What does vitamin A do in the body?

Vitamin A regulates gene expression through nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs) in virtually every cell type. Its critical functions include: immune regulation (required for innate and adaptive immunity, particularly mucosal barrier defense and T regulatory cell function), vision (retinal is the chromophore in rhodopsin, the photoreceptor pigment in rods; night blindness is the first sign of deficiency), skin and mucosal integrity (retinoic acid maintains epithelial differentiation; deficiency produces keratinization), gene expression for cell differentiation and apoptosis, reproduction (required for spermatogenesis and fetal development), and thyroid hormone receptor sensitization.

What are the symptoms of vitamin A deficiency?

Night blindness (difficulty seeing in low light) is the earliest and most specific symptom. Other manifestations include xerophthalmia (dry eyes and Bitot's spots on the conjunctiva), increased susceptibility to infections particularly respiratory and gastrointestinal (vitamin A is required for mucosal immunity), follicular hyperkeratosis (rough, bumpy skin from abnormal keratinocyte differentiation), impaired wound healing, and in children, growth retardation. Severe deficiency causes corneal ulceration and blindness, primarily affecting children in developing countries.

Is vitamin A important for autoimmune thyroid disease?

Yes. Vitamin A is a critical regulator of immune tolerance through its role in generating regulatory T cells (Tregs) in gut-associated lymphoid tissue. Retinoic acid produced from vitamin A is essential for the differentiation of naive T cells into Tregs rather than pro-inflammatory Th17 cells. Vitamin A deficiency impairs this Treg/Th17 balance, promoting autoimmune activation. Low vitamin A status is associated with worsening autoimmune thyroid disease, and optimization to adequate levels (50 to 80 mcg/dL) is an important component of the Hashimoto's root-cause protocol.

Lab Reference Library / Vitamin A (Retinol) Nutritional & Micronutrient

Vitamin A (Retinol)

Q: Can vitamin A be toxic?

Yes. Preformed vitamin A (retinol and retinyl esters from animal sources, cod liver oil, or supplements) accumulates in the liver and can be toxic at chronically elevated doses. Acute toxicity from single doses above 200,000 IU causes headache, nausea, and increased intracranial pressure. Chronic toxicity from daily doses above 10,000 IU over months to years causes liver fibrosis, bone pain, hypercalcemia, and hair loss. In pregnancy, preformed vitamin A above 10,000 IU daily is teratogenic. Beta-carotene from plant foods (provitamin A) is not directly toxic because conversion to retinol is regulated by the body, though very high intake produces harmless carotenemia (orange skin).

Retinol · Serum Vitamin A · Plasma Retinol

Reference range, optimal functional medicine levels, and why vitamin A is essential for immune function, vision, gene expression, and skin integrity, why both deficiency and toxicity are clinically significant, and why retinol testing is important before high-dose vitamin A supplementation.

Most SearchedFat-Soluble Vitamin

Standard Range30 to 95 mcg/dL

FM Optimal50 to 80 mcg/dL

Toxicity RiskAbove 100 mcg/dL

Unitsmcg/dL

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Category: Nutritional & Micronutrient | Also known as: Retinol, Serum Vitamin A, Plasma Retinol

1. What This Test Measures

Serum retinol measures the circulating concentration of retinol, the primary form of preformed vitamin A in the blood, transported bound to retinol-binding protein (RBP) and transthyretin. Vitamin A encompasses retinoids (retinol, retinal, retinoic acid) and provitamin A carotenoids (primarily beta-carotene). Retinoic acid acts through nuclear retinoic acid receptors (RARs and RXRs) to regulate gene expression in virtually every cell type, making vitamin A one of the most broadly influential micronutrients in the body.

