Learning Objective
Describe the forms, functions, clinical uses, and toxicity of vitamin A, and recognize the key manifestations of deficiency and excess.
Forms
- Retinol (vitamin A)
- Retinal (visual cycle)
- Retinoic acid (gene transcription, epithelial differentiation)
USMLE Pearl: Retinol = vitamin A → think Retin-A (topical retinoid used for acne and wrinkles).
Functions
- Vision:
- Retinal is a component of visual pigments (rhodopsin) → essential for dark adaptation
- Epithelial differentiation:
- Maintains normal differentiation of epithelial cells (eg, pancreatic cells, mucus-secreting cells)
- Prevents squamous metaplasia
- Antioxidant activity
- Immune function
Activity
Sources
- Animal: Liver
- Plant: Leafy green vegetables (β-carotene precursors)
Clinical Uses
- Vitamin A supplementation: Improves outcomes in vitamin A-deficient children with measles
- Isotretinoin (oral): Treatment of severe cystic acne
- All-trans retinoic acid (ATRA): Treatment of acute promyelocytic leukemia (APL) by inducing differentiation
Activity
Deficiency
- Night blindness (nyctalopia) – the earliest manifestation
- Xerosis cutis (dry, scaly skin)
- Xerophthalmia (dry eyes)
- Conjunctival squamous metaplasia
- Bitot spots – foamy, keratinized patches on the conjunctiva
- Keratomalacia – corneal softening and degeneration
- Immunosuppression
High-yield association: Vitamin A deficiency → night blindness + dry eyes/skin
Toxicity (Excess Vitamin A)
Acute Toxicity
- Nausea, vomiting
- Increased intracranial pressure
- Headache, vertigo, blurred vision
Chronic Toxicity
- Alopecia
- Dry, scaly skin
- Hepatotoxicity and hepatomegaly
- Arthralgias
- Idiopathic intracranial hypertension
Teratogenicity
- Interferes with homeobox gene expression
- Causes cleft palate and cardiac abnormalities
- Isotretinoin is highly teratogenic
- Requires pregnancy testing and two forms of contraception before prescription
