Some healthy foods may have unintended negative effects on your thyroid function.
The thyroid is an endocrine gland with a pivotal role in regulating different metabolic processes in fetal, childhood, and adult life. Several diseases affect the thyroid function and deem the body either deficient or in excess of the thyroid hormones; causing hypo- or hyperthyroidism. While either of the disease states could be caused by different mechanisms, nutritional factors play an important role in affecting the function of the thyroid gland.
Not enough? ... Trouble. Too much? ... Trouble. Iodine can be tricky when it comes to thyroid health. Among the several micronutrient considerations for thyroid, iodine stands out. It is a major constituent of T3 and T4 hormones, which are both made by the thyroid gland. Iodine deficiency is one of the most common causes of hypothyroidism among children and adults, worldwide. Low iodine can result in enlargement of the thyroid gland, called 'goiter'. The enlargement follows from the thyroid receiving excess stimulatory signals from the pituitary gland, which promotes the growth in an attempt to get more thyroid hormones made, usually in the face of impaired thyroid function. So not having enough can be a big problem. That is why table salt became iodized a century ago. And too much iodine? That has been associated with increased rates of autoimmune thyroiditis.2
When I measure iodine on a patient's labs and we find it low, the person is almost always consuming non-iodized forms of salt and/or not using a good quality comprehensive multi-mineral supplement. There are not too many good natural dietary sources of iodine, apart from seafood. Seafood is the primary source. Yet interestingly, "in the U.S., iodine is present in dairy foods (due to the iodophor cleansers of milk cans and teats) and occasionally in bread dough (due to the use of iodate as bread conditioners). Iodine is only one of several teat dip formulations available in the industry and represents an “accidental” but important source of iodine nutrition".3
Apart from low intake of iodine, there are foods that can interfere with its utilization. These are called 'goitrogenic foods'. They naturally contain compounds that limit the body's ability to maintain proper thyroid function and include cruciferous vegetables, bamboo shoots, soy (in animals, yes, but multiple recent human meta-analyses not finding a correlation), cassava, corn, lima beans, millet, peanuts, pears, peaches, pine nuts, spinich and strawberries.
Cruciferous, or 'Brassica' vegetables are of particular interest on this topic. They contain glucosinolates, which are responsible for the sharp flavor of such plants as horseradish, various mustards, watercress, and radish. Glucosinolates can ultimately be transformed into goitrin and thiocyanate, which are what interfere with iodine uptake. Cruciferous vegetables include broccoli, brussel sprouts, cauliflower, watercress, kale, collard greens, turnips, cabbage, radishes and arugula and have been much maligned on this topic.
The good news is two-fold. One, the consumption of typical amounts of most of these vegetables is far below the threshold scientists found to trigger inhibition of iodine in humans. This gives vegetables like broccoli, cabbage turnip, bok choy, broccoli rabe and others the 'green light'. In contrast, 1.5 cups of kale and collard greens had higher concentrations of glucosinolates and brussel sprouts were found to have varying amounts, depending on the soil in which they were grown, so they too were potentially more problematic. So there patients with thyroid concerns may want to use moderation. But one study in humans found that the consumption of 150 g/day (5 oz/day) of cooked Brussels sprouts for four weeks had no adverse effects on thyroid function. 4
Secondly, cooking the vegetables denatures the goitrogens. Other forms of preparation reduce goitrogen content as well, such as soaking or boiling.5
Ultimately, normal consumption of cruciferous vegetables among the healthy population does not appear to increase the risk of hypothyroidism and among those that already have hypothyroidism, several strategies can likely lead to consumption without any consequences. That includes, limited servings, preparation methods such as cooking or boiling, as well as considering additional iodine supplementation, which itself has shown to be protective.6
With regard to soy, decades of animal studies documented anti-thyroid effects and older studies with infants fed soy formula mounted concerns about the use of soy and thyroid health. Since then, multiple investigations through the literature have not found the associations. "In 2015, the European Food Safety Authority (EFSA) conducted a risk assessment that included the effect of soy protein and soy isoflavones on thyroid function (and) ... concluded soy isoflavones were without effect on thyroid function. ... Similarly, this meta-analysis (from 2019) suggests that soy supplementation has no effect on the thyroid hormones and modestly raises TSH levels, the clinical significance, if any, of the rise in TSH is unclear".7
Another identified antagonist of thyroid hormone metabolism, though itself not a food, is acetyl-L-carnitine. Acetyl-L-carnitine is an acetylated form of its amino acid namesake and is used in cardiovascular health, brain health and weight loss. Acetyl-L-carnitine has been shown to lower thyroid hormone entry into the nucleus of hepatocytes, neurons and fibroblasts. High levels of carnitine in hypothyroid patients could result in an exacerbation of symptoms. One study demonstrated that supplementation with L-carnitine provided successful treatment of patients with hyperthyroidism. 8 Thus use of this supplement among hypothyroid patients should be done with care and clinical supervision of a physician trained in natural medicine.
1. Babiker, A. et al. The role of micronutrients in thyroid dysfunction. Sudanese Journal of Paediatrics. 2020.
8. Bevenga, S., et al. Usefulness of L-carnitine, a naturally occurring peripheral antagonist of thyroid hormone action, in iatrogenic hyperthyroidism: a randomized, double-blind, placebo-controlled clinical trial. J Clin Endocrinol Metab. 2001.