Iodine

Iodine is a trace element with one primary job in the human body: serving as the essential building block for thyroid hormones. Your thyroid gland actively concentrates iodine from your blood — trapping it at concentrations 20-50 times higher than blood levels — and incorporates it into thyroxine (T4) and triiodothyronine (T3). Without adequate iodine, thyroid hormone production falters, and every cell in your body feels the impact since thyroid hormones regulate metabolism, energy, growth, and development.

The global public health story of iodine is both a success and an ongoing challenge. Before widespread salt iodization, iodine deficiency was endemic in many regions, causing goiter (enlarged thyroid), hypothyroidism, and — most devastatingly — cretinism (severe intellectual disability from prenatal/early childhood deficiency). Salt iodization has dramatically reduced these conditions in many countries. Yet iodine deficiency disorders remain the world’s leading cause of preventable intellectual disability, affecting populations where iodized salt isn’t used, where soil iodine is depleted, or where dietary patterns have shifted away from iodine-rich foods.

In developed countries, a different concern has emerged: mild-to-moderate iodine deficiency in specific populations, particularly pregnant women who have dramatically increased requirements. Studies in the US, UK, and Australia have found concerning rates of inadequate iodine status even in supposedly iodine-sufficient countries. Testing identifies individuals whose iodine intake isn’t meeting their needs — before thyroid dysfunction or developmental consequences occur.

Order Your Iodine Test

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Key Benefits of Testing

Iodine testing provides direct assessment of this essential nutrient’s availability for thyroid hormone synthesis. While TSH and thyroid hormone levels show the downstream consequences of iodine status, iodine testing itself reveals whether the raw material supply is adequate.

For pregnant women and those planning pregnancy, iodine assessment is particularly valuable. The fetus depends entirely on maternal thyroid hormone during early pregnancy and on maternal iodine supply for fetal thyroid function throughout gestation. Deficiency during pregnancy can impair fetal brain development even when the mother shows no obvious thyroid dysfunction. Testing identifies women who need supplementation beyond their prenatal vitamin.

When thyroid dysfunction is detected — particularly hypothyroidism or goiter — iodine testing helps determine if deficiency is the cause. In iodine-sufficient regions, autoimmune thyroiditis (Hashimoto’s) is the most common cause of hypothyroidism. But in areas with borderline iodine intake, deficiency contributes significantly. Treatment differs: iodine supplementation for deficiency-related hypothyroidism, versus thyroid hormone replacement for autoimmune destruction.

For those following restricted diets — vegans, those avoiding dairy and seafood, those on salt-restricted diets without iodized salt — testing confirms whether alternative iodine sources are adequate or supplementation is needed.

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What Does Iodine Testing Measure?

Iodine status can be assessed through several methods, each with different strengths and clinical applications.

Urinary Iodine Concentration (UIC)

The most commonly used assessment method. Since over 90% of absorbed iodine is eventually excreted in urine, urinary iodine reflects recent iodine intake. It’s the standard method used in population surveys and epidemiological studies.

Spot urine: A single urine sample measuring iodine concentration. Convenient but highly variable — a single measurement can be influenced by recent meals, hydration status, and time of day. Best interpreted as part of population assessment rather than individual diagnosis.

24-hour urine collection: Measures total daily iodine excretion, reducing variability from hydration and timing. More accurate for individual assessment but less convenient.

Interpretation note: Urinary iodine reflects recent intake (past 24-48 hours), not body stores. A single low value might reflect a recent low-iodine day rather than chronic deficiency. Multiple measurements or 24-hour collection improve accuracy for individual assessment.

Serum Iodine

Measures iodine circulating in blood. Less commonly used than urinary iodine because blood levels are tightly regulated and may not reflect tissue status or total body iodine. However, serum iodine can detect very recent intake and may be useful in certain clinical contexts.

Thyroglobulin

Thyroglobulin is a protein produced by the thyroid that serves as the scaffold for thyroid hormone synthesis. Serum thyroglobulin rises in iodine deficiency (as the thyroid works harder to trap available iodine) and falls with adequate iodine status. It’s used in population studies as a complementary indicator and may help assess individual status when combined with other measures.

