Thyroid hormone regulates metabolism, yet the ability of thyroid to properly function and regulate metabolism can be interrupted by improper diet. Dietary studies are difficult to do as food, medication, lifestyle, exercise, and quantity of nutrients in our whole diet interact to make the data hard to interpret. Even birth control can affect your thyroid gland. There are foods that naturally release goitrin which can naturally lower thyroid synthesis. This can potentially be a benefit in a mildly overactive thyroid patient, but otherwise is potentially a negative for women with normal or under active thyroid. And in diets that are supplemented with iodine generally do not have much effect from these foods, and heating foods can break down the harmful goitrogen substances. Soy foods can be a goitrogen, and millet, even with normal iodine intake can suppress thyroid gland substances according to some studies.