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What You Need to Know About The Thyroid Iodine Connection

Medically reviewed by 
What You Need to Know About The Thyroid Iodine Connection

Iodine deficiency is the leading cause of preventable intellectual disability in the world today. While severe individual outcomes are certainly possible, on a population level, there are collective costs as well. For example, an analysis of studies from the most iodine-deficient areas shows a collective loss of 13.5 IQ points in these areas. This is averaged through the entire population.

Of course, the costs of iodine deficiency span the life cycle from increased risk of miscarriage to hypothyroidism-induced fatigue and heart disease in the elderly. Across all ages, cognitive functioning tends to be impaired. Areas near oceans are naturally higher in iodine, so inland, especially in remote areas, are commonly the most affected.

Iodine deficiency was such a public health concern that some countries, such as the USA, have been adding iodine to salt since the 1920s. This practice has been largely successful at reducing the deficiencies in areas where it has been adopted.


What is The Thyroid?

The thyroid gland sits low on the front of the neck. With a form that resembles a butterfly, it cradles the windpipe. When it is normal-sized, you usually can not feel it or may barely perceive the texture of soft rubber.

The thyroid produces chemical messengers (thyroid hormones). Thyroid hormones increase the metabolism in many organs in the body. This is key to normal development and proper function throughout life.

The primary hormone released by the thyroid is called thyroxine (T4). The thyroid also produces a smaller amount of the active form, triiodothyronine (T3). The T4 form circulates through the bloodstream and is converted to the active T3 form in the liver and destination organs based on current needs.

The thyroid is told how much hormone to make based on a feedback loop that includes the hypothalamus in the brain and the pituitary gland.

What is Iodine?

Iodine is a non-metallic element, one of the halogens in the periodic table, like chlorine and bromine. It was discovered by accident by French chemist Bernard Courtois in 1811 while he was purifying saltpeter while making gunpowder. When heated, the liquid vapor forms of iodine are bright purple. Thus, the name was based on the Greek word ioeidēs, meaning violet.

Iodine is relatively rare as an element in the environment. It is generally found in higher concentrations in the seawater and thus also various sea life forms, such as kelp and fish. It tends to be quite low in concentration in inland areas far from seas, such as the American Midwest and Central Europe.

Iodine makes its way into living systems through the soil and food chain, where it is essential for many forms of life. In humans, it is a major structural component of the hormones made by the thyroid gland.

What is The Relationship Between The Thyroid and Iodine?

As blood flows through the thyroid, iodine is pulled from the bloodstream into the cells, where it is concentrated and ready to be integrated into the hormone structure.

Iodine makes up more than half of the thyroid hormones by weight, 65% for T4, which has four atoms of iodine as part of its structure. The difference between T4 and the active form T3 is that T3 has one less iodine atom. As it is a significant part of the structure, iodine is essential for the thyroid gland to produce thyroid hormones.

If a person is low in iodine, the thyroid will work harder, trying to still produce enough thyroid hormones. This causes a condition of thyroid enlargement called goiter. This can cause a choking feeling and can be visible as a swelling in the neck, but in many cases does not cause deeper problems. If the iodine deficiency becomes severe enough, the thyroid will be unable to keep up, and hormone levels will fall, causing additional symptoms.  

But if too little is a problem, is this a situation where more is simply better? It turns out too much iodine can also cause problems. These include paradoxically suppressing hormone production and potentially triggering autoimmune thyroid disease in vulnerable individuals. It may also trigger the thyroid to produce too much hormone or develop cancer. Those with existing thyroid conditions may be more vulnerable to the negative effects of too much iodine.

So, the thyroid can compensate to some degree for less-than-ideal conditions, but with such an important system, ensuring optimal levels of iodine can make an enormous difference in health.

What are The Symptoms of Iodine Deficiency?

Iodine deficiency generally occurs when intake is less than 20 mcg/day over a prolonged period of time. Mild or moderate iodine deficiency may cause an enlarged thyroid gland (Goiter) with no other symptoms. If iodine deficiency is severe enough to prevent adequate production of thyroid hormones (hypothyroidism), this causes many symptoms due to a decreased metabolic rate:

  • Fatigue
  • Depression
  • Mental sluggishness or brain fog
  • Weight gain or inability to lose weight
  • Dry skin
  • Hair loss
  • Constipation
  • Slow reflexes
  • Decreased fertility
  • Low libido
  • Decreased body temperature
  • Decreased heart rate

Iodine Deficiency Symptoms During Pregnancy:

  • Increased risk of miscarriage or stillbirth
  • Maternal anemia
  • Muscle pain
  • Congestive heart failure
  • Pre-eclampsia
  • Placental abnormalities
  • Postpartum bleeding

Iodine Deficiency Symptoms In Babies of Women With Hypothyroidism During Pregnancy:

  • Impaired growth and brain development
  • Sometimes birth defects

Iodine Deficiency Symptoms In Congenital Hypothyroidism:

  • Impaired physical and mental development, potentially severe if not identified early.
  • Deaf-mutism
  • Difficulty walking
  • Short stature

What Are The Symptoms of Too Much Iodine?

