Iodine has a publicity problem. On one hand, it's the reason we put it in salt — universal iodization was one of the great public health victories of the 20th century, virtually eliminating goiter and cretinism in developed nations. On the other hand, the wellness world has swung to the opposite extreme, with some practitioners recommending milligram doses of iodine that would make an endocrinologist's eye twitch.
The truth is that iodine's therapeutic window is narrow, and getting the dose wrong in either direction has real consequences for your thyroid.
Why Iodine Matters: The Non-Negotiable Nutrient
Iodine is a structural component of thyroid hormones. That's not a supporting role — it's the defining feature. T4 (thyroxine) contains four iodine atoms. T3 (triiodothyronine) contains three. Without iodine, your thyroid literally cannot manufacture its primary products.
The thyroid gland actively concentrates iodine from the blood through the sodium-iodide symporter (NIS), achieving intracellular concentrations 20-40 times higher than plasma. This iodine is then incorporated into thyroglobulin by thyroid peroxidase (TPO) to synthesize T4 and T3.
When iodine is insufficient:
- The thyroid enlarges in an attempt to capture more iodine from the blood, creating a goiter
- Thyroid hormone production drops, leading to hypothyroidism
- During pregnancy, maternal iodine deficiency impairs fetal brain development — iodine deficiency remains the leading preventable cause of intellectual disability worldwide, according to the WHO
The Global and Local Landscape
Iodine deficiency was largely eliminated in the US after the introduction of iodized salt in 1924. But it hasn't disappeared:
- NHANES data shows that median urinary iodine concentrations (a marker of population iodine status) in the US have declined by about 50% since the 1970s
- The shift away from iodized salt toward sea salt, Himalayan salt, and kosher salt — none of which contain added iodine — has contributed to this decline
- About 35% of the world's population still has insufficient iodine intake, primarily in developing countries
A study in The Journal of Clinical Endocrinology & Metabolism (2011) found that mild iodine deficiency may be re-emerging in certain US subpopulations, particularly young women of childbearing age — precisely the group where iodine adequacy matters most.
The Other Side: Too Much Iodine Is Also a Problem
This is where iodine gets tricky. Excessive iodine intake can cause the same problems as deficiency — hypothyroidism and goiter — plus additional issues:
Wolff-Chaikoff effect. When the thyroid is suddenly exposed to a large iodine load, it temporarily shuts down hormone production as a protective mechanism. In healthy people, this resolves within days. In people with underlying thyroid dysfunction, it can cause persistent hypothyroidism.
Iodine-induced hyperthyroidism (Jod-Basedow phenomenon). In people with autonomous thyroid nodules or latent Graves' disease, excess iodine provides substrate for unregulated hormone production, potentially triggering thyrotoxicosis.
Autoimmune thyroiditis. Epidemiological data consistently shows that excessive iodine intake is associated with increased rates of autoimmune thyroid disease. In countries that have introduced aggressive iodine fortification, rates of Hashimoto's thyroiditis have risen in parallel.
The RDA for adults is 150 mcg/day. The UL is 1,100 mcg/day. For most healthy adults, staying within 150-300 mcg/day from food and supplements is the sweet spot.
Food Sources of Iodine
| Food | Serving | Iodine (mcg) |
|---|---|---|
| Seaweed (nori) | 1 sheet | 16-43 |
| Seaweed (kelp) | 1 g | 500-8,000 (highly variable) |
| Cod | 3 oz | 99 |
| Iodized salt | 1/4 tsp | 71 |
| Shrimp | 3 oz | 35 |
| Egg | 1 large | 24 |
| Tuna | 3 oz | 17 |
| Milk | 1 cup | 56 |
| Yogurt | 1 cup | 75 |
Kelp is in a category by itself — and that's not a compliment for supplementation purposes. A single gram of certain kelp species can contain 500-8,000 mcg of iodine, easily exceeding the UL. Kelp-based iodine supplements have caused documented cases of thyrotoxicosis and hypothyroidism. If you supplement with iodine, use a standardized product — not kelp.
