On May 25 each year, the world observes World Thyroid Day to create awareness about the small gland in the neck that governs growth, metabolism, and, most crucially, the developing brain. The theme for 2026 continues the global emphasis on early detection and equitable access to thyroid care, but there is a deeper story here that is worth telling. It is the story of iodine -- a trace but heavy element required for normal biological function -- the discovery and public health use of which, increased the intellectual quotient of humanity in the last century.

Wartime accident

The story begins not in a hospital but in the context of war. During the Napoleonic Wars (1803–1815), particularly under the British blockade (1806–1814), France faced shortages of potassium nitrate, an essential component of gunpowder. This led to a search for alternative chemical sources. Among those working in this setting was Bernard Courtois, who processed seaweed ash to obtain alkali used in nitrate production. In 1811, he observed violet vapours that later proved to be iodine. The substance did not serve a military role but later entered scientific study. The name “iodine” was subsequently given by Joseph Louis Gay-Lussac, from the Greek word ioeides, meaning violet-coloured. In 1813, Humphry Davy confirmed it as a new element.

In its early years, iodine had a mixed reputation. It was used in tinctures for wound care, but also feared for its toxicity in excess. In 1820, in Geneva, Jean-François Coindet was the first to notice patients using seaweed remedies for neck swelling. Suspecting iodine as the active substance, he proposed iodine solutions for shrinkage of goitres. But Jean’s incorrect dosage led to failure of treatment. Nearly a century later in 1914, it was Heinrich Hunzikar who correctly identified iodine as vital micronutrient that’s required in small amounts through diet and not as a “drug”, effectively decoding the role of iodine in goitre.

Geography of deficiency

Iodine is not uniformly distributed in nature. It is abundant in the sea (about 50 billion tons estimated), but scarce in inland soils, especially in mountainous regions where glaciation and flooding have washed it away. Crops grown in such soil carry little iodine. Populations living in areas far from the sea develop a chronic deficiency. This is why regions like the Himalayas (Kashmir to Arunachal Pradesh) have historically reported high rates of goitre and developmental delay.

The thyroid gland

The real breakthrough in understanding the thyroid gland came through a sequence of observations across centuries. In 1656, Thomas Wharton described the gland anatomically, but its function remained unknown. Then Theodor Kocher, through surgeries performed between 1872 and 1883, observed that complete removal of the thyroid led to a syndrome of physical and mental slowing, later termed myxoedema (awarded Nobel prize in 1909). In 1891, George R. Murray treated such patients with thyroid extract, demonstrating that the gland produced a vital internal secretion. Around 1895, Eugen Baumann showed that iodine was concentrated in thyroid tissue. Finally, in 1914, Edward Calvin Kendall isolated thyroxine, establishing the biochemical basis of thyroid function by discovering T3 (triiodothyronine) and T4 (thyroxine).

The T3 and T4 hormones are built around iodine molecules. Without iodine, the gland cannot function. The pituitary gland releases thyroid-stimulating hormone (TSH), which prompts the thyroid to produce T3 and T4. When levels are sufficient, a feedback loop suppresses further stimulation. In iodine deficiency, this system falters. The gland enlarges in an attempt to compensate, but hormone production remains inadequate. For a growing child, especially in the womb, this deficiency has lasting consequences. During the first six months of pregnancy, the foetus depends entirely on the mother’s thyroid hormones. If iodine is lacking, brain development is compromised even before birth, producing children with compromised intelligence. Iodine deficiency is identified as a preventable cause of intellectual disability. However, excess iodine intake can lead to thyroid dysfunction, including iodine-induced hyperthyroidism (Jod-Basedow phenomenon). Thus, iodine improves cognitive outcomes only when it corrects deficiency. Any additional intake leads to clinical harm.

Salt as a vehicle

The question then arose: how to deliver iodine to millions in a simple, sustainable way? The idea of using salt as a vehicle for iodine was strongly advanced by David Marine in the United States in 1917. Thus, salt emerged as the most practical medium for several reasons. It is consumed daily by nearly all individuals, regardless of income or geography, in relatively consistent quantities. The addition of iodine, typically as potassium iodate, does not significantly alter the taste, colour, or cooking properties of salt, ensuring acceptability. It is also inexpensive, making it suitable for population-wide programmes. These characteristics made salt iodisation a reliable method for achieving uniform iodine intake without requiring behavioural change or repeated medical interventions. In India, potassium iodate is preferred over potassium iodide because it is more stable in hot and humid conditions.

A quiet revolution

A 2005 paper by Qian revealed that maternal supplementation recovers up to 12 IQ points in children. By 2013, Bougma’s systematic review confirmed early childhood treatment prevents a 10-point intelligence loss, while Gordon 2009 study showed reasoning gains in school-aged children. Crucially, this cognitive rescue remains strictly time-sensitive; while adult supplementation effectively corrects thyroid health, it cannot reverse established deficits or produce measurable intelligence gains.

India’s own experience offers a powerful illustration from the Kangra Valley of Himachal Pradesh. High rates of goitre and developmental delay were documented in the 1950s. This led to one of the earliest large-scale RCTs on iodine supplementation. The results were transformative. Over time, goitre prevalence declined, and developmental outcomes improved. The Kangra study became a cornerstone in India’s decision to pursue universal iodisation.

Few interventions match iodisation in cost-effectiveness. For a minimal increase in salt costs, societies gain measurable improvements in cognition, productivity, and quality of life. Yet, the story is not complete. Even today, an estimated two billion people worldwide remain at risk of iodine deficiency despite 124/196 countries mandating iodisation of salts through legislation. In India, while coverage has improved, gaps persist in certain regions and among vulnerable populations.

The story of iodine, therefore, is serendipitous. What began as a useless by-product of war chemistry would increase the global intelligence level of humanity a century later though sustained public health efforts.

(Dr. C. Aravinda is an academic and public health physician. The views expressed are personal. aravindaaiimsjr10@hotmail.com)