Excess iodine can be consumed due to prolonged high intake, such as from the anti-arrhythmic, Amiodarone, or high dose supplementation, including excess amounts of iodized salt. At extremely high intakes, such as supplementing with high dose potassium iodide, the result can be iodine toxicity, which can look like:

  • Burning in the mouth and throat
  • Brassy taste in the mouth
  • Produce too much saliva
  • Fever
  • Gastrointestinal (GI) Irritation (Nausea and Vomiting)
  • Acne-like skin breakouts

More Severe Levels of Excess Iodine Symptoms:

  • Swelling of airways
  • Weak pulse
  • Coma

Too High of Prolonged Iodine Intake Can Result in Hyperthyroidism, Which Can Cause:

  • Anxiety
  • Difficulty focusing
  • Rapid heart rate
  • Excessively warm body temperature
  • Unexplained weight loss
  • Hair loss
  • Eyes appearing bulging  

Paradoxically, excess iodine uptake by the thyroid can inhibit the production of thyroid hormones. This is known as the Wolff-Chaikoff effect. It can cause low thyroid hormone levels.

It is also possible for excessive iodine intake to trigger an autoimmune response against the thyroid, which can lead to both elevated and decreased hormone levels.

Thyroid cancer may also be triggered or worsened by excessive iodine consumption.

How to Test for Iodine Deficiency

The most accurate lab assessment of potential iodine deficiency is the 24-hour urine iodine test. This indicates very recent dietary intake, as about 90% of the iodine consumed will end up in the urine. A 24-hour collection is far more informative than one random urine sample.  

Another way of evening out natural fluctuations is with an AM and PM dried urine test. This has the benefit of being far more manageable than collecting all urine for 24 hours in a large container for the sample. The downside of all urine iodine tests is that they are highly affected by very recent iodine intake, thus, highly vulnerable to distortion. They also may not provide information on recent but resolved iodine deficiencies that may have still impacted hormone levels.  

In most cases, it makes sense to test thyroid function if iodine deficiency is suspected. This would be especially true for individuals at risk of low levels of iodine, such as those living in inland areas, those not consuming iodized salt, those who are pregnant or lactating, or those not eating seaweed or seafood. ALL neonates should be screened for hypothyroidism with a TSH test.

Sometimes imaging, such as ultrasound, will be required to further assess a goiter or other abnormal physical presentations of the thyroid gland.

How to Test for Iodine Excess

If very high iodine intake is suspected or known to have occurred, a serum (blood-based) iodine test can be useful to monitor levels in the blood. Serum testing is, however, not particularly useful in assessing normal to low iodine levels.

A random or 24-hour urine iodine test will also reflect higher intakes as long as they are not more than a few days in the past.  

If iodine overload has likely or certainly occurred, it would be prudent to also test thyroid function to know if any imbalances will require management. As autoimmunity can be triggered by excess iodine, antibodies should be included, as well as TSH, free T4, free T3, and reverse T3.  

If there are abnormal physical findings of the thyroid gland, such as goiter or nodules, imaging may also be a part of the testing.

How to Test for Thyroid Dysfunction

Since lab tests only test what is in the blood or the urine, not the cells themselves, they can only provide a window into what is actually happening in the cells. For this reason, it is helpful to be thorough in testing, getting various windows into the situation.

In the case of testing for thyroid dysfunction, this generally means a panel of tests, including:

  • Free T4 (thyroxine)  — The predominant form of thyroid hormone produced and present in the blood
  • Free T3 (triiodothyronine) — The active form of thyroid hormone present in the blood. Some hormone is produced in this form in the thyroid. Some hormone is converted from T4 to T3 in the liver, and some hormone is converted at the destination. This is the most important indicator of the actual effect thyroid hormones are having in the body.
  • TSH (thyroid stimulating hormone) — Produced by the pituitary gland, it stimulates the thyroid to make more thyroid hormones. It is a part of the feedback loop controlling thyroid hormone levels.
  • Reverse T3 — An alternative and inactive form of T3 that blocks activity. It is produced in times of illness or intense stress and reduces the metabolism to shunt energy for healing.
  • Early in the course of illness, testing for thyroid autoantibodies (anti-TPO and anti-TG) can provide information on how the illness is progressing or stabilizing. They may be less helpful later.
  • Thyroid Peroxidase Antibodies (anti-TPO antibodies) — Antibodies attacking an enzyme involved in the production of thyroid hormone. These are commonly found in Hashimoto's Thyroiditis.
  • Thyroglobulin Antibodies (anti-TG) — Antibodies produced against a storage form of thyroid hormone. These are associated with hypothyroid conditions.  

