Vitamin D is the generic name for a group of closely related steroids. Vitamin D is a hormone rather than a vitamin. The majority of Vitamin D is produced by the action of UV light on 7 dehydrocholesterol in the skin to produce Vitamin D3 - cholecalciferol. A minor proportion comes from food as Vitamin D2-ergocalciferol.
Vitamin D itself is biologically inert. It is metabolised to 25-OH Vitamin D in the liver and thence to the active form 1,25-OH Vitamin D by 1a hydroxylase in the kidneys. In the circulation most of the Vitamin D is bound to Vitamin D binding protein and a smaller amount to albumin.
Vitamin D plays a major role in calcium homeostasis. It increases calcium absorption from the gut, promotes bone mineralisation by provision of calcium, reduces 1a hydroxylase activity in the kidneys and reduces PTH production by the parathyroid glands.
It has other functions that are not as well understood. These include regulation of cellular differentiation and immunomodulation. Measurement of 25-OH-Vitamin D3 assesses the principal storage form of the vitamin.
Causes of low Vitamin D
Reduced production - lack of sunlight
Reduced intake - lack of intake in diet
Reduced absorption - biliary disease
Catabolism - anticonvulsant therapy
Reduced metabolism - chronic renal failure
Causes of high Vitamin D
Increased ingestion - diet / iatrogenic
Increased production - granulomatous disease / lymphoma
Vitamin D is assayed by tandem mass spectrometry
<30 nmol/L - Vitamin D deficiency,
30-50 nmol/L - Borderline low vitamin D
>50 nmol/L - Adequate vitamin D
>300 nmol/L - may indicate Vitamin Toxicity