IUPAC Name: (1S,3Z)-3-[(2E)-2-[(1R,3aS,7aR)-7a-methyl-1-[(2R)-6-methylheptan-2-yl]-2,3,3a,5,6,7-hexahydro-1H-inden-4-ylidene]ethylidene]-4-methylidenecyclohexan-1-ol 
Empirical Formula: C₂₇H₄₄O
Molecular Weight: 384.6377
EINECS: 215-797-2  
Classification Code: Bone Density Conservation Agents; Growth Substances; Human Data; Micronutrients; Vitamins
Index of Refraction: 1.523
Molar Refractivity: 121.27 cm³
Molar Volume: 396.9 cm³
Surface Tension: 37 dyne/cm
Density: 0.97 g/cm³
Melting Point:
Flash Point: 8.9 °C
Boiling Point: 105.1 °C at 760 mmHg
Vapour Pressure: 34.9 mmHg at 25°C
Enthalpy of Vaporization: 32.98 kJ/mol
Refractive Index:
Storage temp.: Refrigerator
Following is the molecular structure of  Vitamin D (CAS NO.1406-16-2) is:

 Vitamin D (CAS NO.1406-16-2) plays a number of other roles in human health including inhibition of calcitonin release from the thyroid gland. Calcitonin acts directly on osteoclasts, resulting in inhibition of bone resorption and cartilage degradation. It can also inhibit parathyroid hormone secretion from the parathyroid gland, modulate neuromuscular and immune function and reduce inflammation.

The skin consists of two primary layers: the inner layer called the dermis, composed largely of connective tissue, and the outer, thinner epidermis. The epidermis consists of five strata; from outer to inner they are: the stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale.Cholecalciferol is produced photochemically in the skin from 7-dehydrocholesterol; 7-dehydrocholesterol is produced in relatively large quantities in the skin of most vertebrate animals, including humans. The few exceptions are some bat species, mole rats, cats, and dogs, which produce little vitamin D. In most animals the highest concentrations of 7-dehydrocholesterol are found in the epidermal layer of skin, specifically in the stratum basale and stratum spinosum.The production of pre-vitamin D3 is therefore greatest in these two layers, whereas production in the other layers is less.
A critical determinant of vitamin D3 production in the skin is the presence and concentration of melanin. Melanin functions as a light filter in the skin, and therefore the concentration of melanin in the skin is related to the ability of UVB light to penetrate the epidermal strata and reach the 7-dehydrocholesterol-containing stratum basale and stratum spinosum. Under normal circumstances, ample quantities of 7-dehydrocholesterol (about 25-50 µg/cm² of skin) are available in the stratum spinosum and stratum basale of the skin to meet the body's vitamin D requirements, and melanin content does not alter the amount of vitamin D that can be produced. Thus, individuals with higher skin melanin content will simply require more time in sunlight to produce the same amount of vitamin D as individuals with lower melanin content. The amount of time an individual requires to produce a given amount of vitamin D may also depend upon the person's distance from the equator and on the season of the year.In some animals, the presence of fur or feathers blocks the UV rays from reaching the skin. In birds and fur-bearing mammals, vitamin D is generated from the oily secretions of the skin deposited onto the fur and obtained orally during grooming.

Reported in EPA TSCA Inventory.

Human systemic effects by ingestion: changes in tubules (including acute renal failure, acute tubular necrosis), depressed renal function tests, calcium level changes. When heated to decomposition it emits acrid smoke and irritating vapors.

General description about Vitamin D (CAS NO.1406-16-2) ,it is a cream colored powder. It insoluble in water.
Air & Water Reactions: Sensitive to moisture, air and light.
Reactivity Profile: It may react vigorously with strong oxidizing agents. May react exothermically with reducing agents to release hydrogen gas. 
Also, it obtained from sun exposure, food, and supplements, is biologically inert and must undergo two hydroxylation reactions to be activated in the body. Calcitriol (1,25-Dihydroxycholecalciferol)  is the active form of  Vitamin D  found in the body. The term it also refers to these metabolites and other analogues of these substances.
Vitamin D is naturally produced by the human body when exposed to direct sunlight. Season, geographic latitude, time of day, cloud cover, smog, and sunscreen affect UV ray exposure and vitamin D synthesis in the skin, and it is important for individuals with limited sun exposure to include good sources of vitamin D in their diet. Extra vitamin D is also recommended for older adults and people with dark skin. Individuals having a high risk of deficiency should consume 25 μg (1000 IU) of vitamin D daily to maintain adequate blood concentrations of 25-hydroxyvitamin D.
As civilization and the Industrial Revolution enabled humans to work indoors and wear more clothes when outdoors, these cultural changes reduced natural production of vitamin D and caused deficiency diseases. In many countries, such foods as milk, yogurt, margarine, oil spreads, breakfast cereal, pastries, and bread are fortified with vitamin D2 and/or vitamin D3, to minimize the risk of vitamin D deficiency. In the United States and Canada, for example, fortified milk typically provides 100 IU per glass, or a quarter of the estimated adequate intake for adults over age 50.A 1992 study, however, found that the actual vitamin D content of milk varies widely. Supplementation of 100 IU (2.5 microgram) vitamin D3 raises blood calcidiol levels by 2.5 nmol/litre (1 ng/ml).