Other Lab Test to Check  

As it turns out, the thyroid can be functioning just fine, yet a person experiences symptoms that would suggest it is not. If thyroid tests look ok, but there are symptoms suggesting a problem, here are a few other things to investigate.

The liver is the site for 60% of the conversion from T4 to T3. If it is not working well, such as may be the case with fatty liver disease, this conversion may not be happening at optimal levels, leading to reduced active T3 in circulation.

The liver also produces thyroid binding globulin, which helps to carry the active hormone through the bloodstream to the cells where it will be used. Screening for liver enzymes in the serum in a comprehensive metabolic panel (CMP) may make sense in some situations.

A number of nutrients are also essential in the production of thyroid hormones and related processes. Testing for these levels can provide insight into the bigger nutritional picture and allow targeted nutritional supplementation if levels are low. These would include Tyrosine, iron, selenium, zinc, and manganese.

How is Iodine Deficiency Treated?

Given the severe consequences of untreated iodine deficiency, it should be urgently addressed once identified. This is particularly true during pregnancy, infancy, or early childhood when the effects can cause the most irreversible damage.

The treatment for iodine deficiency recommended in the Merck Manual involves both iodine and thyroxine in combination and differs slightly based on the individual's age.


  • Levothyroxine 3 mcg/kg orally once a day for seven days.
  • Iodide 50 to 90 mcg orally once a day for several weeks


  • Levothyroxine until effectively synthesizing T4
  • Iodide 90 to 120 mcg once a day


  • Iodide 150 mcg once a day.
  • May also give levothyroxine for a period of time.

Women who are pregnant or breastfeeding

  • Iodide 250 mcg once a day
  • Levothyroxine if labs indicate low thyroid function.

For all ages, serum TSH should be monitored until it normalizes.

How is Iodine Deficiency Prevented?

Awareness is key in preventing iodine deficiency in some groups, such as vegans/vegetarians and pregnant women in areas with naturally low iodine levels in water and soil. In such cases, a combination of dietary attention to consuming iodine-rich foods and supplementation through prenatal vitamins can help prevent deficiency.

On a population level, there have been numerous projects to supplement iodine in everyday staples. This has led to a prevalence of iodized salt in the USA since the 1920s. In other places, different staples are the primary route of supplementation. It is worth considering, though, that many people aiming to eat a whole-food diet may not be consuming foods that have been fortified with iodine. Vegans or vegetarians especially need to pay careful attention to getting adequate dietary iodine.  

Usually, mild deficiencies can be fixed with simple attention to a healthy diet and iodine supplementation.

Recommended daily allowances (RDAs) for Iodine have been created, and in general, consuming this amount through food and supplements combined should prevent deficiency.

Table 1: Recommended Dietary Allowances (RDAs) for Iodine [2]

How is Iodine Excess Treated?

If iodine excess has been identified, treatment will consist of two parts: Correction of thyroid dysfunction and reduction in iodine intake (if relevant).

Reducing iodine intake would involve using salt that is not fortified with iodine, avoiding supplements that contain iodine, and limiting the consumption of high-iodine foods, such as seaweed, seafood, and dairy products. The following table shows tolerable upper intake levels (ULs) for different groups.

Table 3: Tolerable Upper Intake Levels (ULs) for Iodine

If hypothyroidism is due to simply consuming too much iodine, it can often resolve with reduced intake. However, in some cases, thyroid hormones must be taken long-term. Thus, monitoring labs is very important.


The thyroid gland is small but mighty. Its function or dysfunction affects many aspects of health in all phases of life. Working properly, the thyroid supports growth, development, energy, vitality, and reproduction. When not working well, the consequences can be profoundly painful and costly to individuals and society. Given that one of the main root causes of dysfunction is as simple as ensuring appropriate intake of a nutrient, iodine, this is worth pursuing on both a personal and societal level.

In cases where the thyroid is not working well, a functional medicine approach can bring additional clarity to the root causes so that treatment can be directed by the individual's circumstances, including nutritional status and the health of other organs, such as the liver. Restoring proper thyroid levels can help people feel like themselves again with a good mood, energy, and vitality for living.

The information provided is not intended to be a substitute for professional medical advice. Always consult with your doctor or other qualified healthcare provider before taking any dietary supplement or making any changes to your diet or exercise routine.
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