Excessive Dietary Intake of Iodine & Hyperthyroid TSH Levels

Iodine is a trace mineral that is essential for normal metabolic function. Many natural and unprocessed foods are low in iodine, but your body only needs a small amount each day. In his book, “Staying Healthy with Nutrition,” Dr. Elson Haas states there is no significant danger of iodine toxicity from a natural diet. However, adding supplemental iodine to your daily routine or eating large amounts of iodine-rich foods, such as seaweed, could adversely affect your thyroid function.

Sushi in seaweed wrappers. (Image: DAJ/amana images/Getty Images)

Thyroid Hormones

Your thyroid gland is located in the front of your neck, just beneath your “Adam's apple.” Its sole function is to trap iodine from your bloodstream and incorporate it into thyroid hormones, principally triiodothyronine, or T3, and thyroxine, or T4. When released into your circulation, T3 and T4 interact with receptors in your cells that guide the production of enzymes, proteins, DNA and other important molecules. The secretion of thyroid hormones is normally controlled by the release of thyroid stimulating hormone, or TSH, from your pituitary gland.

Thyroid Stimulating Hormone

Pituitary production and secretion of TSH is influenced by your hypothalamus, which manufactures and releases a substance called thyrotopin releasing factor, or TRF, whenever your blood levels of T3 and T4 decrease. TRF triggers the release of TSH from your pituitary, and TSH travels through your bloodstream to “tell” your thyroid gland to trap more iodine and make more T3 and T4. Thus, low T3 and T4 levels - hypothyroidism - lead to increased TSH production. Conversely, if your T3 and T4 levels are too high - hyperthyroidism - your TSH level falls.

Iodine-Induced Hyperthyroidism

If you are iodine-deficient for any length of time, your thyroid can “escape” the normal mechanism of pituitary control and begin to function independently in order to trap what little iodine is available in your bloodstream. If iodine supplementation is then started, your thyroid begins producing T3 and T4 at an accelerated rate, leading to hyperthyroidism. This, in turn, causes suppression of TSH. According to the Linus Pauling Institute at Oregon State University, iodine doses as low as 150 to 200 mcg per day can trigger “iodine-induced hyperthyroidism” in people who have previously been iodine deficient.

Iodine-Induced Hypothyroidism

Excessive dietary iodine intake can actually reduce T3 and T4 production if you already have sufficient stores of iodine in your body. As your thyroid hormone levels fall, your pituitary produces more TSH to stimulate increased production of T3 and T4, but your thyroid is impaired by iodine excess and cannot respond normally to TSH. Worsening hypothyroidism ensues, despite your high intake of iodine. This situation can occur when you have adequate stores of iodine and you increase your daily iodine intake to around 1,700 mcg per day.

Considerations and Precaution

Iodine is essential for production of thyroid hormones, which exert widespread metabolic effects in your body. Recommended dietary allowances for iodine vary from 110 mcg daily for newborns to 290 mcg for nursing mothers. In order to prevent iodine-induced hypothyroidism in a U.S. population that already consumes sufficient amounts of iodine, the Institute of Medicine has set a tolerable upper limit for iodine at 1,100 mcg per day for adults. Ask your doctor if you need additional iodine before you take supplements or change your diet to include large amounts of iodine.