Vitamin D 3 (colecalciferol) is formed from its precursor 7-dehydrocholesterol in the skin by ultraviolet irradiation. In the liver the vitamin D 3 is hydroxylated to form 25-hydroxyvitamin D 3 , which is metabolised to its active metabolite 1,25-dihydroxyvitamin D 3 preferentially in the kidney. Vitamin D 3 may also be provided in the diet, which is a significant source of supply only in the case of insufficient exposure to sunlight. Blue fish naturally contains large quantities of vitamin D 3 , while other foods contain significant quantities of vitamin D only after being fortified. For fortification, in many countries vitamin D 2 is used (ergocalciferol) obtained from vegetable sources 1 . The presence of the enzyme CYP27B1, which drives the synthesis of dihydroxyvitamin D from its principal substrate, 25-hydroxyvitamin D, and the vitamin D receptor (VDR), distributed almost universally in the cells and tissues of the organism, confers on vitamin D (although it would be increasingly more correct to say the endocrine system of vitamin D) a broad role in health. This is not only in the regulation of calcium and bone metabolism but also in relation to the cardiovascular system, innate or acquired immunomodulation, the regulation of cell growth, etc., such that around 3% of the human genome is regulated by the hormone 1,25(OH) 2 vitamin D 3 1-3 . It is therefore not surprising that basic scientific and clinical interest in vitamin D 4 , as well as interest in the non-specialist press and in the general population 5 , has increased almost exponentially in the last decade. 25-hydroxyvitamin D, marker for the status of vitamin D in the bodyFor some years there has been universal consensus that the measurement of the levels of the metabolite 25-hydroxyvitamin D in the blood is the marker for the status of vitamin D in the body, including the endogenous synthesis due to exposure to sunlight, dietary consumption in foods, supplemented or not, and pharmacological treatments 1,2

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