Chronic iodine excess tied to hypothyroidism
Chronic exposure to high levels of iodine, either in salt or in water, increases the risk of developing hypothyroidism, a new meta-analysis reveals.
PubMed, Ovid Medline and the Cochrane Central Register of Controlled Trials were accessed for the current meta-analysis. Observational studies and intervention trials that investigated the effects of excess iodine on thyroid diseases were included, whereas those that investigated unnatural sources of iodine (ie, medication and radiation) were excluded. The quality of the studies were assessed using the Cochrane Handbook for Systematic Reviews of Intervention.
After applying the inclusion and exclusion criteria, 50 studies were found eligible for inclusion. Of these, meta-analysis was only possible for the 35 cross-sectional studies and only for cases of subclinical hypothyroidism (SCH) in children, adults and pregnant women, and of overt hypothyroidism (OH) in adults.
Adult studies showed that the cases and the odds ratio (OR) of SCH were significantly greater in the group that received excess iodine than in those that received adequate levels. The corresponding ORs for SCH and OH were 2.03 (95 percent CI, 1.58 to 2.62) and 2.78 (1.47 to 5.27).
Studies in children and in pregnant women showed high levels of heterogeneity and thus did not yield reliable conclusions. Furthermore, there was only one study eligible for inclusion that investigated hyperthyroidism.
The number of included intervention trials, case-control and follow-up studies was too small, and this prevented integration of their results in the meta-analysis.
Overall, the present data highlight the lack of and need for high-quality studies investigating the relationship between iodine intake and thyroid diseases.