deficiency may lead to a variety of disorders. This
LCD identifies the indications and limitations of Medicare coverage and
reimbursement for these services.

Vitamin D is called a "vitamin" because of its exogenous source, predominately
from oily fish in the form of vitamin D2 and vitamin D3. It is really a
hormone, synthesized by the skin and metabolized by the kidney to an active
hormone, calcitriol, which then acts throughout the body. In the skin,
7-dehydrocholesterol is converted to vitamin D3 in response to sunlight, a
process that is inhibited by sunscreen with a skin protection factor (SPF) of
8 or greater. Once in the blood, vitamin D2 and D3 from diet or skin bind with
vitamin D binding protein and are carried to the liver where they are
hydroxylated to yield calcidiol. Calcidiol then is converted in the kidney to
calcitriol by the action of 1a-hydroxylase (CYP27B1). The CYP27B1 in the
kidney is regulated by nearly every hormone involved in calcium homeostasis,
and its activity is stimulated by PTH, estrogen, calcitonin, prolactin, growth
hormone, low calcium levels, and low phosphorus levels. Its activity is
inhibited by calcitriol, thus providing the feedback loop that regulates
calcitriol synthesis.

An excess of vitamin D is unusual, but may lead to hypercalcemia. Vitamin D
deficiency may lead to a variety of disorders, the most infamous of which is
rickets. Evaluating patients' vitamin D levels is accomplished by measuring
the level of 25-hydroxyvitamin D. Measurement of other metabolites is
generally not medically necessary.  
  
**Indications:**  
  
Measurement of vitamin D levels is indicated for patients with:

  * chronic kidney disease stage III or greater;
  * osteoporosis;
  * osteomalacia;
  * osteopenia;
  * hypocalcemia;
  * hypercalcemia;
  * hypercalciura;
  * hypoparathyroidism;
  * hyperparathyroidism;
  * malabsorption states;
  * cirrhosis;
  * hypervitaminosis D;
  * obstructive jaundice;
  * osteosclerosis/petrosis;
  * rickets;
  * low exposure to sunlight; and
  * vitamin D deficiency to monitor the efficacy of replacement therapy
  * Obesity

**Limitations:**  
For Medicare beneficiaries, screening tests are governed by statute. Vitamin D
testing may not be used for routine screening.

Once a beneficiary has been shown to be vitamin D deficient, further testing
is medically necessary only to ensure adequate replacement has been
accomplished. Thereafter, annual testing may be appropriate depending upon the
indication and other mitigating factors.  
  

_Obesity_

Obesity has been linked to vitamin D deficiency thought to be due the
sequestration of vitamin D in body fat leading to reduced availability of
vitamin D and lower dietary intake of vitamin D containing supplements [1, 2].
Early studies found lower serum Vitamin D levels in non-operative morbidly
obese patients, with 62% having deficiencies in serum 25-hydroxyvitamin D
(25-OHD) levels [3]. A prospective cohort study comparing obese to non-obese
subjects found a serum 25-OHD and 1,25-dihydroxy vitamin D was negatively
correlated with body mass index (BMI) in Caucasian and African-American adults
(p<0.0001 for both groups) [4]. A large Canadian cohort with 5,569 individuals
reported that a BMI >30 (obese) was strongly associated with lower levels of
serum 25-OHD (<75nmol/L) in both males and females. A multivariate regression
analysis that included dietary intake of vitamin D, sunlight exposure, and
supplementation showed the impact of BMI was an independent variable (-11.12
(-14.04;-8.21) females and -8.17 (-13.49; -2.85) males [5]. National Health
and Nutrition Examination Survey (NHANES) data reported lower concentrations
of 25-OHD levels among obese white women compared to leaner counterparts [2].
Obese adults also have been found to have difficulty raising their vitamin D
levels by sunlight, ultraviolet light exposure, or supplementation as compared
to nonobese adults [6, 7].

_Obesity_

The American Association of Clinical Endocrinologists, The Obesity Society and
American Society for Metabolic, and Bariatric Surgery Guidelines recommend
vitamin D supplements titrated to therapeutic 25-hydroxyvitamin D levels
>30ng/ml after bariatric surgery (Grade A recommendation)[8]. Up to Date
includes obese patients at a high-risk for vitamin D insufficiency[9]. Obese
individual are considered high-risk for vitamin D deficiency. The relationship
between obesity and vitamin D deficiency has been clearly reported and
supported by societal guidance. Obesity is a reasonably and medically
necessary indication for Vitamin D Assay measurement.