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What Is Iodine Deficiency?

Inadequate dietary iodine intake can lead to iodine deficiency, which means the thyroid gland doesn't have enough iodine to work properly. 

What is Iodine?

Iodine is a trace element that serves as an essential micronutrient to help the thyroid gland produce thyroid hormones. Our bodies need iodine in small amounts. However, they can't produce iodine on their own. Therefore, you obtain iodine from food sources. Additionally, iodine is added to iodized salt (a type of table salt) and is available as iodine supplements. 

Notably, about a third of the world's population is at risk for iodine deficiency; however, it is much rarer in the U.S. 

What’s The Link Between Iodine and Thyroid Hormones?

The main function of iodine is that the body needs it to make thyroid hormones called thyroxine (T4) and triiodothyronine (T3). These hormones are produced by the thyroid gland, which is a butterfly-shaped endocrine gland in the neck. 

The pituitary gland in the brain secretes a hormone called thyroid-stimulating hormone (TSH) or thyrotropin to maintain normal thyroid function in the body. This prevents hypothyroidism (low thyroid hormone levels) or hyperthyroidism (high thyroid hormone levels). 

TSH increases the uptake of iodine by thyroid tissue and stimulates the production of T3 and T4. When there isn’t enough iodine in the body, TSH levels stay high. This can lead to an enlargement of the thyroid gland called goiter. It is the body’s way of trying to grab more iodine from the bloodstream to make more thyroid hormones. Also, low thyroid hormone level (hypothyroidism) occurs when there is an iodine deficiency, and the thyroid gland can’t make enough T3 and T4.

Physiological Functions Impacted by Iodine Deficiency

Thyroid hormones thyroxine (T4) and triiodothyronine (T3) regulate many important processes in the body, including metabolism, protein synthesis, enzyme activity, muscle function, brain development, and physical and mental growth.

In addition to its vital role in thyroid hormone synthesis, iodine also has additional functions in the body, including immune system function and beneficial effects in people with fibrocystic breast disease and mammary dysplasia.

Iodine Metabolism in the Body

Iodine is found in the form of sodium and potassium salts in dietary sources and is called iodate or iodide. Iodide is rapidly absorbed from the gut and distributed to the extracellular fluid (the fluid surrounding cells in the body). However, the concentration of iodide in the extracellular fluid remains low because most of it is rapidly taken up by the thyroid gland. In other words, the thyroid gland quickly traps and concentrates circulating iodide and uses it to synthesize thyroid hormones. Scientists have found that 75% of the iodide ingested each day goes to the thyroid gland. A healthy adult who does not have iodine deficiency has about 15 to 20 milligrams (mg) of iodine in their body, of which 70-80% is present in the thyroid gland.

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What Are Iodine Deficiency Disorders (IDD)?

As mentioned, iodine deficiency occurs when there isn’t adequate iodine intake from dietary sources and/or iodized salt. This leads to inadequate thyroid hormone production, which affects many organ systems in the body, including the heart, muscles, liver, and kidneys, as well as central nervous system development (brain development).

The abnormalities that occur due to low levels of thyroid hormones are collectively termed iodine deficiency disorders or IDD. They include:

• Goiter: A visible lump in the neck caused by an enlarged thyroid gland.

• Hypothyroidism (underactive thyroid gland or low thyroid hormone levels).

• Hyperthyroidism (overactive thyroid gland or high thyroid hormone levels).

• Decreased fertility rate.

• Increased risk of follicular thyroid cancer from chronic iodine deficiency.

Severe iodine deficiency during pregnancy is associated with an increased risk of: 

• Miscarriage or spontaneous abortion.

• Stillbirth.

• Congenital abnormalities (birth defects).

• Increased infant mortality.

• Neurological cretinism (severe neurological abnormalities in the fetus), including severe mental retardation, deaf-mutism, spastic diplegia (motor spasticity), and squint.

Causes and Risk Factors of Iodine Deficiency

Common Causes of Iodine Deficiency 

The most common cause of iodine deficiency is insufficient iodine intake. This can be due to a lack of sufficient iodine intake from food sources and/or a lack of iodized salt intake. 

Groups At Risk of Insufficient Iodine Intakes

Iodine deficiency may occur in:

• People who don’t have access to iodized salt (approximately 10% of the world’s population).

• People who live in areas inland or at high altitudes where the soil, and consequently the crops contain very little iodine.

• People who eat restricted diets, such as vegans. Seafood and dairy are some of the richest food sources of iodine.

• Pregnant women (iodine intake needs to be 50% more in pregnant women to support the development of the fetus).

Signs and Symptoms of Iodine Deficiency Disorder

Symptoms of goiter:

• A lump in the neck, just below the Adam’s apple

• Hoarseness or scratchy voice

• A sensation of choking or tightness in the throat

• Swelling of neck veins

• Dizziness when raising your arms above your head

• Difficulty breathing

• Coughing

• Wheezing

• Difficulty swallowing

Symptoms of hypothyroidism (low thyroid hormone):

• Tiredness

• Increased sensitivity to cold

• Dry skin

• Coarse hair

• Thinned hair

• Constipation

• Hoarseness

• Puffiness of the face

• Muscle pain 

• Muscle weakness

• Weight gain

• Irregular menstrual cycles

• Slowed heart rate

• Memory problems

• Depression

Symptoms of hyperthyroidism (elevated thyroid hormone):

In some cases, a goiter can become toxic and start producing more thyroid hormones, resulting in hyperthyroidism (overactive thyroid). Symptoms of hyperthyroidism may include:

• Fast heartbeat 

• Unexplained weight loss

• Diarrhea

• Sweating

• Shakiness

• Agitation

How is Iodine Deficiency Diagnosed? 

