I recently had a patient ask me how diet and/or supplements could affect thyroid function. This particular patient has mild hypothyroid, but was reluctant to start on any hormone therapy (natural or synthetic). This was some advice given her on her quest for a better balanced thyroid function with just diet/herbal control
Researchers have identified that the isoflavones in soy act as potent anti-thyroid agents, and are capable of suppressing thyroid function, and causing or worsening hypothyroidism. Soy is a phytoestrogen, and therefore acts in the body much like a hormone, no surprise that it interacts with the balance of the thyroid's hormonal systems. High consumption of soy products (either by high dose soy supplements or a high concentration of soy and soy-products) in our diet can be detrimental. Very high doses have been proven to cause goiter. I would suggest limits on soy and soy-based foods. In adults, just 30 mg of soy isoflavones per day is the amount found to have a negative impact on thyroid function. This amount of soy isoflavones is found in just 5-8 ounces of soy milk, or 1.5 ounces of miso. So use these products sparingly and definitely NOT daily.
Of course Iodine is very important to thyroid function. Low iodine intake can cause goiter and low thyroid hormones, but in America we have iodinated (iodized) salt to help obliterate this cause of hypothyroid. Make sure you use occasional iodinated salt in you cooking (most processed foods with salt in them don't use iodinated salt). And also a good source of Iodine is sea food (cod, sea bass and shellfish). Kelp is a vegetarian source of iodine.
Additionally, the lauric acid in coconut oil has a stimulant affect on thyroid function. This was discovered in the 1940's, so using coconut oil in your cooking or taking it as a daily supplement may help stimulate higher levels of T4 and T3 naturally.
With fluoridation of water (and fluoride toothpaste) we can see an impact on iodine. Fluoride displaces iodine in the body, so intake of iodine (by iod. salt and seafood) is important to counter the unavoidable fluoridation of municipal water systems and toothpaste.
Some herbals that have a history of increasing thyroid function are: bladderwrack, iceland moss, oat straw, saw palmetto berries, calamus root. There are preparations out there that have these as single or grouped supplements for thyroid health.
A 1/4 teaspoon of iodized table salt provides 95 micrograms of iodine. A 6-ounce portion of ocean fish provides 650 micrograms of iodine. Most people are able to meet the daily recommendations by eating seafood, iodized salt, and plants grown in iodine-rich soil. Recommendations for females age 14 and older: 150 mcg/day of iodine.