Who Found the Role of Iodine, and How Seaweed Became a Salt Story
The history of iodine runs from a seaweed remedy for swollen necks, through the 1811 discovery of the element, to iodized salt and the thyroid hormones that need it. It also explains why more iodine is not always better.
The history of iodine is a story about a remedy that seemed to work long before anyone knew why, and a salt that later made the fix routine. For centuries, people treated swollen necks with seaweed and sponge ash. In 1811, Bernard Courtois noticed a violet vapor rising from burning seaweed ash, and that vapor proved to come from a new element, iodine. In 1813, the Swiss physician Coindet argued that seaweed worked because of its iodine ([PubMed](https://pubmed.ncbi.nlm.nih.gov/18936198/); [DOI](https://doi.org/10.1093/jn/138.11.2060)). Iodine matters for a reason that is still current. The thyroid gland needs it to make its hormones, and when iodine runs short, the gland enlarges into a goiter. ## In this article - Seaweed was the remedy before iodine had a name - Courtois found the element, and Coindet tied it to goiter - Salt turned a treatment into a public health tool - Thyroxine showed why the thyroid needs iodine - Too much iodine has a record, and pregnancy raises the target ## Seaweed was the remedy before iodine had a name Seaweed shows up in goiter history centuries before the element was named. A [2021 review of goiter in traditional Chinese medicine](https://pubmed.ncbi.nlm.nih.gov/34061810/) describes seaweed decoction in ancient Chinese literature ([DOI](https://doi.org/10.1097/JCMA.0000000000000547)). A [2022 review in *Hormone Research in Paediatrics*](https://pubmed.ncbi.nlm.nih.gov/36446327/) dates the seaweed remedy to 2697 BCE, when, in its account, a figure traditionally known as the Yellow Emperor described it. Treat that date as the review's account, not a documented record. A [2008 history in French](https://pubmed.ncbi.nlm.nih.gov/19579547/) says seaweed and sponge ashes were prescribed in ancient times. Until the 19th century, according to a [2014 review by Michael Weissel](https://pubmed.ncbi.nlm.nih.gov/24652010/), the working knowledge was that something in food, especially seaweed, kept goiters from developing ([DOI](https://doi.org/10.1007/s00508-014-0515-7)). The remedy rested on that observation, not on a known ingredient. The [vitamin C history](/blog/history/history-of-vitamin-c-from-scurvy-to-ascorbic-acid) follows a similar route, from a disease to a named nutrient. ## Courtois found the element, and Coindet tied it to goiter The element came from the same kind of ash. According to the [2008 review in *The Journal of Nutrition*](https://pubmed.ncbi.nlm.nih.gov/18936198/), Courtois saw the violet vapor in 1811, and Gay-Lussac later identified it as iodine, a new element. The surprise is that the ash people had prescribed for goiter held the element that researchers would soon link to it. Coindet also treated goitrous patients with iodine itself. Two decades later, the French chemist Boussingault, working in the Andes, was the first to advocate salt with iodine to prevent goiter. Another French chemist, Chatin, proposed in 1851 that iodine deficiency caused goiter. In 1896, Baumann and Roos found iodine in the thyroid gland itself ([DOI](https://doi.org/10.1093/jn/138.11.2060)). ## Salt turned a treatment into a public health tool The prevention plan came from a survey of schoolgirls in Akron, Ohio. In 1917, David Marine and O. P. Kimball surveyed thyroid enlargement in grades 5 through 12 and proposed a plan of prevention. The title carries the 1917 date, and [PubMed lists a 1990 reprint](https://pubmed.ncbi.nlm.nih.gov/2405081/). A [2022 review](https://pubmed.ncbi.nlm.nih.gov/36446327/) credits Marine's studies with showing that iodine treatment prevented endemic goiter, and says that work led to salt iodination. Switzerland's iodized salt program has run without interruption since 1922, according to the [2008 review](https://pubmed.ncbi.nlm.nih.gov/18936198/). In the United States, salt makers have added iodine to table salt since the 1920s, although the practice is still voluntary ([NIH ODS](https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/)). Today the NIH says salt iodization has dramatically reduced iodine deficiency worldwide, and that about 88% of households worldwide use iodized salt. Other mineral supplements followed their own routes, as the [calcium history](/blog/history/history-of-calcium-supplements-from-chalk-to-bone-trials) and the [iron history](/blog/history/history-of-iron-supplements-and-the-ferritin-test) show. ## Thyroxine showed why the thyroid needs iodine Edward Kendall discovered thyroxine on Christmas Day in 1914, according to the [2022 review](https://pubmed.ncbi.nlm.nih.gov/36446327/). Its synthesis and correct structure followed about 10 years later, and T3, the most biologically active thyroid hormone, was detected in the 1950s ([2014 review](https://pubmed.ncbi.nlm.nih.gov/24652010/)). The NIH explains the chain. Iodine is an essential component of thyroxine (T4) and triiodothyronine (T3). Thyroid-stimulating hormone raises the thyroid's uptake of iodine, and when iodine is short, TSH stays elevated and the gland enlarges. That enlarged gland, or goiter, reflects the body's attempt to trap more iodine from the circulation ([NIH ODS](https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/)). The NIH also calls iodine deficiency the most common cause of preventable intellectual disability in the world. It describes thyroid hormones as critical determinants of metabolic activity, so for the cell-level energy side of that picture, see our [mitochondrial health guide](/health/mitochondrial-health-complete-guide). ## Too much iodine has its own record A 1948 paper by Wolff and Chaikoff is titled "Plasma inorganic iodide as a homeostatic regulator of thyroid function," and [PubMed indexes it](https://pubmed.ncbi.nlm.nih.gov/18865621/). PubMed has no abstract for it, so this page cites the title only. The modern