Do a little Googling, and you might turn up a page or two claiming that cruciferous vegetables can cause thyroid troubles. The truth is a little murkier. While it's true that these veggies contain compounds called glucosinolates, which might interfere with your body's production of thyroid hormones in high amounts, it's pretty unlikely that they'll harm your thyroid if you're eating normal-size servings. One case report in the New England Journal of Medicine highlighted the story of an 88-year-old woman who showed up to the ER with hypothyroidism after eating about 2 or 3 pounds of bok choy a day—but, as Ilic points out, "that's not a normal amount."
When it comes to thyroid medications, it’s important for RDs to know the medications can interact with common nutritional supplements. Calcium supplements have the potential to interfere with proper absorption of thyroid medications, so patients must consider the timing when taking both. Studies recommend spacing calcium supplements and thyroid medications by at least four hours.21 Coffee and fiber supplements lower the absorption of thyroid medication, so patients should take them one hour apart.22 Dietitians should confirm whether clients have received and are adhering to these guidelines for optimal health.
Although relatively low serum T3 levels could contribute to these residual manifestations, the higher serum T4:T3 ratio should also be considered. This has been well-established for 4 decades (28, 50, 59), but only recently has it been recognized as a relevant measure given that higher serum T4 levels will impair systemic T3 production via downregulation of a deiodinase pathway (9). Thus, some emphasis has recently been directed toward establishing the clinical significance of this ratio (1, 5).
Medications that are used to treat an overactive thyroid (hyperthyroidism) may cause hypothyroidism. These drugs include methimazole (Tapazole) and propylthiouracil (PTU). The psychiatric medication, lithium (Eskalith, Lithobid), is also known to alter thyroid function and cause hypothyroidism. Interestingly, drugs containing a large amount of iodine such as amiodarone (Cordarone), potassium iodide (SSKI, Pima), and Lugol's solution can cause changes in thyroid function, which may result in low blood levels of thyroid hormone.
If you have been diagnosed with both hypothyroidism and iodine deficiency, there are some things you can do to make these vegetables less harmful. Cooking them can reduce the effect that cruciferous vegetables have on the thyroid gland, and limiting your intake of these (cooked) vegetables to 5 ounces a day may help as well, since that amount appears to have no adverse effect on thyroid function.

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