Iodine: Clinical Use

Increased iodine intake can be achieved through dietary modification and supplementation with tablets. Dietary modification usually refers to increased intake of iodised salt, but may also refer to use of iodised water, iodised vegetable oil or seafood.

TREATMENT AND PREVENTION OF DEFICIENCY

Iodine deficiency is accepted as the most common cause of brain damage worldwide, with IDD affecting 740 million people. Although it is well accepted that severe deficiency is responsible, evidence is now emerging that mild deficiency during pregnancy is also important and can have subtle effects on brain development, lowering intellectual functioning and inducing psychomotor deficits in early childhood. Preliminary data are also emerging to suggest an association between iodine deficiency hypothyroidism of pregnancy and the incidence of ADHD in the offspring; however, this still requires confirmation in larger studies.

PREGNANCY

Severe iodine deficiency is uncommon in Western countries, such as Australia and New Zealand, but several local surveys have identified that mild to moderate deficiency is more prevalent than once thought. A research group at Monash Medical Centre in Melbourne screened 802 pregnant women and found that 48.4% of Caucasian women had urinary iodine excretion (UIE) concentrations below 50 µg/L compared to 38.4% of Vietnamese women and 40.8% of Indian/Sri Lankan women. These figures are disturbing when the WHO defines healthy urinary iodine excretion levels as greater than 100 µg/L, mild deficiency is diagnosed at 51-100 µg/L and moderate to severe deficiency at <50 µg/L. A study conducted at a Sydney hospital involving 81 women attending a ‘high’ risk clinic found moderate to severe iodine deficiency in 18.8% of subjects and mild iodine deficiency in another 29.6%, the former clearly too close to the WHO maximum acceptable level of 20%. This study also revealed that almost 5% of the sample had UIE <25 µg/L

Based on such results it may well be expected that endemic cretinism could emerge, and that it has not yet occurred in Australia may be due to a low to moderate intake of goitrogens and adequate selenium levels.

INFANTS

Results of a study investigating TSH levels in a group of infants within 72 hours of birth at the Royal North Shore Hospital in Sydney suggest that endemic IDD may be emerging. Currently, the WHO recommends that less than 3% of newborns should have TSH levels greater than 5 mlU/Land of the 1773 infants enrolled in the study, 5-10% had a TSH reading >5 mlU/L, which is clearly outside WHO recommendations and provides evidence of insufficient iodine levels during pregnancy.

CHILDREN AND ADOLESCENTS

Evidence of iodine deficiency is not limited to pregnant women and newborns and has also been demonstrated in Australian schoolchildren. Iodine status in schoolchildren is based on median urinary iodine excretion values and is categorised as normal (UIE >100 µg/L)orasmild (UIE 50-99 µg/L), moderate (UIE 20-49 µg/L) and severe deficiency (UIE <20 µg/L). The UIE is considered in combination with the child’s sex, year of school and presence of goitre.

A study of Melbourne schoolchildren aged 11-18 years found that 76% (439/577) had abnormal urinary iodine excretion values, with 27% (156/577) having values consistent with moderate-severe iodine deficiency. The median UIE value in girls was lower than that in boys (64 µg/Lvs 82 µg/L), and girls had significantly lower urinary iodine excretion values overall (P < 0.002). A study of 324 schoolchildren aged 5-13 years from the Central Coast of New South Wales produced similar results and there was a median UIE concentration of 82 µg/L, with 14% of children having levels below 50 µg/L.

More recently, these findings were confirmed in the Australian National Iodine Nutrition Study, which identified inadequate iodine intake in the Australian population and called for the urgent implementation of mandatory iodisation of all edible salt in Australia. The study consisted of a survey of 1709 schoolchildren aged 8-10 years in the five mainland Australian States and was conducted between July 2003 and December 2004. It found that, overall, children in mainland Australia are borderline iodine deficient, with a national median urinary iodine excretion of 104 µg/L. On a State basis, children in Victoria and New South Wales are mildly iodine deficient, with median UIE levels of 89 µg/Land 73.5 µg/L, respectively, South Australian children are borderline iodine deficient, with a median urinary iodine excretion of 101 µg/L, whereas both Queensland and Western Australian children are iodine sufficient, with median UIE levels of 136.5 µg/L and 142.5 µg/L, respectively. Researchers attributed the decline in iodine intake to changes within the dairy industry, with chlorine-containing sanitisers now replacing iodine-containing sanitisers and decreased intake of iodised salt.

In 2001 an iodine supplementation program was initiated in Tasmania because it was identified as an area of endemic goitre by the Department of Health Services. The programme involves the use of iodised salt in 80% of Tasmania’s bread production and aims to reduce the incidence of iodine deficiency. Despite encouraging preliminary data, iodine levels are still inadequate according to the WHO standards. There have been conflicting opinions about the success of this programme, with the largest study demonstrating evidence of ongoing iodine deficiency.

Iodine deficiency in children and adolescents is associated with poorer school performance, reduced achievement motivation and a higher incidence of learning disabilities. A meta-analysis of 18 studies from 8 countries of people aged between 2 and 30 years showed that iodine deficiency alone reduced mean IQ scores by 13.5 points in children. From both a public health and a socioeconomic perspective, these findings have significant repercussions.

ADULTS

A study of non-pregnant adults in 1999 demonstrated iodine deficiency in 25.3% of ‘healthy’ subjects and 34.1% of diabetic subjects.

NON-TOXIC GOITRE THYROIDECTOMY

One 12 month study involving 139 patients who had undergone thyroidectomy for non-toxic goitre identified that supplementing L-thyroxine therapy with iodised salt produced significant improvements in thyroid function compared with stand alone L-thyroxine therapy.