Lab Reference Library / Iodine (Urinary)
Nutritional & Micronutrient
Iodine (Urinary)
UIE · Urinary Iodine Excretion · 24-Hour Urine IodineReference range, optimal functional medicine levels, and why urine iodine is the standard population-level iodine status measure, why both deficiency and excess are clinically significant for thyroid health, and why iodine supplementation should never be started without baseline testing.
Most SearchedThyroid Nutrient
Standard (Spot Urine)100 to 199 mcg/L
FM Optimal150 to 250 mcg/L
DeficientBelow 100 mcg/L
Unitsmcg/L
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Category: Nutritional & Micronutrient | Also known as: Urinary Iodine Excretion, Spot Urine Iodine, UIE
1. What This Test Measures
Urinary iodine excretion (UIE) measures iodine concentration in a spot or 24-hour urine sample. Because approximately 90% of dietary iodine is excreted in urine within 24 to 48 hours, urinary iodine is the WHO-endorsed standard for assessing iodine status at both population and individual levels. Iodine is the essential mineral substrate for thyroid hormone synthesis: thyroid peroxidase incorporates iodine into thyroglobulin to produce T4 (four iodine atoms) and T3 (three iodine atoms). The thyroid gland actively concentrates iodine from the bloodstream to approximately 30 to 60 times the plasma concentration, reflecting the mineral's unique criticality for thyroid function.
Spot urine iodine (urinary iodine concentration, UIC) is the most practical measurement but is highly variable based on hydration status. Creatinine correction (iodine-to-creatinine ratio) or 24-hour collection provides more accurate individual assessment. For population screening, WHO uses spot UIC with median values above 100 mcg/L as the adequacy threshold for adults.
2. Optimal Range and Risk Stratification
| Urinary Iodine | Status |
|---|---|
| Below 20 mcg/L | Severe deficiency: high goiter and hypothyroidism risk |
| 20 to 49 mcg/L | Moderate deficiency |
| 50 to 99 mcg/L | Mild deficiency |
| 100 to 199 mcg/L | Adequate (WHO standard) |
| 150 to 250 mcg/L | Functional medicine optimal |
| Above 300 mcg/L | Excess: risk of Wolff-Chaikoff hypothyroidism; caution in Hashimoto's |
3. The Iodine-Thyroid Connection
- Deficiency: inadequate iodine substrate reduces T4 and T3 synthesis, raising TSH (compensatory stimulation); goiter forms from prolonged TSH-driven thyroid hypertrophy; the brain and fetus are most vulnerable
- Excess (Wolff-Chaikoff effect): high intracellular iodine transiently inhibits thyroid peroxidase, blocking hormone synthesis; healthy thyroids escape this effect in 24 to 48 hours; Hashimoto's patients and those with subclinical dysfunction often cannot escape, developing worsening hypothyroidism from iodine excess
- Iodine and Hashimoto's: excess iodine in selenium-deficient individuals promotes hydrogen peroxide accumulation in thyroid follicles, increasing oxidative thyroid cell damage and accelerating TPO antibody production; always optimize selenium before supplementing iodine in Hashimoto's patients
4. What Drives Iodine Deficiency
- Avoidance of iodized salt (reduced salt intake without replacement from seafood or dairy)
- Dairy-free diet (dairy is a significant iodine source due to iodine-containing udder sanitizers)
- Minimal seafood consumption (coastal populations historically have high iodine intake)
- Goitrogenic food excess (raw cruciferous vegetables, cassava, and millet contain thiocyanates that competitively inhibit iodine uptake by the thyroid; less clinically relevant than dietary deficiency)
- Geographic iodine-depleted soils (inland regions where iodization programs are insufficient)
- Pregnancy: dramatically increases iodine requirements (the developing fetal brain requires substantial maternal iodine; the WHO recommends 220 to 250 mcg iodine daily during pregnancy)
5. How to Optimize Iodine Status
Dietary Sources
- Seafood (cod, shrimp, tuna, salmon): 30 to 100 mcg per serving
- Dairy products (milk, yogurt): 50 to 100 mcg per cup
- Eggs: 24 mcg per large egg
- Iodized salt: approximately 45 mcg per quarter teaspoon
- Seaweed: variable and often extremely high; kelp can provide 1,000 to 8,000 mcg per gram (risk of excess); nori is more moderate at 20 to 60 mcg per sheet
Supplementation
- Standard supplemental dose: 150 to 220 mcg potassium iodide daily; matches the RDA and provides adequate thyroid substrate without excess
- Pregnancy: 220 to 250 mcg daily through prenatal vitamin or additional potassium iodide
- High-dose iodine (above 1,000 mcg daily): use only under physician supervision with baseline urinary iodine testing; risk of Wolff-Chaikoff hypothyroidism, particularly in Hashimoto's patients
- Always ensure selenium adequacy (110 to 150 ng/mL serum) before initiating iodine supplementation in thyroid patients
Always Test Alongside
- Selenium: must be adequate before optimizing iodine in Hashimoto's; selenium protects thyroid cells from hydrogen peroxide generated during iodine organification
- TSH, Free T3, Free T4: clinical thyroid function context for iodine status interpretation
