Impact of iodine supplementation in mild-to-moderate iodine deficiency

Although the detrimental effects of severe iodine deficiency are well recognised the benefits of correcting mild-to-moderate iodine deficiency are uncertain. Taylor, Okosieme, Dayan and Lazarus undertook a systematic review of the impact of iodine supplementation in populations with mild-moderate iodine deficiency.

Impact of iodine supplementation in mild-to-moderate iodine deficiency: systematic review and meta-analysis
Peter N Taylor, Onyebuchi E Okosieme, Colin M Dayan, John H Lazarus

Interesting quotes

In a recent study from the Netherlands low maternal urinary iodine during pregnancy was associated with impaired executive functioning in children at 4 years of age.

Low urinary iodine excretion during early pregnancy is associated with alterations in executive functioning in children, The Generation R Study
Nina H van Mil et al

The advantages of correcting mild iodine deficiency will no doubt be less dramatic than for severe deficiency but substantial returns in productivity and reduced health care costs will still be made.

A major concern with iodine supplementation has been the risk of iodine induced thyroid dysfunction. We found no evidence of an excess of thyroid dysfunction in the controlled iodine intervention trials in pregnancy. Furthermore, the incidence of postpartum thyroid dysfunction observed in these trials was not higher than published rates in the general population.

Epidemiological studies however show that sharp increases in iodine intake in severely iodine deficient populations may precipitate hyperthyroidism especially in elderly individuals with longstanding thyroid autonomy. Less striking manifestations are reported in marginally iodine deficient areas or where iodine prophylaxis has been gradually introduced.

[...] transient increases in the incidence of hyperthyroidism were recorded in the aftermath of iodisation but with reversal to baseline rates occurring within years of iodisation.

[...] increases in the occurrence of thyroid dysfunction or autoimmunity in the wake of iodisation. In addition the prevalence of both TPOAb and TgAb (albeit low titre) was higher 4:5 years after cautious iodine fortification of salt was introduced in Denmark, particularly in young women.

These population level increases in the adult incidence of autoimmune thyroiditis thus seem an inevitable byproduct of iodisation but should not deter future efforts at iodisation as the potential adverse effects of iodine deficiency on child development far outweighs the risk of correctable hypothyroidism in adults.

Long-term exposure to excessive iodine from water is associated with thyroid dysfunction in children
Sang Z, Chen W, Shen J, Tan L, Zhao N, Liu H, Wen S, Wei W, Zhang G, Zhang W

Thyroid function and serum lipids of adults living in areas of excessive iodine in water in Hebei province
Li H, Sang Z, Tan L, Zhao N, Wei W, Zhang G, Liu H, Wen S, Zhang W

However salt may not suffice as the sole vehicle of iodisation in some countries. Retail outlet surveys conducted in the United Kingdom for example showed that most commercial salt brands lacked adequate iodine and iodised salt was unlikely to contribute substantially to overall iodine nutrition. In addition recent successful public health campaigns aimed at preventing cardiovascular disease through reduced salt consumption may have instigated further reductions in population iodine intake.

However, a recent WHO forum has indicated that strategies to reduce salt intake and increase iodine fortification should not necessarily be contradictory and such strategies could support each other [...]

We therefore agree that whilst awaiting results from current trials of iodine supplementation in pregnancy, pregnant and breastfeeding women should be offered iodine supplementation [...]

For instance some authors suggest that self reported supplement intake in excess of 150 µg daily is associated with impaired foetal neurodevelopment. This is a concern given that the adaptive mechanisms to counteract the thyroid inhibitory actions of an acute iodide load, or Wolff-Chaikoff effect, do not fully develop in the foetus until late gestation.


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