Your thyroid is one of the largest endocrine, or hormone-secreting, glands in your body. Located just beneath your Adam's apple, your thyroid traps iodine from your bloodstream and incorporates it into hormones that control cellular energy consumption, protein synthesis and metabolic function. When your thyroid becomes inflamed or overstimulated by certain disorders, such as Hashimoto's thyroiditis or Graves' disease, the iodine from seafood can trigger overproduction of thyroid hormones, leading to hyperthyroidism.
Iodine
Iodine is an essential trace element found in high concentrations in very few foods. Although iodized salt is widely available, seafood and marine vegetables provide the best natural sources of iodine. Iodine's primary role in your body is to help form thyroid hormones, specifically triiodothyronine and thyroxine, T3 and T4, respectively, which are required for normal metabolism, neurologic function, growth and development. In children, iodine deficiency interferes with normal brain development. According to the World Health Organization, iodine deficiency is the world's leading preventable cause of brain damage.
Hashimoto's Thyroiditis
Hashimoto's thyroiditis is a condition that develops when your immune system attacks one of the enzymes involved in T3 and T4 production. This triggers an inflammatory response that usually destroys the hormone-producing cells in your thyroid. Early in the disease's course, ongoing thyroid damage can trigger the release of stored thyroid hormones, leading to hyperthyroidism. In addition, your thyroid may trap iodine -- such as that acquired from eating seafood -- more avidly than is normally the case in an attempt to convert it to thyroid hormones. Eventually, most people with Hashimoto's thyroiditis become hypothyroid and must take thyroid hormones.
Seafood
Iodine concentrations in different kinds of seafood vary widely. According to the Linus Pauling Institute at Oregon State University, 3 oz. of tuna contain 17 mcg of iodine, while a quarter-ounce of seaweed could come with as much as 4,500 mcg. The recommended dietary allowance for adults is 150 mcg daily, and the tolerable upper intake level -- the dosage at which normal, healthy adults could develop signs of iodine toxicity -- is 1,100 mcg a day. If your thyroid is "iodine-sensitive," which could occur at certain stages of Hashimoto's disease, the iodine from seafood could prompt a temporary spike in your thyroid hormone levels.
Considerations
Hashimoto's thyroiditis is among the most common causes of hypothyroidism, or low thyroid function, in North America. However, during the early stages of this disorder, your thyroid function can actually be higher than normal due to destruction of thyroid cells and inappropriate release of thyroid hormones. Consumption of seafood or other sources of iodine can also trigger temporary increases in thyroid hormone levels during certain stages of inflammatory thyroid diseases. If your thyroid function remains elevated even when you decrease your iodine intake, see your doctor for an evaluation.
References
- World Health Organization: Iodine Deficiency Disorders
- "The Merck Manual of Diagnosis and Therapy, 18th Edition: Hashimoto's Thyroiditis"; Mark H. Beers, M.D., Editor-in-Chief; 2006
- Linus Pauling Institute at Oregon State University: Iodine