Thyroid Hormone Levels

TSH, Free T4, and Free T3 reflect thyroid function, which depends on iodine status. However, thyroid hormones may remain normal in mild-to-moderate iodine deficiency as the thyroid compensates. Abnormal thyroid hormones indicate that deficiency has progressed to cause dysfunction.

Which Test to Choose?

For most clinical purposes, urinary iodine (preferably 24-hour collection or multiple spot samples) provides the best assessment of iodine status. Combining urinary iodine with thyroid function tests (TSH, Free T4) gives a complete picture of iodine status and its functional consequences.

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Why Iodine Testing Matters

Pregnancy and Fetal Development

Iodine requirements increase dramatically during pregnancy — by approximately 50% — due to increased maternal thyroid hormone production, transfer of iodine to the fetus, and increased renal clearance. The developing fetal brain is exquisitely sensitive to thyroid hormone levels, which depend on adequate maternal iodine.

Even mild-to-moderate maternal iodine deficiency during pregnancy has been associated with subtle but measurable cognitive deficits in offspring. Severe deficiency causes cretinism — a devastating syndrome of intellectual disability, deaf-mutism, and motor spasticity. While cretinism is rare in developed countries, the subtler effects of marginal deficiency are increasingly recognized.

Testing pregnant women (or those planning pregnancy) identifies inadequate iodine status while there’s still time to correct it. Most prenatal vitamins contain iodine, but not all, and some women have higher requirements.

Thyroid Dysfunction Investigation

When hypothyroidism is detected, determining its cause guides treatment:

Iodine deficiency: The thyroid lacks raw material to make hormones. Treatment is iodine supplementation, which can restore normal function if the gland hasn’t been permanently damaged.

Autoimmune thyroiditis: The immune system destroys thyroid tissue. Treatment is thyroid hormone replacement, since the gland can’t recover regardless of iodine availability.

Mixed or unclear cases: Some patients have both inadequate iodine and autoimmune disease. Testing clarifies the contribution of each.

Goiter (enlarged thyroid) also warrants iodine assessment. Iodine deficiency is the most common cause of goiter worldwide. The thyroid enlarges as it works harder to trap limited iodine. Adequate iodine can prevent further enlargement and may cause regression of diffuse goiter.

At-Risk Populations

Certain groups are prone to iodine deficiency and benefit from testing:

Pregnant and breastfeeding women: Dramatically increased requirements make deficiency more likely even with typical diets.

Vegans and vegetarians: If avoiding dairy and seafood (major iodine sources) without using iodized salt or supplements, deficiency risk is substantial.

Those avoiding iodized salt: People using sea salt, kosher salt, or specialty salts (most of which are not iodized) miss this important dietary source.

Those on salt-restricted diets: Limiting salt for blood pressure can reduce iodine intake if iodized salt was the primary source.

Residents of iodine-deficient regions: Some geographic areas have iodine-depleted soil, affecting locally grown foods.

Those consuming goitrogens: Certain foods (cruciferous vegetables, soy, cassava) contain compounds that interfere with iodine utilization. Usually not problematic with adequate iodine intake, but can exacerbate marginal deficiency.

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What Can Affect Iodine Levels?

Causes of Low Iodine

Inadequate dietary intake: The most common cause globally. Iodine content of food depends on soil iodine where plants are grown or animals are raised. Seafood and seaweed are reliable sources; dairy is significant in countries where iodine is added to cattle feed or dairy equipment sanitizers. Without these sources or iodized salt, intake may be inadequate.

Not using iodized salt: Salt iodization is the primary intervention preventing deficiency worldwide. Using non-iodized specialty salts (sea salt, Himalayan salt, kosher salt) eliminates this source. In the US, salt iodization is voluntary and not universal, so checking labels matters.

Increased requirements: Pregnancy increases iodine needs by ~50%; breastfeeding continues elevated requirements. These life stages can unmask marginally adequate intake as insufficient.

