Iodine supplementation

There are different ways of supplying iodine, with the forms of iodide as sodium iodide being superior to the iodate forms.

Intake and deficiencies

Deficiencies and excess

Iodine deficiency. Its concentration depends on external inputs. Iodine deficiency is very common in the population.

TSH is higher in patients with deficient iodine intake, and the elevation is directly proportional to the iodine deficiency. Thyroid size is also a sensitive marker of deficiency.

Some 275 million people in Europe are at risk of iodine deficiency, and 130 million suffer from goiter. Spain is among the countries with iodine deficiency. Between 30% and 40% of pregnant women have lower than recommended iodine intakes.

According to the ENIDE survey, the median daily iodine intake is 87–97 μg/day in both men and women. This indicates that there is a risk of inadequate iodine intake, especially for women (AESAN, 2011).

In Spain, only iodized and fluoride iodized salt contain iodine.

Diets with little fish and shellfish and no iodized salt can be deficient in iodine, particularly if the fruits and vegetables consumed are grown far from the sea.

Foods that influence iodine deficiency. If iodine intake is slightly low, habitually eating certain foods can create an iodine deficiency. These foods are: cabbage, Brussels sprouts, cauliflower, broccoli, escarole, carrots, spinach, peaches, pears, strawberries, mustard, turnips, corn, peanuts, and soy.

Nutrients and iodine deficiency. Soy isoflavones (genistein, daidzein) can inhibit synthesis of the thyroid hormone.

Selenium deficiency can contribute synergistically to iodine deficiency in the development of goiter or hypothyroidism.

Selenium, iron, and vitamin A deficiency exacerbate the effects of iodine deficiency.

If iodine and selenium deficits exists, both should be corrected to normalize thyroid function. Otherwise, the hypothyroidism may be aggravated.

Iodine Deficiency Disorders (IDD). The most important are: goiter, hypothyroidism, mental retardation, cretinism, lower IQ, lower auditory threshold, and increased neonatal and child mortality. Schoolchildren with iodine deficit improve their school performance when their intake is adjusted.

Iodine deficiency reduces the production of thyroid hormones, decreasing thyroid function with all of its consequences: fatigue, constipation, dry skin, intolerance to cold, weight gain, apathy, poor memory, etc.

Symptoms of hypothyroidism. The most common symptoms of hypothyroidism and thyroid hormone resistance are the following: feeling cold, dry skin and mucous membranes, hair loss, weight gain, fatigue, headaches and neck and back pain, and painful points on the body. Low heart rate and body temperature.

Excess iodine. It can be caused by any the following: iodine supplements, medications (amiodarone, expectorants, etc.), and/or eating seaweed with a high iodine content. It can raise T4 and lower T3 levels due to reduced peripheral conversion of T4 to T3.

Excess iodine can inhibit synthesis of thyroid hormones, as in the case of deficiency, or produce hyperthyroidism. It is toxic goiter.

Reference values

NRV, PRI, UL

The NRV is 150 µg. The PRI for adults is 150 mg. The minimum intake to prevent goiter is 50 µg/day.

The UL is 600 µg.

High iodine intake entails a risk of thyroid and hyperkalaemia disorders. Toxicity is rare at doses below 5,000 µg and exceptional at doses below 1,000 µg.

NRV: Nutrient Reference Value. These are the daily amounts recommended for vitamins and minerals (micronutrients only) considered sufficient in the European Union to prevent a deficit in most healthy individuals in the population. They are the values used for labeling and claims.

PRI: Population Reference Intake. These are the reference values for a specific population group, defined as the amount of energy and/or nutrients (macronutrients and micronutrients) recommended to cover the nutritional needs of practically the entire population. They refer to healthy population groups. The values are expressed per person per day, as a 15-day average. To account for individual differences in digestive and metabolic processes, the PRI figure used for each nutrient is the average plus twice the standard deviation (except for energy, for which the average is used), thereby including almost all healthy individuals in the group (97.5%).

UL: Tolerable Upper Intake Level: the maximum level of total daily intake of a nutrient in a context of long-term or chronic consumption which does not pose a risk to health.

Foods with iodine

The foods that contain the most iodine are seaweed and shellfish in general, followed by fish. The iodine content of fruits and vegetables is much lower and depends on where they are grown, with levels being very low when grown far from the sea. In Spain, only iodized and iodine- and fluoride-enriched salt contain iodine.

The foods with the highest iodine content are the following:

• Seaweed. Wakame, kombu, dulse, Icelandic kelp, etc., with a high protein, iron, and iodine content. Nori, sea spaghetti, fucus, kelp, etc., with low or moderate protein content and high iron and iodine content.
• Shellfish in general, mainly: clams, cockles, shrimp, cuttlefish, Norway lobster, prawns, and mussels.
• Fish, mainly cod and red mullet, but also the following: mackerel, herring, flounder, salmon, sardines, tuna, anchovies, sole, megrim, whiting, etc.

The foods with moderate iodine content are the following:

• chard, broccoli, watercress, leeks, carrots, spinach, endives, green garlic, onions, spring onions, asparagus, etc.
• Mushrooms in general (except canned).
• Eggs, especially the yoke.
• Milk and all milk products, such as yogurt and cheese.
• Peanuts.
• Oats.
• Pineapple.
• Beef and pork liver.

Industrialized and packaged products and some drugs with iodized preservatives (read the label) should also be taken into account.