Association between radiation dose, thyroid hormone, and IQ levels in children exposed to radiation in utero after the Chernobyl accident.

Few studies have explored the effects of n utero radiation exposure on human health and cognition and none have taken into account thyroid hormone levels (T3), which have shown to affect cognitive performance. We investigated mechanisms of possible radiation effects on IQ in two cohorts of 250 persons each: exposed n utero after the Chernobyl accident: a 'higher exposure group (HEG)', whose mothers resided in more heavily contaminated territories at the time of the Chernobyl accident, and a 'lesser exposure group (LEG)' whose mothers resided in less contaminated areas. The dataset included information on estimated prenatal thyroid radiation dose, gestation week at the time of the accident (ATA); thyroid hormones: T3 (triiodothyronine) and T4 (thyroxine) levels measured at age 11-12 years and general IQ measured at three time points: t1: 6-7 years old; t2: 11-12 years old and t3: 15-16 years old. Descriptive and inference analyses were used to explore the dynamic of changes through time and the associations between key variables at the three time points. Estimated radiation doses to the thyroid gland were substantially higher in the HEG than in the LEG (mean 391 vs 25 mGy respectively). Significant differences in thyroid hormones levels were observed between the two groups, with lower values in T3 (higher in T4) in the LEG. At t1, the general IQ, as well as verbal and non-verbal IQ scores, were lower in the HEG than in the LEG. In the HEG, analyses adjusting simultaneously for radiation dose, gestational week ATA and T3 levels suggest that all three variables are associated with IQ, with the latter being highest among those exposed later during gestation and decreasing with increasing level of dose and of T3. No significant association was observed between IQ and T4 levels. No effect of exposure on IQ was seen in the LEG. Further investigation of this hypothesis will be important to understand the relation between n utero exposure radiation dose to thyroid, thyroid hormone levels and IQ, taking into account effects of potential confounding factors (physiological stress, maternal anxiety related evacuation).