Is a cup of plain yogurt everyday good for you? i heard plain yogurt is rich in iodine which is good to treat hypothyroidism naturally.

Yes, ideally organic: Yogurt is one of the best dietary sources of iodine- see http://www.whfoods.com/genpage.php?tname=nutrient&dbid=69. Plain yogurt is a good choice as sweetened yogurt tends to be high in sugar (though you can add fruit). I advise choosing organic yogurt if possible. Iodine is not only essential for thyroid function but has other important health benefits- see http://www.mbschachter.com/Iodine.htm