Goitrogen exposure: Certain foods contain substances that inhibit iodine uptake or utilization. Cruciferous vegetables (broccoli, cabbage, cauliflower), soy, cassava, and millet contain goitrogens. Cooking reduces goitrogen content. With adequate iodine intake, normal consumption of these foods isn’t problematic, but they can worsen marginal deficiency.

Environmental exposures: Perchlorate (found in some water supplies and food), thiocyanate (from tobacco smoke), and nitrate can compete with iodine for thyroid uptake, effectively increasing iodine requirements.

Selenium deficiency: Selenium is required for enzymes that convert T4 to active T3 and that protect the thyroid from oxidative damage. Combined iodine and selenium deficiency is worse than either alone.

Causes of High Iodine

Excessive supplementation: Over-the-counter iodine or kelp supplements can provide doses far exceeding requirements. While the thyroid can adapt to moderate excess, very high intake can paradoxically suppress thyroid function (Wolff-Chaikoff effect) or trigger thyroid dysfunction in susceptible individuals.

High seaweed consumption: Certain seaweeds (especially kelp/kombu) contain extremely high iodine concentrations. Regular consumption of these foods can cause iodine excess. Japanese populations consuming traditional seaweed-rich diets have some of the highest iodine intakes globally.

Iodine-containing medications: Amiodarone (heart rhythm medication) contains substantial iodine and commonly causes thyroid dysfunction. Iodinated contrast agents used in CT scans provide massive iodine loads that can affect thyroid function.

Topical iodine exposure: Povidone-iodine (Betadine) and other topical iodine preparations can be absorbed, especially through damaged skin or mucous membranes.

Too Much Iodine: The U-Shaped Curve

Iodine has a “U-shaped” relationship with thyroid health — both deficiency and excess can cause problems. While deficiency causes hypothyroidism and goiter, excess iodine can cause:

Iodine-induced hypothyroidism: High iodine can suppress thyroid function, especially in those with underlying autoimmune thyroiditis.

Iodine-induced hyperthyroidism: Particularly in older adults with nodular goiter, sudden iodine excess (from contrast agents or supplements) can trigger hyperthyroidism (Jod-Basedow phenomenon).

Thyroiditis: Excess iodine may exacerbate autoimmune thyroid inflammation.

The optimal range for iodine intake is neither too little nor too much — adequate but not excessive.

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Understanding Your Results

Interpreting Urinary Iodine

Urinary iodine results are typically expressed as micrograms per liter (μg/L). Because spot urine iodine varies considerably, interpretation differs for individuals versus populations.

For population assessment (the context most reference values are based on), median urinary iodine indicates community iodine status. The World Health Organization uses median urinary iodine to classify populations from severely deficient to excessive.

For individual assessment, single spot urine values are less reliable. A low single value might reflect recent low intake rather than chronic deficiency. Multiple measurements, 24-hour collection, or combining with thyroglobulin and thyroid function tests improves accuracy.

Your healthcare provider can interpret your results in context of your diet, symptoms, thyroid function tests, and clinical situation. Low urinary iodine with elevated TSH or goiter strongly suggests iodine deficiency. Low urinary iodine with normal thyroid function suggests marginal status that may benefit from dietary improvement or supplementation.

Iodine Testing with Thyroid Function

Combining iodine status assessment with thyroid function tests provides the complete picture:

Low iodine, normal TSH: Early or mild deficiency. The thyroid is compensating but may be stressed. Dietary improvement or supplementation appropriate.

Low iodine, elevated TSH, low T4: Iodine deficiency causing hypothyroidism. Iodine supplementation may restore function if thyroid tissue remains healthy.

Low iodine, goiter present: Classic iodine deficiency goiter. Supplementation can prevent progression and may cause regression.

Normal iodine, hypothyroidism: Iodine isn’t the problem — look for autoimmune thyroiditis or other causes.

High iodine, thyroid dysfunction: Consider iodine-induced thyroid disease, especially if recent contrast exposure or supplement use.