Your healthcare provider can make a diagnosis of iodine deficiency based on your symptoms, medical history, physical examination, and test results. 

A goiter (enlarged thyroid gland) may be visible as a lump in your neck during a physical exam. Your healthcare provider may also order imaging studies, such as thyroid ultrasound or thyroid scan. 

Blood tests called thyroid function tests measure circulating levels of thyroid hormones (T3 and T4) and thyroid-stimulating hormone (TSH) and can detect thyroid dysfunction. High levels of TSH might indicate an iodine deficiency. All newborns in the United States undergo a screening blood test for hypothyroidism.

How is Iodine Deficiency Treated?

Improvement of Iodine Status Through Diet and Preventive Measures

You can ensure iodine sufficiency (enough iodine in your body) and prevent iodine deficiency by eating foods that are natural sources of iodine (seafood, fish, eggs, dairy products). 

You can also ensure optimal iodine nutrition by using iodized table salt for cooking and adding to foods. Iodized salt is widely available in the United States. 

Keep in mind that processed foods rarely contain iodized salt. Also, specialty salts (kosher salt, sea salt, Himalayan salt, and fleur de sel) do not typically contain iodine.

In addition, you can talk to your doctor about taking an iodine-containing dietary supplement. 

Pregnant women should talk to their doctor about adequate iodine intake. Some prenatal vitamins, but not all, contain iodine.

Treatments for Iodine Deficiency

Iodine supplements are used to treat iodine deficiency. They contain potassium iodide or sodium iodide salts. Iodine is also included in many multivitamins and mineral supplements. In addition, there are dietary supplements containing seaweed (kelp), a rich natural source of iodine, and can be used to control iodine deficiency.

Note: Check with your healthcare provider before taking an iodine supplement. Iodine-induced hyperthyroidism, also called Jod-Basedow syndrome, is a rare condition that occurs from too much iodine in the body. Excess iodine has also been linked to autoimmune thyroid disease. Too much iodine can occur from use of iodinated contrast media during imaging studies, use of drugs that contain iodine such as amiodarone, povidone-iodine (Betadine), or Iodo-Niacin, or from taking dietary supplements containing seaweed.

You may also need to take thyroid hormone supplements such as levothyroxine (Synthroid), liothyronine (Cytomel), or desiccated thyroid extract (Armour, NatureThyroid, NP Thyroid) if you have an iodine deficiency.

Sources of Dietary Iodine

Recommended Daily Intake Levels of Iodine

Recommended dietary allowances (RDAs) or iodine requirements vary by age and are as follows (in micrograms (mcg) per day):

Dietary Sources of Iodine

Iodine-rich foods include:

• Seaweed (kelp, kombu, nori, wakame)

• Fish (cod, tuna)

• Seafood (shrimp, oysters)

• Eggs 

• Dairy products (milk, yogurt, cheese)

• Iodized salt (the iodine content of ½ teaspoon iodized table salt is 156 mcg, which is the recommended iodine intake for most adults)

• Human breast milk 

• Infant formula

Complications and Long-Term Effects of Iodine Deficiency

Potential Complications from Prolonged Deficiency

Untreated iodine deficiency can cause iodine deficiency disorders such as goiter and hypothyroidism. Iodine deficiency during pregnancy can lead to serious complications in the baby, including severe mental retardation and birth defects. Maternal iodine supplementation can prevent these serious complications.

Effects Across Different Life Stages

Iodine nutrition is important for people of all age groups. However, research has shown that the fetus (developing baby) during pregnancy and young children up to 2 years of age are the most severely affected by iodine deficiency because this essential nutrient is needed for brain development and mental and physical growth.

Population-Level Impacts: WHO Criteria for Iodine Insufficiency Disorders

Salt iodization is one of the best ways to prevent iodine deficiency across populations. The World Health Organization (WHO) uses median urinary iodine concentrations (urinary iodine levels) as an indicator of iodine deficiency in populations. 

Most of the iodine in the body is eliminated through urine. The test for median urinary iodine concentration is a non-invasive, inexpensive, and easy way to measure iodine excretion data and iodine status in a population. It is commonly measured in school-age children 6-12 years of age. In this age group, a population median urinary iodine concentration indicates the following:

• 100-199 micrograms per liter (μg/L) indicates an adequate iodine level.

• Less than 100 μg/L indicates inadequate iodine status or iodine deficient status. 

• Between 50 and 99 μg/L indicates mild iodine deficiency. 

• Between 20 and 49 μg/L indicates moderate iodine deficiency.

• Below 20 μg/ indicates severe iodine deficiency.