The biologically active form of vitamin D is 1,25-dihydroxyvitamin D (1,25(OH)2 D). Measuring serum levels of 1,25(OH)2 D should be considered upon suspicion of deficiency or excess of 1,25(OH)2 D.
Reference ranges of 1,25(OH)2 D may be reported as either pg/mL or pmol/L. The molecular weight of 1,25(OH)2 D is approximately 416.7, yielding the following conversion factors: 1 pmol/L = 0.42 pg/mL; conversely, 1 pg/mL = 2.4 pmol/L. Equivalencies are as follows:
Reference ranges of 1,25(OH)2 D vary since different laboratories may use different methods for measuring 1,25(OH)2 D and different units of measurement.
It is important to refer to each specific laboratory’s reference range before interpreting a test result.
Plasma 1,25-dihydroxyvitamin D (1,25(OH)2 D) is tightly controlled by plasma parathyroid hormone (PTH), serum calcium, serum phosphate, and fibroblast-like growth factor 23 (FGF-23).
Decreased 1,25-dihydroxyvitamin D levels
Chronic kidney disease: Low 1,25(OH)2 D levels have been shown to present even in early stages of kidney failure. The decrease of 1,25(OH)2 D level is more prominent when kidney failure progresses. In a study by Levin et al (2007), 13% of patients with an estimated glomerular filtration rate (eGFR) greater than 80 mL/min and more than 60% of patients with an eGFR of less than 30 mL/min had low serum levels of 1,25(OH)2 D.  Impaired production of the enzyme 1α-hydroxylase in kidney failure was thought to be the main mechanism. However, phosphate retention and FGF-23 also contribute to the decreased synthesis of 1,25(OH)2 D. 
Heritable disorders associated with low 1,25(OH)2 D levels include vitamin D–dependent rickets type 1 (inactivating mutation in the 1-hydroxylase gene),  autosomal-dominant hypophosphatemic rickets (mutation of the gene coding for FGF-23, which prevents its breakdown),  and X-linked hypophosphatemic rickets (mutations that elevate levels of FGF-23). 
In tumor-induced osteomalacia, tumor-secreted FGF-23 inhibits enzyme 1α-hydroxylase and subsequently results in decreased 1,25(OH)2 D synthesis. 
HIV protease inhibitors have been reported to markedly suppress the activities of 25- and 1α-hydroxylase and thus affect 1,25(OH)2 D synthesis.  In a cohort study including 671 patients, progression to bone demineralization was observed in 28% of the patients over a median of 2.5 years. Patients who were concurrently using protease inhibitors were at greater risk for worsening bone demineralization than those who were not using protease inhibitors (OR 1.64; 95% CI, 1.35-2.04; P < 0.0001). 
Severe vitamin D deficiency: 25(OH)D is the main substrate of 1,25(OH)2 D. Vitamin D deficiency can affect the production of 1,25(OH)2 D owing to the lack of substrate. A positive correlation between serum levels of 25(OH)D and 1,25(OH)2 D was observed during seasonal changes. Treatment with 25(OH)D can normalize 1,25(OH)2 D concentrations in patients with vitamin D deficiency. 
Increased 1,25-dihydroxyvitamin D levels
Increased 1,25(OH)2 D levels can result from extrarenal 1α-hydroxylation or hereditary vitamin D–resistant rickets.
In granulomatous disease such as lymphoproliferative disorders, sarcoidosis, tuberculosis, and inflammatory bowel disease, 1α-hydroxylase enzyme activity was found in macrophages as the extrarenal source of 1,25(OH)2 D. When 1α-hydroxylase is activated, it converts 25(OH)D to 1,25(OH)2 D, just as what occurs under physiologic conditions in the kidneys.  However, unlike the kidney, the 1α-hydroxylase in the macrophages in granulomatous diseases is not controlled by the usual physiologic regulators. Moreover, not all conditions or all patients with increased macrophage activity manifest increases in 1α-hydroxylase activity. In vitro studies of monocytes/macrophages indicate that gamma interferon is an important regulator of 1α-hydroxylase but only when other key signaling pathways are also activated (eg, JAK-STAT and MAP-Kinase). 
Hereditary vitamin D-resistant rickets is a very rare autosomal recessive disorder in which mutations of vitamin D receptor (VDR) coding genes cause failure or abnormal binding of vitamin D to VDRs. [14, 15] Patients usually present with hypocalcemia, early-onset rickets, alopecia, and other ectodermal anomalies.
Collection and Panels
Specimen: Blood (0.25 mL room-temperature serum)
Container: Red-top tube, serum separator tube (also acceptable: lavender [EDTA] or pink [K2 EDTA])
Collection method: Routine venipuncture
Storage/transport temperature: Refrigerated (Note: Some laboratories require a frozen specimen; one should check with the specific laboratory before ordering the test)
Stability (collection to initiation of testing; after separation from cells): Ambient, 72 hours; refrigerated, 1 week; frozen, 6 months
Panels: 1,25(OH)2 D is not included in routine comprehensive metabolic panels; it should be ordered as a separate blood test
Other instructions: Collect blood in a standard red-top tube; allow blood to clot at room temperature; centrifuge and separate the serum from the cells immediately within 2 hours of collection
Assays for serum 1,25(OH)2D
Compared to 25(OH)D, 1,25(OH)2 D circulates in the human body at a very low concentration, making its serum levels challenging to assess.
First introduced in 1974, the radioreceptor assay (RRA) originally included several purification steps. Chicken intestinal VDR was used as the binding agent.  In 1984, Reinhardt et al presented a significant improvement in the RRA method whereby the high performance liquid chromatography step was eliminated. Measurement of 1,25(OH)2 D was achieved by a nonequilibrium assay using 1,25(OH)2 D receptor from calf thymus. 