2. Optimal Range and Risk Thresholds

Serum Retinol	Interpretation
Below 20 mcg/dL	Severely deficient: night blindness, xerophthalmia, high infection risk
20 to 40 mcg/dL	Deficient: impaired immune function, mucosal integrity, and vision
40 to 50 mcg/dL	Low-normal: borderline; supplementation likely beneficial
50 to 80 mcg/dL	Optimal: adequate for immune regulation, vision, and gene expression
80 to 100 mcg/dL	Adequate but upper range; monitor if supplementing preformed retinol
Above 100 mcg/dL	Elevated: hepatotoxicity and teratogenicity risk from preformed retinol; evaluate supplement intake

Beta-carotene from plant foods does not cause vitamin A toxicity because conversion to retinol is tightly regulated. Preformed retinol from animal foods (liver, cod liver oil) and supplements accumulates in the liver and can be toxic at sustained high doses. Always distinguish dietary source when evaluating elevated retinol.

3. Critical Functions of Vitamin A

Immune regulation: retinoic acid is required for the differentiation of naive T cells into regulatory T cells (Tregs) in gut-associated lymphoid tissue; without adequate vitamin A, the Treg-to-Th17 balance shifts toward autoimmunity; deficiency dramatically increases susceptibility to respiratory and gastrointestinal infections
Vision: retinal is the chromophore in rhodopsin, the photoreceptor pigment in rod cells; night blindness is the earliest clinical sign of deficiency
Mucosal barrier integrity: retinoic acid maintains epithelial differentiation throughout respiratory, gastrointestinal, and genitourinary tracts; deficiency causes keratinization of mucosal surfaces and impaired barrier function
Gene expression: RARs and RXRs regulate hundreds of genes affecting cell differentiation, proliferation, and apoptosis; vitamin A status influences thyroid hormone receptor sensitivity, immune gene expression, and epithelial cell turnover
Reproduction: vitamin A is required for spermatogenesis and normal fetal development; deficiency impairs fertility; excess is teratogenic in early pregnancy

4. Vitamin A and Thyroid Autoimmunity

Vitamin A is a critical regulator of immune tolerance. Retinoic acid drives differentiation of naive T cells toward regulatory T cells (Tregs) rather than pro-inflammatory Th17 cells in gut-associated lymphoid tissue. Vitamin A deficiency impairs this Treg/Th17 balance, promoting autoimmune activation. Low retinol (below 40 to 50 mcg/dL) is associated with worsening autoimmune thyroid disease. Optimizing vitamin A to 50 to 80 mcg/dL is an important component of the Hashimoto's root-cause protocol alongside selenium, vitamin D, and zinc.

5. Supplementation Considerations

Dietary Sources

Beef liver: 26,000 to 30,000 IU per 3 oz (highest preformed source; consume no more than once weekly)
Cod liver oil: 4,000 to 14,000 IU per teaspoon (variable; monitor total intake)
Egg yolks: 270 IU per large egg
Dairy (whole milk, butter, cheese): 100 to 500 IU per serving
Sweet potato (provitamin A, not preformed): 21,000 IU beta-carotene per medium potato (safe; conversion regulated)
Carrots, pumpkin, leafy greens: high in provitamin A carotenoids; not toxic

Supplementation Safety

For confirmed deficiency: 5,000 to 10,000 IU preformed retinol daily is safe for non-pregnant adults; recheck serum retinol at 3 months
Do not exceed 10,000 IU preformed retinol daily long-term without serum monitoring
In women of childbearing age: limit preformed retinol to below 5,000 IU daily; beta-carotene is safe
Chronic toxicity signs: headache, bone pain, hair loss, dry skin, liver enzyme elevation; onset at sustained doses above 25,000 IU daily for months to years
Mixed carotenoid supplements (beta-carotene, alpha-carotene, lycopene): provide provitamin A safely without preformed retinol risk

Synergistic Nutrients

Vitamin D: retinoid receptors (RXR) form heterodimers with vitamin D receptors; vitamin A and D work together in immune gene regulation; optimize both together
Zinc: required for retinol-binding protein synthesis and for the alcohol dehydrogenase enzyme that converts retinol to retinal; zinc deficiency impairs vitamin A mobilization from liver stores even when liver stores are adequate
Dietary fat: vitamin A is fat-soluble; supplement with the largest meal of the day for optimal absorption
Vitamin K2: retinoid signaling interacts with vitamin K2 through shared RXR receptor pathways; pairing A, D, and K2 provides synergistic immune regulation