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Health Connections

Thyroid Disorders

Hypothyroidism: Iodine deficiency is the leading cause of preventable hypothyroidism worldwide. The thyroid can’t produce enough T4 and T3 without sufficient iodine. Symptoms include fatigue, weight gain, cold intolerance, constipation, dry skin, and cognitive slowing.

Goiter: Thyroid enlargement is the body’s attempt to compensate for iodine deficiency by increasing the gland’s iodine-trapping capacity. Endemic goiter (affecting large portions of populations) characterized iodine-deficient regions before salt iodization.

Thyroid nodules: Chronic iodine deficiency increases nodule formation. Some nodules can become autonomous, producing hormones independently and potentially causing hyperthyroidism, especially if suddenly exposed to iodine excess.

Pregnancy and Child Development

Cretinism: Severe prenatal iodine deficiency causes irreversible intellectual disability, growth retardation, and neurological deficits. While rare in developed countries, it remains a concern in areas without iodine supplementation programs.

Cognitive development: Even mild-to-moderate maternal iodine deficiency during pregnancy is associated with subtle reductions in offspring IQ and school performance. The first trimester — before the fetal thyroid is functional — is particularly critical, as the fetus depends entirely on maternal thyroid hormone.

Pregnancy complications: Iodine deficiency is associated with increased risk of miscarriage, stillbirth, and preterm birth in some studies.

Cognitive Function in Adults

Brain function: Thyroid hormones (dependent on iodine) remain important for adult brain function. Hypothyroidism from any cause, including iodine deficiency, causes cognitive slowing, memory problems, and “brain fog.” These symptoms typically improve with correction of thyroid status.

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Why Regular Testing Matters

For most people in iodine-sufficient countries eating varied diets including dairy, seafood, and iodized salt, routine iodine testing isn’t necessary. Standard thyroid function tests (TSH) can detect thyroid dysfunction that might result from deficiency.

However, certain situations warrant iodine assessment:

Pregnancy and preconception: Given the critical importance of iodine for fetal brain development and the increased requirements of pregnancy, testing ensures supplementation is adequate. All pregnant women should use prenatal vitamins containing iodine, but testing can identify those needing additional supplementation.

Restricted diets: Vegans, those avoiding dairy and seafood, and those not using iodized salt should periodically assess iodine status to ensure alternative sources are adequate.

Thyroid disease investigation: When hypothyroidism or goiter is present, iodine testing helps determine if deficiency is contributing.

Residents of historically deficient regions: Some areas have iodine-depleted soil, and local food may be insufficient even with generally iodine-sufficient national status.

Before high-iodine medical procedures: Some clinicians check iodine status and thyroid function before administering iodinated contrast agents or amiodarone, particularly in those with thyroid nodules.

For those with identified deficiency, follow-up testing confirms that dietary changes or supplementation have achieved adequacy.

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Related Biomarkers Often Tested Together

TSH (Thyroid Stimulating Hormone) — The primary thyroid function test. Rises when thyroid hormone is insufficient (hypothyroidism), which can result from iodine deficiency. TSH is more sensitive than T4/T3 for detecting early thyroid dysfunction.

Free T4 (Thyroxine) — The main thyroid hormone circulating in blood. Falls in hypothyroidism. Requires iodine for synthesis — each T4 molecule contains four iodine atoms.

Free T3 (Triiodothyronine) — The active thyroid hormone. Contains three iodine atoms. May fall later than T4 in developing deficiency as the body preferentially maintains T3.

Thyroid Antibodies (TPO, Thyroglobulin) — Help distinguish autoimmune thyroid disease from iodine deficiency as the cause of thyroid dysfunction.

Thyroglobulin — Rises in iodine deficiency as the thyroid works harder. Useful as a complementary marker of iodine status.

Selenium — Required for thyroid hormone metabolism. Combined iodine-selenium deficiency is worse than either alone.

Note: Information provided in this article is for educational purposes and doesn’t replace personalized medical advice.