- TPO antibodies and TgAb: Hashimoto's status guides iodine supplementation caution level
- Retest urinary iodine 6 to 8 weeks after any supplementation change to confirm target achievement without excess
6. Related Lab Tests
SeleniumMust Optimize Before Iodine Supplementation
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TSHPrimary Thyroid Function Assessment
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TPO AntibodiesHashimoto's Activity Guides Iodine Caution
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Free T3Active Thyroid Hormone Requiring Iodine
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Vitamin DCo-nutrient for Thyroid Autoimmune Management
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ZincZinc Required for Thyroid Hormone Synthesis
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7. Clinical Perspective
Clinical Perspective
Iodine is the nutrient I approach with the most caution in my thyroid patients, specifically because the consequences of getting it wrong in Hashimoto's can be significant. I never start iodine supplementation in a thyroid patient without knowing their baseline urinary iodine, their selenium status, and their current antibody titers. The sequence matters: selenium first, then iodine, never the reverse. When I find genuine iodine deficiency in a patient with low-normal thyroid function, cautious iodine repletion to the 150 to 250 mcg/L urinary range alongside maintained selenium adequacy is safe and often meaningfully improves thyroid hormone production. But blindly prescribing high-dose iodine to every thyroid patient, which I see recommended frequently, can trigger antibody flares and worsening hypothyroidism in a patient with Hashimoto's who had stable, manageable disease.
Brian Lamkin, DO | Founder, The Lamkin Clinic | Edmond, Oklahoma
8. Frequently Asked Questions
What is the optimal urinary iodine level?
In functional medicine, optimal urinary iodine is 150 to 250 mcg/L on a spot urine sample. The WHO defines adequacy as 100 to 199 mcg/L for general adults. Below 100 mcg/L indicates iodine deficiency. Above 300 mcg/L indicates excess that can suppress thyroid function through the Wolff-Chaikoff effect, particularly in those with Hashimoto's thyroiditis.
Can too much iodine cause thyroid problems?
Yes. Iodine excess above 300 mcg/L consistently can paradoxically suppress thyroid function through the Wolff-Chaikoff effect. In healthy thyroids, this is temporary. In patients with Hashimoto's or subclinical thyroid dysfunction, the inhibitory effect may be sustained, worsening hypothyroidism. High-dose iodine supplementation also increases hydrogen peroxide generation in thyroid follicles, accelerating thyroid cell oxidative damage and TPO antibody production in selenium-deficient individuals.
Why must selenium be adequate before supplementing iodine?
Selenium is required for the glutathione peroxidase system (GPx) in thyroid follicular cells that neutralizes hydrogen peroxide generated during iodine organification (the process of incorporating iodine into thyroglobulin). Without adequate selenium, hydrogen peroxide accumulates, causing oxidative thyroid cell damage, releasing thyroid antigens, and amplifying Hashimoto's autoimmune activity. Selenium also is required for iodothyronine deiodinase (DIO) enzymes that convert T4 to active T3. Selenium optimization before iodine supplementation is therefore protective on multiple levels.
Should iodine be tested before starting supplementation?
Yes, always. Testing baseline urinary iodine before supplementation establishes whether deficiency is actually present, guides appropriate dose, and identifies patients already at excess who should not supplement further. Many patients with thyroid symptoms are not iodine-deficient, and supplementing iodine without confirmed deficiency can worsen their condition, particularly in Hashimoto's. Retest urinary iodine 6 to 8 weeks after starting or changing supplementation to confirm target range without excess.
Content authored and clinically reviewed by Brian Lamkin, DO, founder of The Lamkin Clinic in Edmond, Oklahoma. Brian Lamkin, DO has 25+ years of experience in functional and regenerative medicine. This page reflects current functional medicine practice standards and is updated as new clinical evidence becomes available.
Iodine supplementation without testing is not precision medicine. It is guessing with your thyroid.
A baseline urinary iodine test and selenium assessment should precede any iodine supplementation decision. Schedule a consultation for a complete thyroid nutrient evaluation.
Schedule a ConsultationMedical Disclaimer: This content is provided for educational purposes only and is not intended as a substitute for professional medical advice, diagnosis, or treatment. Lab interpretation should always be performed in clinical context by a qualified healthcare provider. Reference ranges and optimal targets may vary based on individual patient history, clinical presentation, and laboratory methodology. Schedule a consultation to discuss your specific results with Dr. Lamkin.