Iodine, or iodide, is a trace mineral found in ocean water, in ocean-dwelling plants and animals, and in soils adjacent to oceans. Iodine's principal role in human metabolism is in the production of thyroid hormones. These hormones – thyroxine, or T4, and triiodothyronine, or T3 – are necessary for normal growth and development, neurologic function, cellular energy production and protein synthesis. Every cell and tissue in your body is influenced by thyroid hormones, and a lack of thyroid hormone production leads to a general slowdown of your bodily functions. Although iodine is essential for your health, supplementation with this nutrient is usually not recommended when you take levothyroxine for a poorly functioning thyroid gland.
Iodine in Hormone Synthesis
Your body contains only several milligrams of iodide, the ionic form of iodine, and much of this is concentrated in your thyroid gland. The cells of your thyroid are adept at trapping iodide from your bloodstream and combining it with the amino acid tyrosine to produce T3 and T4. Your thyroid gland then stores T3 and T4 until their release is triggered by thyroid stimulating hormone, or TSH, a hormone secreted by your pituitary gland. Whenever your blood levels of thyroid hormones decrease, your pituitary releases more TSH, which prompts your thyroid to trap more iodine and release more thyroid hormones.
Hypothyroidism, a condition resulting from insufficient production of thyroid hormones, affects nearly 2 percent of the American population. Hashimoto's thyroiditis, an autoimmune condition that damages the cells of your thyroid gland, is the most frequent cause of hypothyroidism in the U.S. Surgical removal of your thyroid gland and radiation therapy to treat other thyroid disorders are also common causes of hypothyroidism. Once your thyroid gland stops functioning, it no longer traps iodine or produces T3 or T4, and you have to take thyroid hormones to remain healthy.
Levothyroxine is a synthetic version of T4. It is used to treat individuals who have hypothyroidism due to any cause. Levothyroxine can be converted to T3 in your body, so you only have to take one medication to supply both hormones. Since levothyroxine already contains iodine, you do not need to take additional iodine to treat hypothyroidism. Any extra iodine you consume will not be incorporated into new thyroid hormones, because your thyroid's function is being replaced by levothyroxine. In fact, according to a 1992 study published in "International Journal for Vitamin and Nutrition Research," any additional iodine you obtain from supplements will be eliminated in your urine, feces and sweat.
If you develop hypothyroidism and begin taking levothyroxine, your doctor will order blood tests to ensure your dosage is correct. The most useful indicator of your body's response to levothyroxine is your TSH level, because your pituitary gland "senses" when your blood levels of T3 and T4 are adequate to support your metabolism. A high TSH indicates that you need more levothyroxine, whereas an abnormally low TSH means you need to reduce your levothyroxine dosage. Once your dosage is adjusted to maintain your TSH within a normal range, further monitoring is performed annually or even less frequently. Iodine or iodide supplements do not offer any additional benefits in managing hypothyroidism.
- The National Academies Press: Iodine – Physiology of Absorption, Metabolism, and Excretion
- “American Family Physician”; Treatment of Hypothyroidism; W. J. Hueston; November 2001
- “International Journal for Vitamin and Nutrition Research”; Urinary Excretion of Iodine and Fluoride From Supplemented Food Grade Salt; S. K. Nath, et al.; 1992