case record is more specific about harm. A [2023 case report](https://pubmed.ncbi.nlm.nih.gov/37273337/) describes the Jod-Basedow phenomenon, also called iodine-induced hyperthyroidism, in which excess iodine increases hormone synthesis and, without auto-regulation, can lead to thyrotoxicosis ([DOI](https://doi.org/10.7759/cureus.38415)). A [2018 case report](https://pubmed.ncbi.nlm.nih.gov/29466241/) describes a 17-year-old with an intermittent goiter who had taken an iodine supplement at least seven times the RDA for five years. Methimazole did not control the hyperthyroidism over six weeks, and the thyroid tests normalized after steroid therapy ([DOI](https://doi.org/10.1515/jpem-2017-0485)). ## Pregnancy raises the iodine target The NIH says the RDA rises from 150 to 220 mcg a day in pregnancy, and to 290 mcg while breastfeeding. Surveys indicate that many pregnant women in the United States might not get enough iodine. The NIH says the effect of that shortfall on fetal development is not known. The American Thyroid Association recommends 150 mcg a day, as potassium iodide, for women who are planning a pregnancy, pregnant or breastfeeding ([NIH ODS](https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/)). ## Bottom line Treat iodized salt and supplements as two sources to add up, not two separate decisions. Read the supplement label for the form and the amount, and keep the total under the NIH's 1,100 mcg adult upper limit unless a clinician sets a different target. If you are pregnant, planning a pregnancy, breastfeeding or taking antithyroid medication, check with a clinician first, because the right amount depends on those facts. ## Frequently asked questions ### Why did ancient healers use seaweed for swollen necks? Because it seemed to work, and nobody could yet say why. A [2021 review](https://pubmed.ncbi.nlm.nih.gov/34061810/) describes seaweed decoctions in ancient Chinese literature, and a [2008 history](https://pubmed.ncbi.nlm.nih.gov/19579547/) says seaweed and sponge ashes were prescribed in ancient times. Until the 19th century, according to a [2014 review](https://pubmed.ncbi.nlm.nih.gov/24652010/), the working knowledge was that something in food, especially seaweed, kept goiters from developing. Iodine did not surface in the record until 1811. ### Who first used iodine as a treatment, and when? The earliest iodine treatment in the sources we checked is Coindet's. In 1813 he argued that seaweed worked because of its iodine, and he treated goitrous patients with iodine, according to the [2008 review in *The Journal of Nutrition*](https://pubmed.ncbi.nlm.nih.gov/18936198/). That work built on Courtois's 1811 observation of a violet vapor from seaweed ash, which Gay-Lussac later identified as iodine. The next step, a proposal to use iodine-rich salt, came about two decades later. ### How did iodized salt change goiter rates? The NIH says salt iodization programs have dramatically reduced the prevalence of iodine deficiency worldwide, and that about 88% of households worldwide use iodized salt. Switzerland's program has run without interruption since 1922, according to the [2008 review](https://pubmed.ncbi.nlm.nih.gov/18936198/). A [2022 review](https://pubmed.ncbi.nlm.nih.gov/36446327/) credits Marine's studies with the move to salt iodination. Insufficiency still persists in Southeast Asia, sub-Saharan Africa and Eastern Europe, so the gains are large but uneven. ### Why is iodine a supplement topic in pregnancy? The need rises. The NIH says the RDA goes from 150 to 220 mcg a day in pregnancy, and it notes that iodine deficiency during pregnancy and early infancy can cause irreversible effects. Surveys indicate that many U.S. pregnant women might not get enough iodine, although the effect of that shortfall on fetal development is not known. The American Thyroid Association recommends 150 mcg a day, as potassium iodide, for women planning a pregnancy, pregnant or breastfeeding. A clinician can help decide whether a supplement fits your situation. ### How does iodine connect to thyroid hormones? Iodine is an essential component of thyroxine (T4) and triiodothyronine (T3), according to the NIH Office of Dietary Supplements. Thyroid-stimulating hormone from the pituitary raises the thyroid's uptake of iodine and drives the release of both hormones. When iodine runs short, TSH stays elevated and the gland enlarges into a goiter. Thyroxine was discovered in 1914, and T3 was detected in the 1950s, according to a [2022 review](https://pubmed.ncbi.nlm.nih.gov/36446327/) and a [2014 review](https://pubmed.ncbi.nlm.nih.gov/24652010/). ### Which iodine forms show up in salt and supplements? In U.S. iodized salt, the FDA has approved potassium iodide and cuprous iodide for iodization. The WHO recommends potassium iodate because it holds up better in warm, damp or tropical climates, according to the NIH. Supplements often use potassium iodide or sodium iodide, and some use kelp, a seaweed that contains iodine. In a small study, people absorbed potassium iodide almost completely, at 96.4 percent. The label gives the form and the amount, and the amount is what counts against the 1,100 mcg adult limit. ### What did early iodine excess reports teach? The reports we could verify are modern case reports, not early studies, but they point the same way. Excess iodine can increase thyroid hormone production. A [2023 case report](https://pubmed.ncbi.nlm.nih.gov/37273337/) describes the Jod-Basedow phenomenon as iodine-induced hyperthyroidism, and a [2018 case report](https://pubmed.ncbi.nlm.nih.gov/29466241/) describes a 17-year-old whose supplement, at seven times the RDA for five years, contributed to hyperthyroidism that resisted methimazole. The NIH sets the adult upper limit at 1,100 mcg a day. ### Related reading - [The potassium origin story, from wood ash to supplement](/blog/history/history-of-potassium-from-ash-to-supplement) - [The selenium origin story, another trace mineral with a narrow margin](/blog/history/history-of-selenium-from-poison-to-keshan-disease)