Dairy is an underappreciated iodine source. The iodine in milk comes from iodine-containing sanitizers used in dairy processing and from iodine-supplemented cattle feed. A cup of milk provides about 56 mcg — roughly a third of the RDA.
Iodine and Selenium: The Thyroid Power Couple
Iodine and selenium have a critical combined effect in thyroid function:
- Iodine provides the substrate for thyroid hormone synthesis
- Selenium (via glutathione peroxidase) protects the thyroid from the hydrogen peroxide generated during that synthesis
- Selenium (via deiodinases) converts T4 to the active T3
Supplementing iodine without adequate selenium may actually worsen thyroid inflammation, because you're increasing hydrogen peroxide production without the selenoprotein buffer to neutralize it. Conversely, severe selenium deficiency can exacerbate the effects of iodine deficiency.
If you supplement one, ensure you're adequate in the other.
Testing Your Iodine Status
Unlike most minerals, there's no single reliable blood test for individual iodine status. Urinary iodine concentration (UIC) is the standard population-level assessment used by the WHO, but a single spot urine test in an individual is unreliable because iodine intake varies dramatically day to day.
For clinical purposes, thyroid function tests (TSH, free T4, free T3) serve as proxies. Elevated TSH with low T4 suggests hypothyroidism that could be iodine-related — but could also be autoimmune. Thyroid antibodies (TPO-Ab, TG-Ab) help distinguish between the two.
If you suspect iodine insufficiency, a 24-hour urine collection provides the most accurate individual assessment. Levels below 100 mcg/L suggest insufficiency; below 50 mcg/L indicates moderate deficiency.
For most people in iodized-salt countries, testing isn't necessary. The pragmatic approach: use iodized salt regularly, eat seafood or dairy a few times per week, and you're likely covered.
Pregnancy: When Iodine Becomes Critical
Iodine requirements increase by nearly 50% during pregnancy (from 150 to 220 mcg/day) and further during lactation (290 mcg/day). The fetal thyroid begins producing its own hormones around week 12 of gestation, and the fetus depends entirely on maternal iodine supply.
The American Thyroid Association recommends that all pregnant and breastfeeding women in North America take a supplement containing 150 mcg of iodine daily (in addition to dietary sources). Many prenatal vitamins now include iodine, but not all — check the label.
When to Talk to a Pro
Consult a healthcare provider if:
- You suspect thyroid dysfunction (fatigue, weight changes, cold intolerance, hair loss)
- You're pregnant or planning pregnancy and not taking a prenatal with iodine
- You're considering iodine supplementation above 300 mcg/day
- You have Hashimoto's or Graves' disease (iodine supplementation in autoimmune thyroid disease is controversial and needs medical guidance)
- You consume large amounts of seaweed or kelp supplements
FAQ
Does sea salt contain iodine? Naturally, very little. Sea salt contains trace amounts of iodine (roughly 2 mcg per gram) but far less than iodized table salt (about 45 mcg per gram, depending on brand). If sea salt is your primary salt, you're likely not getting meaningful iodine from it.
Can iodine supplements help with hypothyroidism? Only if iodine deficiency is the cause of hypothyroidism — which is relatively uncommon in countries with iodine-fortified salt. Most hypothyroidism in developed nations is caused by Hashimoto's thyroiditis, where iodine supplementation may actually worsen the condition. Get tested before supplementing.
Are iodine drops and lugol's solution safe? Lugol's solution (potassium iodide + iodine) delivers milligram doses — hundreds to thousands of times the RDA. This far exceeds the UL and can cause serious thyroid dysfunction. It has legitimate medical uses (preoperative thyroid preparation, radiation emergencies) but is not appropriate for routine supplementation.
A note from Living & Health: We're a lifestyle and wellness magazine, not a doctor's office. The information here is for general education and entertainment — not medical advice. Always talk to a qualified healthcare professional before making changes to your health routine, especially if you have existing conditions or take medications.
