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Iodine
Iodine does one thing in your body: build thyroid hormone, which runs your metabolism, energy, and mood. Most people in iodised-salt countries get enough without trying. But three groups have quietly fallen off the default and don't know it: pregnant women, vegans, and anyone who swapped table salt for pink Himalayan or fancy sea salt. Here's what you didn't know you didn't know: sea salt has almost no iodine, and the wellness kelp capsule sold to fix that is the most common way people overdose.
Do · Daily Evidence Strong Chapter Food

The floor is well settled. Iodised salt rolled out across 130-plus countries and took endemic goitre from epidemic to near-zero within a generation 1. The higher stakes are in pregnancy: children of UK mothers in the lowest first-trimester iodine band were 60% more likely to land in the bottom quartile of verbal IQ at age 8 2. Australian kids of mildly deficient mothers kept lower spelling scores at 9, and adding iodine to the salt afterward didn't reverse it 3.

More is not better. Across three Chinese regions, autoimmune thyroiditis rose stepwise with intake 4. There's a comfortable middle and a real cost on either side.

Who has to think about this, and what they do.

You don't need a test. Spot urinary iodine swings too much day to day to mean anything for one person 7.

What sufficiency buys you. If you were already fine, nothing visible changes, because nothing was broken. For someone actually deficient, energy and cold tolerance come back over the first month or two of restored intake, and a creeping TSH starts coming down 1. For the pregnant woman the payoff lands in the next generation: the child who got enough hormone through the first trimester never shows the verbal-IQ gap the deficient child does, and by the time it would show, the window closed years ago 2.

The fine print — when to skip it, and what people get wrong

Sea salt is a natural iodine source because it's from the sea. False: unfortified sea salt runs below 2 µg/g versus 45 µg/g in iodised salt 8. Kelp is a safe way to top up. Capsule assays have found over ten times the labelled dose 9.

With Hashimoto's, Graves', or a thyroid nodule, don't start high-dose iodine without your endocrinologist; a sudden load can trigger Jod-Basedow hyperthyroidism 10. The 150 µg in a prenatal is fine; the kelp and Lugol's range is the danger.

References
  1. 1Zimmermann MB, Jooste PL, Pandav CS (2008). Iodine-deficiency disorders. The Lancet. link
  2. 2Bath SC, Steer CD, Golding J, Emmett P, Rayman MP (2013). Effect of inadequate iodine status in UK pregnant women on cognitive outcomes in their children: results from the Avon Longitudinal Study of Parents and Children (ALSPAC). The Lancet. link
  3. 3Hynes KL, Otahal P, Hay I, Burgess JR (2013). Mild iodine deficiency during pregnancy is associated with reduced educational outcomes in the offspring: 9-year follow-up of the gestational iodine cohort. Journal of Clinical Endocrinology and Metabolism. link
  4. 4Teng W, Shan Z, Teng X, et al. (2006). Effect of iodine intake on thyroid diseases in China. New England Journal of Medicine. link
  5. 5Trumbo P, Yates AA, Schlicker S, Poos M (2001). Dietary reference intakes: vitamin A, vitamin K, arsenic, boron, chromium, copper, iodine, iron, manganese, molybdenum, nickel, silicon, vanadium, and zinc. Journal of the American Dietetic Association. link
  6. 6Pearce EN, Lazarus JH, Moreno-Reyes R, Zimmermann MB (2016). Consequences of iodine deficiency and excess in pregnant women: an overview of current knowns and unknowns. American Journal of Clinical Nutrition. link
  7. 7WHO, UNICEF, ICCIDD (2007). Assessment of iodine deficiency disorders and monitoring their elimination: a guide for programme managers, 3rd edition. link
  8. 8Pearce EN, Pino S, He X, Bazrafshan HR, Lee SL, Braverman LE (2004). Sources of dietary iodine: bread, cows' milk, and infant formula in the Boston area. Journal of Clinical Endocrinology and Metabolism. link
  9. 9Zava TT, Zava DT (2011). Assessment of Japanese iodine intake based on seaweed consumption in Japan: a literature-based analysis. Thyroid Research. link
  10. 10Laurberg P, Cerqueira C, Ovesen L, et al. (2010). Iodine intake as a determinant of thyroid disorders in populations. Best Practice and Research Clinical Endocrinology and Metabolism. link
Related in the handbook (6)
Related
  • — Like folate, iodine is a prenatal essential for the baby's developing brain; don't assume your prenatal has enough.
  • — Iodized salt is where most people actually get their iodine — this is the practical side of the iodine question.
  • — Selenium and iodine together determine thyroid output — a deficiency in one changes how the other behaves.
  • — Too little iodine drives an underactive thyroid; too much can trigger the autoimmune kind.
  • — Iodine has one job — making thyroid hormone — so both too little and too much show up on thyroid tests.
  • — Vegans run low on iodine for the same reason as B12 — both come mostly from animal foods, so both need a plan.
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