Soy: Studies suggest that phytoestrogens in soybeans and soy-rich foods may inhibit the activity of an enzyme that makes thyroid hormones. One study found that women who consumed soy supplements were three times more likely to develop hypothyroidism.

Iodine-rich foods: Some forms of hypothyroidism are caused by a lack of sufficient iodine. In such cases, using iodized salt or iodine-enriched foods can be beneficia. But eating too much iodine can have the opposite effect and suppress thyroid gland activity. Check with your doctor before taking supplements.

Iron and calcium supplements: Taking iron or calcium supplements can also change the effectiveness of many thyroid medications.

High-fiber foods: Although a high-fiber diet is usually recommended, too much fiber eaten right after taking thyroid medicines may interfere with their absorption. Wait two hours before you eat a high-fiber meal (one with more than about 15 grams of fiber).

Certain vegetables: Cruciferous vegetables that are rich in fiber, like broccoli, cabbage, spinach, kale, and Brussels sprouts, may inhibit thyroid medication absorption. Reducing the amounts of such produce in the morning right after taking your medication may help.

Other foods: Caffeine, tobacco, and alcohol can also influence the effectiveness of thyroid medicine. Ask your doctor for tips on how to regulate or reduce your consumption.