When it comes to nutrients that get attention for their role in healthy thyroid function you would think iodine and selenium were the only players in town. With iodine being a key constituent of thyroxine (T4) and tri-iodothyronine (T3) and selenium being a required cofactor for the de-iodinase enzyme that converts T4 into the more potent T3 there’s no denying the crucial role of these minerals in the production of thyroid hormones. But there’s another nutrient that’s important for a healthy thyroid. It’s time to bring vitamin A out of the shadows.
The mentioning of vitamin A immediately brings to mind its key role in eye health. Basic grade school health textbooks typically point to night blindness as a sign of vitamin A deficiency. Vitamin A is also a major player in the immune and reproductive systems and there’s a role for vitamin A in thyroid function as well.
Vitamin A is required for activation of thyroid hormone receptors and insufficient vitamin A may depress thyroid function. Animal models have shown that vitamin A deficiency interferes with thyroid health starting further upstream in the pituitary gland. Vitamin A insufficiency increases pituitary synthesis and secretion of thyroid-stimulating hormone (TSH) increases the size of the thyroid gland and reduces uptake of iodine by the thyroid gland.
Vitamin A given alone even in the absence of increased iodine has demonstrated a positive impact on thyroid function and gland size. Vitamin A supplementation was shown to reduce serum TSH levels and increase T3 in premenopausal women (ages 17-50) including a cohort of obese women who are at increased risk for subclinical hypothyroidism. Among children with goiter living in areas where iodine deficiency disorders are prevalent the greater the severity of vitamin A deficiency the greater their thyroid gland volume and the higher their TSH levels. Compared to placebo supplemental vitamin A significantly reduced median TSH and the rate of goiter. In rats with concurrent deficiencies in both iodine and vitamin A vitamin A repletion alone was effective in reducing thyroid gland size and hyperstimulation.
However clinicians should not assume that all hypothyroid patients should increase their intake of vitamin A-rich foods or take supplements. It appears that high doses of vitamin A may actually decrease thyroid function. Hyperthyroid patients have been successfully treated with high doses of vitamin A resulting in decreased symptoms of an overactive thyroid and a decreased metabolic rate. It appears that the effect of vitamin A on thyroid function is a U-shaped curve; too little causes problems but too much can also be harmful depending on the individual patient’s pituitary and thyroid status.Perhaps the underappreciated role of vitamin A in thyroid function is a contributing factor as to why thyroid dysfunction is so difficult to treat. It’s rare that the thyroid gland itself is malfunctioning. A sluggish thyroid more often results from long-term chronic stress via feedback from the adrenals pituitary and hypothalamus. But in patients for whom this is not an issue stealth nutrient insufficiencies could be at play. Even among patients who consume a whole-foods diet and avoid processed foods it’s not impossible to be low in iodine and selenium particularly if they avoid iodized salt and don’t consume dairy shellfish or seaweed.
As for vitamin A although overt deficiency is associated with the developing world more than with industrialized nations it may well be that a significant portion of the “First World” population is living with some degree of insufficiency. People vary in their capacity to convert plant precursor carotenes into true vitamin A. This may not be an issue for people who follow omnivorous diets and consume adequate pre-formed vitamin A but it may be problematic for vegetarians and vegans. Even if they consume large amounts of foods high in beta-carotene they may not be making the conversion effectively and this would be exacerbated if the plant-based diet is low in fat.
Moreover as if the relationship between vitamin A status and thyroid function wasn’t complicated enough sluggish thyroid function may impair the conversion of carotenes to true vitamin A creating a double-whammy scenario for a healthy thyroid. Rat studies indicate that a hypothyroid state results in lower conversion of carotenes to vitamin A while an overactive thyroid leads to accumulation of greater vitamin A stores. This could have important implications for dietary recommendations for hypo- and hyper-thyroid patients alike.
When patients with seemingly intractable thyroid disorders have explored multiple therapeutic strategies and are still not improving clinicians may consider evaluating vitamin A status.