This modification of RRA has made it a more practical assay. However, it still has some disadvantages, including the necessity of VDR isolation and the limitations that comes from using3 H-1,25(OH)2 D3 as the reporter. 
The first radioimmunoassay (RIA) that measured 1,25(OH)2 D was introduced in 1978.  Over the years, an RIA for 1,25(OH)2 D using a radio-iodinated (125 I) tracer was developed by Hollis et al.  The assay involves acetonitrile extraction, treatment of the crude extract supernatant with sodium periodate, extraction and purification of endogenous 1,25(OH)2 D by solid-phase chromatography, and quantification by RIA.
Vitamin D is a group of fat-soluble compounds with a four-ringed cholesterol backbone. First described by Whistler and Glisson in the mid 1600s, vitamin D is now recognized as a prohormone.
Vitamin D exists in two major forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). The precursor for vitamin D2 is a plant sterol ergosterol. D2 can be synthesized by ultraviolet irradiation of ergosterol from yeast. Similarly, vitamin D3 is synthesized in the body when sunlight (ultraviolet B, wavelength 280-315 nm) photoisomerizes 7-dehydroxycholesterol found in the skin. D3 is also found in animal-based foods (eg, fatty fish, liver, milk, eggs).
Vitamin D2 and D3, regardless of the source, are biologically inactive. They are transformed into the biologically active molecule 1,25 dihydroxyvitamin D. After being synthesized in the skin or absorbed (in chylomicrons) from the gastrointestinal (GI) tract, most vitamin D is bound to specific carrier proteins in the blood (vitamin D–binding protein [DBP] and albumin) and transported to the liver. In the liver, vitamin D is hydroxylated by the enzyme 25-hydroxylase (CYP2R1) to become 25(OH)D. 25(OH)D is the major circulating form of vitamin D. From the liver, 25(OH)D is transported to the kidneys via the same carrier proteins as above.
1,25 dihydroxyvitamin D (1,25(OH)2 D) is formed when 25(OH)D is hydroxylated by the enzyme 1α-hydroxylase (CYP27B1), which is located in the mitochondria of proximal tubules of the kidney. 1,25(OH)2 D is the biologically active form of vitamin D. As a result of 1 and 25 hydroxylation, the prohormone vitamin D has been transformed into an active hormone.
1,25(OH)2 D is a steroidlike hormone. In target cells, such as classic steroid hormones, it binds to a specific cytoplasmic VDR; the vitamin D bound to VDR then translocates to the nucleus, where its effects are initiated at a transcriptional level.  The main established actions of 1,25(OH)2 D collectively increase calcium in the body and modulate the skeleton. It increases the intestinal absorption of calcium and phosphate, decreases renal excretion of calcium and phosphate, suppresses PTH production, and regulates osteoblast function and bone resorption. It has been suggested that 1,25(OH)2 D has roles beyond the calcium-skeletal axis. 
The identification of VDRs in various cells has prompted the investigation of vitamin D in immunomodulation, cancer prevention and therapy, autoimmune disease, cardiovascular disease, and other nonskeletal organs.
The synthesis of 1,25(OH)2 D is tightly regulated by PTH, serum calcium, serum phosphate, and fibroblast-like growth factor 23 (FGF-23). Increased levels of PTH and hypophosphatemia stimulate the enzyme 1α-hydroxylase and, subsequently, the synthesis of 1,25(OH)2 D. FGF-23 is a circulating hormone synthesized by osteocytes and osteoblasts. 1,25(OH)2 D and phosphate intake stimulates the synthesis of FGF-23, which, in turn, inhibits 1,25(OH)2 D production, reduces the expression of renal sodium–phosphate transporters, and activates the metabolizing of active 1,25(OH)2 D to the inactive metabolite 24,25(OH)2 D. 
As a fat-soluble molecule, vitamin D is stored in adipose tissue; however, the exact mechanism by which vitamin D is regulated and mobilized from adipose tissue has not been elucidated at this time. Most vitamin D products are excreted through bile into the gut. Very little is eliminated via kidneys (see graph below).
Measuring serum levels of 1,25(OH)2 D should be considered upon suspicion of deficiency or excess of 1,25(OH)2 D.
1,25(OH)2 D assessment may be beneficial in patients with chronic kidney failure; early-onset rickets; family history of rickets; or long-term use of protease inhibitors, glucocorticoids, or anticonvulsants who are at risk of hypocalcemia from low concentration of 1,25(OH)2 D.
1,25(OH)2 D may be helpful in diagnosing parathyroid function disorders. A high serum level of 1,25(OH)2 D, for example, may suggest of primary hyperparathyroidism, whereas a normal or low serum level is more likely found in secondary hyperparathyroidism.
Patients with hypercalcemia can undergo testing for both 25(OH)D and 1,25(OH)2 D to rule out vitamin D intoxication or disorders that can enhance 1,25(OH)2 D synthesis, such as lymphoma, sarcoidosis, and other granulomatous diseases.
Serial 1,25(OH)2 D levels can be used in monitoring the efficacy of treatment in patients receiving 1,25(OH)2 D supplementation.
In overweight and obese individuals, serum 1,25(OH)2 D concentrations were reported to be correlated with 25(OH)D levels and seasonal variations. This suggests that measurement of both levels might be beneficial when assessing vitamin D stores in this population. 
The half-life of 1,25(OH)2 D is approximately 4 hours. Therefore, it is not useful in assessing the total-body vitamin D status. Also, it must be noted that a normal level of 1,25(OH)2 D does not exclude a diagnosis of vitamin D deficiency. Measurement of 25(OH)D levels should be used for this purpose.