Is going to the seaside good for your thyroid? Here’s what we know

The idea that a holiday by the sea can benefit thyroid function has no solid scientific basis, but it stems from certain real facts. First of all, it is true that seawater, for the very reason that it is salty, also contains a higher concentration of iodine than that found in fresh water. Rivers flowing into the sea carry with them minerals eroded from rocks, including iodine, and the evaporation of the water under the sun’s rays increases its concentration. The droplets of seawater vaporised into the air – particularly when waves crash violently against the rocks – therefore also contain this mineral.

On the other hand, it is also true that the thyroid gland requires an adequate supply of iodine to produce its hormones, triiodothyronine (T3) and thyroxine (T4). These hormones are vital in the womb and during the early years of life, particularly for the development of the central nervous system, but they are also essential for adults in maintaining a healthy metabolic balance and for the health of hair, nails, skin and teeth. In inland areas, where water tends to be lower in this mineral, local populations are likely to suffer from iodine deficiency. In the most severe cases, this leads to enlargement of the gland and the formation of what is known as a ‘goitre’, which, in this specific instance, precisely because it is common and caused by a widespread environmental factor, is referred to as ‘endemic goitre’.

By combining these two observations – namely, the common deficiency disorder in inland areas and the higher iodine content in seawater – it was believed that stays on the coast allowed people to inhale an extra supply of this mineral, which is essential for thyroid function. In reality, this was not the case. The improvement that used to be observed when people accustomed to living in the mountains travelled to the seaside can instead be explained by the fact that, on the coast, it is easier to include fish, shellfish and other seafood in one’s diet – dietary sources of iodine that were scarcely available at the time to those living in inland areas.

We now know, in fact, that only a minimal amount of iodine can be absorbed through inhalation – by ‘breathing in sea air’ – which is insufficient to make up for any deficiency of this substance. The only situation in which proximity to the sea might, at least in theory, provide iodine through inhalation is the presence of large piles of seaweed on the shore. These, in fact, release gaseous iodine, and some studies have found higher concentrations of iodine in the urine of people living near these areas, even compared with others living on the coast but far from such deposits.

In fact, even so, it is estimated that the amount of iodine that can be inhaled in a day whilst breathing near seaweed remains negligible (1–20 μg/day) compared with the recommended daily intake of 150 μg/day. The most important factor in ensuring an adequate intake of iodine remains diet, whether through the consumption of fish, shellfish or, to an even greater extent, seaweed – where this is served as part of a meal – and, above all, through the use of iodised salt, which is recommended for everyone regardless of where they live.