- Antioxidant- Constituent of visual pigments (combines with opsin to produce rhodopsin)- Essential for normal differentiation of epithelial cells into specialized tissue (pancreatic cells, mucus-secreting cells)- Prevents squamous metaplasia- Used to treat acne (oral isotretinoin), measles, and acute promyelocytic leukemia (APL) (all-trans retinoic acid)

- Found in liver and leafy vegetables

Deficiency:- Night blindness (nyctalopia)- Dry, scaly skin (xerosis cutis)- Corneal degeneration (keratomalacia)- Bitot spots (foamy appearance) on conjunctiva- Immunosuppression

Excess:- Acute toxicity – nausea, vertigo, blurred vision- Chronic toxicity – alopecia, dry skin (eg, scaliness), hepatic toxicity and enlargement, arthralgias, and pseudotumor cerebri- Teratogenic (cleft plate, cardiac abnormalities)