Vitamin A (retinol)

Stored vitamin A, measured as retinol, which the liver releases at a steady rate.

It matters for night vision, skin and immune function, and unlike most vitamins it is genuinely toxic in excess.

The liver holds it so tightly that the blood value stays normal until stores are nearly gone, which makes a normal result weak reassurance. It falls with any inflammation, so check CRP before reading a low value as deficiency.

Worth discussing a high value, which matters as much as a low one.