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Vitamin A shows to regulate thyroid hormone metabolism and inhibit thyroid-stimulating hormone (TSH) secretion. A 4-month randomized, double-blind controlled trial of premenopausal women found that Vitamin A caused a significant reduction in serum TSH. The study concluded that vitamin A supplementation might reduce the risk of subclinical hypothyroidism in premenopausal women.
Vitamin A deficiency can also impact thyroid health by playing a role in the pituitary gland, which sends signals (in the form of TSH) to the thyroid to regulate production. A deficiency in vitamin A may limit pituitary synthesis and secretion of TSH, increase the size of the thyroid gland, or reduce uptake of iodine by the thyroid gland.
Vitamin A can also decrease the risk of Hashimoto’s Disease, the autoimmune condition that underlies most cases of hypothyroidism. The vitamin may decrease autoimmune reactions in the body.
Because vitamin A is a fat-soluble vitamin, the body stores any excess, primarily in the liver. These levels can build up, causing vitamin A toxicity. Vitamin A toxicity can happen if there is a sudden, massive vitamin A intake. It can cause dizziness, nausea, headaches, skin irritation, joint or bone pain. The Tolerable Upper Intake Level (UL)—defined as the highest daily intake level that is likely to pose no adverse health effects—for adults is 3,000 mcg per day for adult men, adult women, and women who are pregnant or breastfeeding.
Vitamin A does have the potential to interact with certain medications, and some medicines may affect vitamin A levels. Orlistat, a weight-loss treatment, can decrease the absorption of vitamin A and other fat-soluble vitamins. And some synthetic retinoids can increase the risk of vitamin A toxicity when taken in combination with vitamin A supplements. People who take either of these medications regularly should talk to their doctor about vitamin A supplementation.
The Recommended Dietary Allowance (RDA) of vitamin A—or, the average daily level of intake sufficient to meet the nutrient requirements of nearly all healthy individuals—is 900 mcg RAE for adult men and 700 mcg RAE for adult women. Women who are pregnant or breastfeeding should take 770 mcg RAE daily and 1,330 mcg RAE daily, respectively.
The body converts both retinol and provitamin A carotenoids into retinol. However, the RDAs for vitamin A are given as retinol activity equivalents (RAE) to account for the different bioactivities of retinol and provitamin A carotenoids. According to the National Institutes of Health, "One mcg RAE is equivalent to 1 mcg retinol, 2 mcg supplemental beta-carotene, 12 mcg dietary beta-car
Age-related macular degeneration (AMD) is a significant cause of vision loss in older people. Research suggests that oxidative stress may play a role in the onset of AMD. So, supplementing with vitamin A with antioxidant properties may be useful for preventing or treating this condition. One large study found that participants at high risk of developing AMD reduced their risk by 25% by taking a daily supplement containing vitamin A (beta-carotene), vitamin E, vitamin C, zinc, and copy compared to the control group.
Vitamin A comes in two primary forms—preformed vitamin A like retinol, retinyl esters, and provitamin A carotenoids like beta-carotene to convert to retinol. Preformed vitamin A comes from animal products, fortified foods, and vitamin supplements. Carotenoids are found naturally in plant foods.
Liver and fish oils have the highest concentrations of preformed vitamin A. You can also find preformed vitamin A in milk and eggs. Most provitamin A comes from leafy greens, orange and yellow vegetables, tomato products, fruits, and some vegetable oils.