Vitamin D, known as the "sunshine vitamin," is crucial for bone health and has been linked to reduced risks of chronic diseases, including autoimmune diseases, cancer, and cardiovascular disease. Vitamin D, which can be produced in the skin or obtained through diet, is biologically inactive and requires two hydroxylations: first in the liver to form 25-hydroxyvitamin D (25(OH)D), and then in the kidneys to form the active form, 1,25-dihydroxyvitamin D (1,25(OH)2D). Serum 25(OH)D levels are used to assess vitamin D status, with levels below 20 ng/ml indicating deficiency and 21-29 ng/ml indicating insufficiency. The goal is to maintain levels above 30 ng/ml to maximize health benefits. The historical perspective on vitamin D began with the industrialization of northern Europe, leading to rickets, and has since been extensively studied and fortified into foods. Various assays for measuring 25(OH)D and 1,25(OH)2D have been developed, including competitive protein binding assays, radioimmunoassays, and liquid chromatography tandem mass spectrometry (LC-MS). Despite the biological activity of 1,25(OH)2D, it is not a reliable indicator of vitamin D status due to its short half-life and the influence of parathyroid hormone (PTH) on its levels. The preferred level of 25(OH)D is now recommended to be > 30 ng/ml, with a normative range of 20-100 ng/ml. Vitamin D intoxication, defined as levels > 150 ng/ml, is rare and typically associated with hypercalcemia and hypercalciuria.Vitamin D, known as the "sunshine vitamin," is crucial for bone health and has been linked to reduced risks of chronic diseases, including autoimmune diseases, cancer, and cardiovascular disease. Vitamin D, which can be produced in the skin or obtained through diet, is biologically inactive and requires two hydroxylations: first in the liver to form 25-hydroxyvitamin D (25(OH)D), and then in the kidneys to form the active form, 1,25-dihydroxyvitamin D (1,25(OH)2D). Serum 25(OH)D levels are used to assess vitamin D status, with levels below 20 ng/ml indicating deficiency and 21-29 ng/ml indicating insufficiency. The goal is to maintain levels above 30 ng/ml to maximize health benefits. The historical perspective on vitamin D began with the industrialization of northern Europe, leading to rickets, and has since been extensively studied and fortified into foods. Various assays for measuring 25(OH)D and 1,25(OH)2D have been developed, including competitive protein binding assays, radioimmunoassays, and liquid chromatography tandem mass spectrometry (LC-MS). Despite the biological activity of 1,25(OH)2D, it is not a reliable indicator of vitamin D status due to its short half-life and the influence of parathyroid hormone (PTH) on its levels. The preferred level of 25(OH)D is now recommended to be > 30 ng/ml, with a normative range of 20-100 ng/ml. Vitamin D intoxication, defined as levels > 150 ng/ml, is rare and typically associated with hypercalcemia and hypercalciuria.