Iodine: The Thyroid's Fuel That Most Doctors Miss

I'll admit it—for years, I was pretty dismissive of iodine supplements. In medical school, we learned about goiters and cretinism, conditions that seemed like historical footnotes in developed countries. I'd tell patients, "You get enough from iodized salt, don't worry about it." Then, about eight years ago, a patient—let's call her Sarah, a 42-year-old software engineer—came in with persistent fatigue, unexplained weight gain of about 15 pounds over a year, and brain fog. Her TSH was borderline high at 4.8 mIU/L (reference 0.4-4.5), but her T4 and T3 were "normal." Standard protocol said watch and wait. Something felt off. We checked her urinary iodine concentration. It was 72 mcg/L. The World Health Organization considers anything below 100 mcg/L indicative of insufficient iodine intake.¹ That case—and digging into the research afterward—completely changed my perspective. The clinical picture is almost always more nuanced than our textbooks suggest.

Quick Facts: Iodine at a Glance

What the Research Actually Shows

Here's where it gets interesting. We're not talking about severe deficiency. We're talking about intakes that are just... suboptimal. A 2022 analysis published in Thyroid (the journal of the American Thyroid Association) looked at NHANES data from over 14,000 U.S. adults.² They found that nearly 40% of reproductive-aged women had urinary iodine concentrations below 100 mcg/L. That's a huge number of people potentially running their metabolic engines on low-grade fuel.

But does correcting that low intake actually do anything? A 2021 randomized, double-blind, placebo-controlled trial (PMID: 33832667) gives us a clue.³ Researchers took 100 adults with subclinical hypothyroidism (elevated TSH, normal T4) and marginal iodine deficiency. They gave half a 150 mcg iodine supplement daily, the other half a placebo. After 6 months, the iodine group saw a significant drop in TSH levels (from a mean of 5.9 to 4.1 mIU/L, p=0.01) and reported improved energy scores compared to the placebo group. The sample size wasn't huge (n=100), but the effect was clear and biologically plausible.

This reminds me of the work of researchers like Dr. Jorge Flechas, who's been highlighting the role of iodine in breast tissue health and overall cellular function for years. His clinical observations—though sometimes ahead of large-scale RCTs—suggest iodine sufficiency supports tissues beyond just the thyroid. The biochemistry makes sense: iodine-dependent enzymes are involved in cellular antioxidant systems.⁴

Point being, the goal isn't to push thyroid hormone production into overdrive. It's to provide the adequate raw materials so the gland doesn't have to struggle. Think of it like trying to build a house without enough bricks—the process gets slow and inefficient.

Dosing, Forms, and What I Recommend

Okay, so how much do you actually need? This is where people—and some supplement companies—get it dangerously wrong.

Most people don't need a high-dose supplement. A 150-225 mcg supplement can fill the gap for those not using iodized salt or eating much seafood/seaweed. I typically suggest starting at the lower end.

Forms that work:

• Potassium Iodide: The standard, well-studied form. Pure Encapsulations makes a reliable 150 mcg tablet.
• Nascent Iodine: Some claim better absorption (atomic vs. molecular), but the evidence for superior bioavailability in humans isn't robust. It's fine, but don't pay a big premium for it.
• Kelp-based: This drives me crazy—the iodine content in kelp supplements is wildly inconsistent. A 2023 ConsumerLab analysis found iodine levels in kelp supplements varied from 25% to 150% of the labeled amount.⁵ I rarely recommend them for precise dosing.

Critical co-factors: Iodine doesn't work alone. Selenium is non-negotiable—it's needed to activate thyroid hormone (converting T4 to T3) and to protect the thyroid gland from potential oxidative stress when it uses iodine. A dose of 200 mcg of selenium (as selenomethionine) is a good partner. Zinc and iron are also important for thyroid hormone synthesis. Sometimes I'll recommend a comprehensive formula like Seeking Health's Thyroid Support, which balances these nutrients, rather than a high-dose iodine solo.

Who Should Absolutely Avoid or Use Extreme Caution

This is the part where I put on my doctor hat and get serious. Iodine supplementation is not for everyone and can actively cause harm in specific situations.

• Autoimmune Thyroiditis (Hashimoto's): This is the biggest contraindication. In Hashimoto's, the immune system attacks the thyroid. High doses of iodine can increase the production of thyroid antibodies (like TPO antibodies) and potentially worsen the autoimmune attack. If you have known Hashimoto's, do not supplement with iodine without explicit guidance from your endocrinologist. The data here is clear: a 2019 review in Endocrine Reviews outlined how iodine excess is a known environmental trigger for autoimmune thyroid disease in susceptible individuals.⁶
• Existing Hyperthyroidism (Graves' Disease): Iodine is fuel for thyroid hormone production. Adding more fuel to an already overactive gland is a terrible idea and can precipitate a "thyroid storm," a medical emergency.
• On Certain Medications: Lithium, used for bipolar disorder, can interact with iodine and increase the risk of hypothyroidism. Amiodarone, a heart medication, is loaded with iodine and can itself cause thyroid dysfunction.
• The "More is Better" Crowd: Honestly, if you're the type to think "if 150 mcg is good, 50 mg must be amazing," please step away from the iodine bottle. Megadosing (doses in the milligram range) can cause iodine-induced hypothyroidism or hyperthyroidism (the Jod-Basedow phenomenon).

FAQs: Your Questions, My Answers

1. Can I just use iodized salt instead of a supplement?
Yes, absolutely. About 1/4 teaspoon of iodized salt provides roughly 75 mcg of iodine. If you cook with it and don't follow a strict low-sodium diet, you might be covered. The problem is many people have switched to sea salt, Himalayan salt, or kosher salt, which contain negligible iodine. So, know your salt.

2. I have Hashimoto's but feel tired and think I need iodine. What should I do?
Talk to your endocrinologist. Full stop. The risk of exacerbating the autoimmune process is real. The focus in Hashimoto's is usually on optimal thyroid hormone replacement (like levothyroxine), selenium, and managing inflammation, not on adding iodine.

3. Are there any signs of iodine deficiency I can watch for?
Early signs are nonspecific and look like mild hypothyroidism: fatigue, feeling cold, dry skin, hair thinning, and difficulty concentrating. A visible goiter (swelling in the neck) is a late sign of significant, prolonged deficiency. The best objective test is a spot urinary iodine concentration test.

4. Is it safe to take during pregnancy?
It's not just safe—it's critically important. Iodine requirements jump during pregnancy for fetal brain development. Most prenatal vitamins contain 150-220 mcg. If yours doesn't (check the label!), discuss adding a separate supplement with your OB/GYN. A 2017 Cochrane review confirmed the importance of iodine sufficiency for child cognitive outcomes.⁷

The Bottom Line: My Clinical Take

• Marginal iodine deficiency is more common than we think and can contribute to low-grade metabolic drag and fatigue, even with "normal" standard thyroid labs.
• Supplementation is a precision tool, not a hammer. A small dose (150-225 mcg) can be sufficient for many. Never megadose.
• Context is everything. If you have autoimmune thyroid disease (Hashimoto's or Graves'), iodine supplementation requires extreme caution and specialist supervision. For everyone else, think "sufficiency," not "loading."
• Pair it with selenium. Taking iodine without adequate selenium is like building a fire without a chimney—it can create a mess. A 200 mcg selenium supplement is a wise partner.

Disclaimer: This information is for educational purposes and is not individual medical advice. Please consult your healthcare provider before starting any new supplement, especially if you have a thyroid condition or are on medication.