A violet element from seaweed ash
In 1811 the Frenchman Bernard Courtois, making saltpetre for gunpowder, treated seaweed ash with acid and saw violet vapour rise; the discovery was published in 1813. Joseph Gay-Lussac showed it was a new element and named it from the Greek for violet. For centuries, East and West had treated goitre — swelling of the neck — with burnt seaweed or sponge, and in 1820 the Swiss physician Jean-François Coindet identified iodine as the active ingredient and used it against goitre. In 1915 Edward Kendall isolated crystals of thyroxine, the iodine-containing hormone of the thyroid.
Iodine deficiency and iodised salt
Thyroid hormones (T4 and T3) contain four or three iodine atoms, so lack of iodine means the gland cannot make them, causing hypothyroidism and goitre. Severe deficiency in pregnancy can permanently harm a baby's brain development, and iodine deficiency has been regarded as a leading preventable cause of intellectual disability. In 1917–20 David Marine and Oliver Kimball showed that giving iodine to schoolgirls in Ohio prevented goitre, and iodised salt went on sale in the United States in 1924. Since the 1990s WHO and UNICEF have promoted universal salt iodisation, greatly reducing the number of deficient countries, though pockets of deficiency remain.
How much, and radioactive iodine
WHO recommends 150 μg a day for adults and 250 μg for pregnant and breastfeeding women. In Korea and Japan, where seaweeds such as kelp and laver are popular, studies suggest excess intake is more common than deficiency; too much iodine can be linked to thyroid dysfunction or worsening hyperthyroidism. The usual view is that normal dietary seaweed need not be avoided outright, but people with thyroid disease should discuss intake with a clinician. In 1941–46 Saul Hertz pioneered radioactive iodine for hyperthyroidism, and it is still used for hyperthyroidism and some thyroid cancers. Potassium iodide tablets issued after nuclear accidents block radioactive iodine from building up in the thyroid and should be taken only when authorities advise. High-dose iodine supplements are not recommended without medical advice. This is general information, not a substitute for medical advice.
This article is general health information and is not a substitute for diagnosis or treatment. If symptoms persist, see a doctor.
FAQ
- Should I avoid seaweed if I have thyroid nodules?
- For most thyroid nodules or thyroid conditions, normal amounts of dietary seaweed need not be cut out completely. Some situations, such as before radioactive iodine therapy, require a low-iodine diet, so follow your doctor's instructions.
- Does antiseptic povidone-iodine contain iodine?
- Yes. Povidone-iodine is an iodine-based antiseptic. Prolonged use on large wounds, or heavy use in newborns or pregnancy, can lead to absorption, so use it with care.
Sources reviewed: 2026-10-26
References
- Courtois, B. (1813). Découverte d'une substance nouvelle dans le Vareck. Annales de Chimie, 88, 304–310.
- Gay-Lussac, J. L. (1814). Mémoire sur l'iode. Annales de Chimie, 91, 5–160.
- Coindet, J.-F. (1820). Découverte d'un nouveau remède contre le goître. Annales de Chimie et de Physique, 15, 49–59.
- Kendall, E. C. (1915). The isolation in crystalline form of the compound containing iodin, which occurs in the thyroid. JAMA, 64(25), 2042–2043.
- Marine, D., & Kimball, O. P. (1920). The prevention of simple goiter in man: a survey of the incidence and types of thyroid enlargements in the schoolgirls of Akron (Ohio). Archives of Internal Medicine, 25(6), 661–672.
- Hertz, S., & Roberts, A. (1946). Radioactive iodine in the study of thyroid physiology. VII. The use of radioactive iodine therapy in hyperthyroidism. JAMA, 131(2), 81–86.
- WHO, UNICEF & ICCIDD. (2007). Assessment of Iodine Deficiency Disorders and Monitoring Their Elimination: A Guide for Programme Managers (3rd ed.). Geneva: World Health Organization.
- Zimmermann, M. B., Jooste, P. L., & Pandav, C. S. (2008). Iodine-deficiency disorders. The Lancet, 372(9645), 1251–1262.
- Zimmermann, M. B. (2009). Iodine deficiency. Endocrine Reviews, 30(4), 376–408.