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Vitamin A is a fat-soluble micronutrient that influences countless biological processes including vision, immune function, skin health, tissue remodeling, and cellular development. A lesser-known function of vitamin A, however, is its role in hormone health. Adequate vitamin A is crucial for healthy thyroid function, sex hormone balance, as well as the absorption of other essential nutrients.
The most bioavailable form of vitamin A comes from retinoids, which are typically found in animal foods including liver, egg yolks, and grass-fed dairy products. Beta-carotene from peppers, carrots, and leafy green vegetables must undergo a more complex conversion process to retinoic acid–the most metabolically active and final form of vitamin A. Conversion takes place in the intestine and is reliant on gut health, liver function, and an individual’s genetic makeup. This makes beta-carotene less available for the body to use when compared to retinoids. Retinyl palmitate is a common retinoid found in supplements and is readily converted to retinoic acid, making it an effective option for increasing vitamin A status in the body.
Vitamin A and the Thyroid
Adequate vitamin A is essential for thyroid health. In fact, many symptoms of low vitamin A are also symptoms associated with hypothyroidism. This includes fatigue, weight gain, dry skin, and chronic infections. Transthyretin (TTR) is a protein that binds to both vitamin A and the thyroid hormone thyroxine (T4). TTR works synergistically with vitamin A for hormone conversion by transporting retinol from stores in the liver to target tissues. In the absence of adequate vitamin A, TTR binds only to T4 and inhibits the conversion of T4 to T3–the active form of thyroid hormone. Without conversion, the body may experience a state of hypothyroidism. This also can result in increased cholesterol levels because sex hormones, which are derived from cholesterol, cannot be converted without adequate thyroid function.
Various studies indicate that vitamin A deficiency can exacerbate thyroid dysfunction. Thyroid hypertrophy is common in individuals with poor vitamin A status. This can present as increased thyroid volume and increased TSH levels, both of which are implicated in metabolic syndrome and obesity. This is because vitamin A plays a critical role in the thyroid’s uptake of iodine, an essential mineral for thyroid hormone synthesis. Without adequate iodine absorption, T3 and T4 cannot act effectively on cells. Studies demonstrate that the administration of retinyl palmitate alleviates thyroid hypertrophy, decreases gland volume, increases iodine uptake, and increases hepatic conversion of T4 to T3.
Vitamin A and Sex Hormones
Because the thyroid influences so many processes in the body, vitamin A deficiency can lead to other hormonal imbalances downstream. The nutrient is essential for an appropriate balance of progesterone in women and testosterone in men. In women, thyroid hormones are essential for ovulation, which stimulates the production of progesterone. The corpus luteum, the temporary endocrine gland that secretes progesterone, contains high concentrations of vitamin A. In animal studies, researchers have found that the corpus luteum is smaller, less developed, and produces less progesterone when animals are deprived of vitamin A. Deficiency also can lead to decreased adrenal hormone production, specifically pregnenolone.
One of retinol’s most important functions is to promote healthy cell differentiation. In men, adequate vitamin A is critical for the development of reproductive cells, specifically Leydig cells which produce testosterone. Vitamin A deprivation has been demonstrated to decrease the secretion of testosterone which can lead to infertility, cardiovascular problems, and weight gain. In women with breast cancer, excess estrogen has been demonstrated to increase the proliferation of cancer cells, while vitamin A has exhibited the opposite effect.
Vitamin A and Other Nutrients
Like iodine, vitamin A allows the body to use iron effectively. Vitamin A deficiency is commonly associated with anemia because vitamin A facilitates iron into hemoglobin in blood cells. Sufficient retinol effectively moves iron out of the liver, which when in excess, can inhibit the conversion of T4 to T3 and disrupt the body’s ability to detox estrogen. Vitamin A prevents iron overload in other tissues as well and allows the body to use copper, which is also necessary for estrogen metabolism, thyroid function, and the conversion of cholesterol into hormones.
Vitamin A and vitamin D have a similar relationship. Adequate vitamin A enhances the actions of vitamin D and signals vitamin D receptors in cells, allowing for adequate absorption. Because vitamin D influences calcium absorption, the immune system, estrogen balance, and parathyroid hormone balance, vitamin A deficiency can exacerbate a vitamin D deficiency as well.
Nutrient status plays an important role in endocrine function. Because all the systems in the body are connected, an imbalance in one nutrient can have multiple downstream effects. Targeted supplementation may be an effective solution when trying to support vitamin A status in the body, thus helping support healthy thyroid function and a proper balance of sex hormones.
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