Iodine

Iodine is not a psychoactive substance in any traditional sense. If your iodine levels are already adequate, taking a supplement will produce no noticeable subjective effect whatsoever. Your body will simply excrete the excess through urine. This is an important distinction from substances where "more equals more" -- with iodine, supplementation is only meaningful if you are actually deficient.

For people who are deficient, however, the experience of correction can be surprisingly noticeable. The most commonly reported change is a reduction in anxiety, sometimes described as a background hum of tension that quietly disappears over days. People also describe improved mental clarity, as if a persistent fog has lifted, along with increased physical energy and reduced sensitivity to cold. Some report that their skin and hair improve over weeks. These changes typically emerge within the first one to two weeks of consistent supplementation at RDA levels (150 mcg/day) and continue to stabilize over a month or two.

Community discussions on Reddit, particularly in r/Supplements and r/vegan, contain hundreds of reports following this pattern. The experiences are not universal -- not every anxious vegan has an iodine problem -- but the subset of people who were genuinely deficient tend to describe the correction in dramatic terms. The key variable is whether you were actually deficient in the first place, which is why a urinary iodine test or thyroid panel (TSH, free T4) is worth getting before attributing symptoms to iodine or expecting supplementation to help.

Iodine is actively concentrated by the thyroid gland through the sodium-iodide symporter (NIS), achieving intracellular concentrations 20-50 times higher than plasma. Inside thyroid follicular cells, iodide is oxidized by thyroid peroxidase (TPO) and incorporated into tyrosine residues on thyroglobulin (organification), forming monoiodotyrosine (MIT) and diiodotyrosine (DIT). Coupling of two DIT molecules produces thyroxine (T4, the prohormone), while coupling of MIT and DIT produces triiodothyronine (T3, the active hormone).

T3 binds to nuclear thyroid hormone receptors (TRalpha and TRbeta), which function as ligand-activated transcription factors regulating the expression of hundreds of genes. In the developing brain, thyroid hormones are essential for: neuronal proliferation and migration, oligodendrocyte differentiation and myelination, dendritic and axonal growth, synaptogenesis, and neurotransmitter system maturation.

Severe iodine deficiency during pregnancy causes cretinism (profound intellectual disability, deaf-mutism, spasticity). Even mild-to-moderate deficiency reduces IQ by 8-13.5 points on average in affected populations, making iodine deficiency the world's leading preventable cause of brain damage.

In 1811, iodine was discovered by French chemist Bernard Courtois, who was born to a family of manufacturers of saltpetre (an essential component of gunpowder). At the time of the Napoleonic Wars, saltpetre was in great demand in France. Saltpetre produced from French nitre beds required sodium carbonate, which could be isolated from seaweed collected on the coasts of Normandy and Brittany. To isolate the sodium carbonate, seaweed was burned and the ashes washed with water. While investigating the cause of corrosion to the copper vessels used in the process, Courtois added an excess of sulfuric acid to the waste remaining and a cloud of violet vapour arose. He noted that the vapour crystallised on cold surfaces, forming dark crystals. Courtois suspected that this material was a new element but lacked funding to pursue it further.

Courtois gave samples to his friends, Charles Bernard Desormes (1777–1838) and Nicolas Clément (1779–1841), to continue research. He also gave some of the substance to chemist Joseph Louis Gay-Lussac (1778–1850), and to physicist André-Marie Ampère (1775–1836). On 29 November 1813, Desormes and Clément made Courtois' discovery public by describing the substance to a meeting of the Imperial Institut de France. On 6 December 1813, Gay-Lussac found and announced that the new substance was either an element or a compound of oxygen and he found that it is an element. Gay-Lussac suggested the name "iode" (anglicised as "iodine"), from the Ancient Greek Ιώδης (iodēs, "violet"), because of the colour of iodine vapour. Ampère had given some of his sample to British chemist Humphry Davy (1778–1829), who experimented on the substance and noted its similarity to chlorine and also found it as an element. Davy sent a letter dated 10 December to the Royal Society stating that he had identified a new element called iodine. Arguments erupted between Davy and Gay-Lussac over who identified iodine first, but both scientists found that both of them identified iodine first and also knew that Courtois is the first one to isolate the element.

In 1873, the French medical researcher Casimir Davaine (1812–1882) discovered the antiseptic action of iodine. Antonio Grossich (1849–1926), an Istrian-born surgeon, was among the first to use sterilisation of the operative field. In 1908, he introduced tincture of iodine as a way to rapidly sterilise the human skin in the surgical field.

In early periodic tables, iodine was often given the symbol J, for Jod, its name in German; in German texts, J is still frequently used in place of I.