Much recent attention has focused on elucidating the effects of iodine deficiency on neurodevelopmental impairment
[13, 14]. However, the effects of iodine excess should not be overlooked. We investigated whether and how a 3-fold increase in the physiological dose of iodine in rat would affect brain development of their offspring. The rat brain at PN7 is considered developmentally equivalent to the human brain at birth
. Rat brain development at PN21 approximates the late toddler stage of humans, and PN45 equates to the teenager stage
. We selected PN7 and PN45 as two crucial time points in the brain development of the rat to explore the effect of maternal excessive iodine intake on the neurodevelopment and cognitive function in their offspring. Our results show for the first time that there were changes in BDNF and NSP-A expression and neurodevelopmental impairment in rat pups whose mothers had a 3-fold higher-than-normal intake of iodine prior to pregnancy and throughout gestation and breastfeeding.
Iodine intake levels that are either lower or higher than the recommended range are associated with an increase in the incidence of thyroid disease in China
. Both thyroid hormone deficiency and excess have detrimental effects on the expression of neuronal proteins during neurodevelopmental stages
Iodine deficiency is the most common cause of hypothyroidism. The results of some studies regarding excess iodine suggested that high iodine intake was associated with increased incidence of hypothyroidism
[19, 20], while others found that it led to hyperthyroidism
[21, 22]. These different results may be related to the duration of high iodine exposure. The incidence of hyperthyroidism after the initiation of the Universal Salt Iodization (USI) program began to rise at about 6 months, peaked at 1–3 years and returned to the baseline within 3–10 years
[23–25]. Iodine-induced thyrotoxicosis lasting the first few years after the implementation of iodine supplementation was reported in Lesotho
 and Poland
. However our epidemiological study demonstrated that there was no difference in either the prevalence or the incidence of hyperthyroidism among mildly deficient, more than adequate, and excessive iodine intake areas (MUI 84, 243, and 651 μg/L, respectively). We found that more than adequate and excessive iodine intake could increase the prevalence of subclinical hypothyroidism
. The USI policy had been implemented for 3 to 8 years in China, since 1996, when we conducted the epidemiological study from 1999 to 2004. Therefore we did not obtain epidemiological data for the early stage of iodine supplementation.
In the present animal study, after 12 weeks of treatment (pre-pregnancy), the serum TT4 and FT4 levels were significantly higher in the adult female rats of the 3HI group compared with those of the NI group. On G17, the TT4 concentration of the adult females remained the higher level in the 3HI group compared with that of the NI. Although no significant changes were found in the TSH levels on pre-pregnancy and G17 in the 3HI rats compared with those of the NI, there were descending trends in the 3HI group.
Before the onset of foetal thyroid function in both humans (10–12 weeks gestation) and rats (G17), early foetal brain development is completely dependent on maternal thyroxine (T4) supply
. Even after the foetal thyroid has begun to secrete hormone, as much as 17.5% of foetal T4 derives from transplacental transfer
. Recent studies have reported that excess thyroid hormone could impair foetal brain development and affect the neurological outcome of rat offspring
[30, 31]. Excessive iodine intake has been shown to inhibit foetal thyroid function and lead to iodine-induced neonatal hypothyroidism in Asian populations
. Our previous study
 found that excessive iodine intake increased the risk of subclinical hypothyroidism in offspring. Furthermore, excessive iodine content from breast milk caused subclinical hypothyroidism in preterm infants
. These studies were consistent with our finding in the present study that maternal iodine excess caused subclinical hypothyroidism of rat pups on PN7, although the thyroid hormone levels of pups in the 3HI group were within the normal range on PN45. Our results indicated that maternal thyroid hormone disruption during sensitive periods of brain development caused by mildly maternal iodine excess could lead to thyroid dysfunction of pups and cause neurodevelopmental defects in offspring.
The proteins c-Fos and c-Jun in the nuclei of neurons are involved in neuroplastic mechanisms and neuronal differentiation, and also have important roles in memory formation and consolidation
[8, 35]. In our study, c-Fos and c-Jun expression in the hippocampal CA1 area of rat pups was not significantly affected by the 3-fold high iodine intake of their mothers during gestation. It is possible that compensatory mechanisms restored c-Fos and c-Jun to normal levels in spite of maternal thyroid dysfunction. Another possibility is that c-Fos and c-Jun were not involved in the neurodevelopmental impairment of pups from the 3HI group.
BDNF, a neurotrophin protein, has significant influence on crucial processes of brain development, including neurogenesis, neuronal differentiation, synaptogenesis, memory formation, and consolidation
[36–39]. Previous reports have documented that maternal thyroid dysfunction could affect the expression of BDNF and cause neurological defects in neonates
 and adult rats
. Our previous studies
[10, 41] reported that maternal subclinical hypothyroidism decreased BDNF expression in rat pup hippocampi and impaired spatial learning; pups required more time during the Morris water maze test to find the hidden platform, compared with pups from NI mothers. The present study found that BDNF protein levels were decreased in the hippocampi of the pups from both the 3HI and LI mothers. On PN45, the BDNF expression in the 3HI pups remained lower than control pups, although circulating levels of thyroid hormone had fully recovered. Our study indicated that a persistently lower BDNF level may contribute to the adverse effect of 3HI on the developing brain. However, the impairment was mild compared with hypothyroidism induced by iodine deficiency.
NSP-A is known to be an important mediator of thyroid hormone effects during brain development and is involved in neuronal differentiation and axonal guidance
. Dowling et al.[12, 43] showed that the expression of NSP-A was regulated by thyroid hormone, and the expression of NSP-A was selectively affected by maternal hypothyroidism in the proliferative zone of the fetal rat brain cortex. Our data also found that the expression of NSP-A in the hippocampus on PN7 was affected by mildly maternal excessive iodine intake. The abnormal expression of NSP-A in the neonatal brain appears to be related to the neurodevelopmental impairment in the pups of the 3HI group, but the impairment was less severe than pups in the LI group.
In this study, spatial learning ability and memory of the rat pups were assessed using the MWM test on PN40-44. Pups in the LI group required more time to find the hidden platform, compared with pups from NI mothers. The severe impairment of spatial and learning ability was associated with the thyroid dysfunction and abnormal levels of proteins related to neurodevelopment, which were not reversed by PN45 in LI group, and the neurodevelopmental impairment thus appeared to be permanent. The mean escape latency of 3HI pups was longer than for the NI control pups, and the difference was significant on the third day (PN42; P < 0.05). Our results suggested that the offspring in the 3HI group may have had a mildly impaired learning capacity, which could be associated with a decrease in BDNF and an increase in NSP-A levels. The mild spatial learning and memory impairment was temporary, which was consistent with the recovery of NSP-A expression on PN45, and the long-term effects of mildly maternal excessive iodine intake on neurodevelopment and cognitive function need to be investigated in our future study.