Vitamin D, known as the "sunshine vitamin," is essential for bone health and has other health benefits, including reducing the risk of chronic diseases such as autoimmune disorders, cancer, and cardiovascular disease. Vitamin D is synthesized in the skin or obtained from the diet, and it undergoes two hydroxylations to become biologically active. The primary circulating form is 25-hydroxyvitamin D (25(OH)D), which reflects vitamin D status. Serum 25(OH)D levels are used to assess vitamin D deficiency, insufficiency, or sufficiency. Levels below 20 ng/ml are considered deficient, 21-29 ng/ml insufficient, and above 30 ng/ml sufficient. The historical context of vitamin D began with the discovery of its role in preventing rickets, a disease caused by vitamin D deficiency. Over time, vitamin D was identified as a hormone, and methods to measure its metabolites were developed. 25(OH)D is the main form measured in blood, while 1,25-dihydroxyvitamin D (1,25(OH)₂D) is the active form but is not a reliable indicator of vitamin D status due to its short half-life and association with secondary hyperparathyroidism. Various assays, including radioimmunoassays and high-performance liquid chromatography (HPLC), are used to measure 25(OH)D. Recent advancements in liquid chromatography-tandem mass spectrometry (LC-MS) allow for more accurate quantification of 25(OH)D₂ and 25(OH)D₃. The preferred level for 25(OH)D is now considered to be above 30 ng/ml for optimal health. The upper limit of normal for 25(OH)D has been debated, with some studies suggesting levels up to 100 ng/ml. However, vitamin D intoxication is rare and typically occurs at levels above 150-200 ng/ml. In conclusion, measuring 25(OH)D levels is crucial for assessing vitamin D status. The most reliable method is the measurement of total 25(OH)D, which includes both 25(OH)D₂ and 25(OH)D₃. Maintaining levels above 30 ng/ml is recommended for optimal health outcomes.Vitamin D, known as the "sunshine vitamin," is essential for bone health and has other health benefits, including reducing the risk of chronic diseases such as autoimmune disorders, cancer, and cardiovascular disease. Vitamin D is synthesized in the skin or obtained from the diet, and it undergoes two hydroxylations to become biologically active. The primary circulating form is 25-hydroxyvitamin D (25(OH)D), which reflects vitamin D status. Serum 25(OH)D levels are used to assess vitamin D deficiency, insufficiency, or sufficiency. Levels below 20 ng/ml are considered deficient, 21-29 ng/ml insufficient, and above 30 ng/ml sufficient. The historical context of vitamin D began with the discovery of its role in preventing rickets, a disease caused by vitamin D deficiency. Over time, vitamin D was identified as a hormone, and methods to measure its metabolites were developed. 25(OH)D is the main form measured in blood, while 1,25-dihydroxyvitamin D (1,25(OH)₂D) is the active form but is not a reliable indicator of vitamin D status due to its short half-life and association with secondary hyperparathyroidism. Various assays, including radioimmunoassays and high-performance liquid chromatography (HPLC), are used to measure 25(OH)D. Recent advancements in liquid chromatography-tandem mass spectrometry (LC-MS) allow for more accurate quantification of 25(OH)D₂ and 25(OH)D₃. The preferred level for 25(OH)D is now considered to be above 30 ng/ml for optimal health. The upper limit of normal for 25(OH)D has been debated, with some studies suggesting levels up to 100 ng/ml. However, vitamin D intoxication is rare and typically occurs at levels above 150-200 ng/ml. In conclusion, measuring 25(OH)D levels is crucial for assessing vitamin D status. The most reliable method is the measurement of total 25(OH)D, which includes both 25(OH)D₂ and 25(OH)D₃. Maintaining levels above 30 ng/ml is recommended for optimal health outcomes.