6. Related Lab Tests

Vitamin DCo-regulates Immune Tolerance with Vitamin A

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ZincRequired for Retinol-Binding Protein and Vitamin A Mobilization

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TPO AntibodiesVitamin A Deficiency Worsens Hashimoto's

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ALTVitamin A Toxicity Causes Hepatocyte Injury

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Vitamin K2Shares RXR Receptor Pathway with Vitamin A

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FerritinVitamin A Deficiency Impairs Iron Mobilization

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7. Clinical Perspective

Clinical Perspective

Vitamin A is the fat-soluble vitamin that gets overshadowed by vitamin D in most functional medicine conversations, but it is equally important for immune regulation, and the two work together through shared receptor pathways. In my Hashimoto's patients, I routinely check retinol alongside vitamin D, selenium, and zinc as the foundational thyroid immune nutrient panel. A retinol of 38 mcg/dL in a patient with elevated TPO antibodies, normal vitamin D at 62 ng/mL, and selenium of 84 ng/mL tells me that we are addressing the wrong nutrients if we only focus on vitamin D and selenium. The Treg-promoting effect of retinoic acid is essential for keeping Hashimoto's autoimmunity in check, and that requires adequate vitamin A in addition to vitamin D, not instead of it.

Brian Lamkin, DO | Founder, The Lamkin Clinic | Edmond, Oklahoma

8. Frequently Asked Questions

What is the optimal vitamin A (retinol) level?

In functional medicine, optimal serum retinol is 50 to 80 mcg/dL. Values below 40 mcg/dL impair immune function, mucosal barrier integrity, and vision. Values consistently above 100 mcg/dL from preformed retinol raise hepatotoxicity and teratogenicity concerns, particularly in women of childbearing age.

Can vitamin A be toxic?

Yes. Preformed vitamin A (retinol from animal sources and supplements) accumulates in the liver and is toxic at sustained high doses. Chronic toxicity occurs with daily doses above 25,000 IU long-term, causing liver fibrosis, bone pain, hypercalcemia, and hair loss. In pregnancy, preformed vitamin A above 10,000 IU daily is teratogenic. Beta-carotene from plants is not toxic because conversion to retinol is tightly regulated.

Does vitamin A affect thyroid autoimmunity?

Yes. Retinoic acid produced from vitamin A is essential for generating regulatory T cells (Tregs) in gut-associated lymphoid tissue, which suppress autoimmune activation. Vitamin A deficiency shifts the immune balance toward pro-inflammatory Th17 cells, worsening autoimmune thyroid disease. Optimizing retinol to 50 to 80 mcg/dL is an important component of the Hashimoto's root-cause protocol.

What is the difference between preformed vitamin A and beta-carotene?

Preformed vitamin A (retinol) from animal sources and supplements is directly bioavailable and accumulates in the liver, creating toxicity risk at high doses. Beta-carotene and other provitamin A carotenoids from plant foods are converted to retinol only as needed, with conversion tightly regulated. Beta-carotene supplementation is not associated with vitamin A toxicity and is safe even at high doses, though it can cause harmless orange skin (carotenemia) at very high intakes.

Content authored and clinically reviewed by Brian Lamkin, DO, founder of The Lamkin Clinic in Edmond, Oklahoma. Brian Lamkin, DO has 25+ years of experience in functional and regenerative medicine. This page reflects current functional medicine practice standards and is updated as new clinical evidence becomes available.

Vitamin A is not just about vision. It is a master regulator of immune tolerance that most thyroid patients have never had tested.

Vitamin A, vitamin D, selenium, and zinc form the foundational thyroid immune nutrient panel. Schedule a consultation for a comprehensive thyroid and immune nutrient assessment.

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Medical Disclaimer: This content is provided for educational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Lab interpretation should always be performed in clinical context by a qualified healthcare provider. Reference ranges and optimal targets may vary based on individual patient history, clinical presentation, and laboratory methodology. Schedule a consultation to discuss your specific results with Dr. Lamkin.