Maternal
thyroid hormones have dynamic trophic actions during the perinatal development
in particular the developing brain and myelination process. It is widely known
that the appropriate myelinated growth, at postnatal day 25 in the rat, is
responsible for protection and insulation of axons and is vital for the
function of Central Nervous System (CNS), in particular the learning and memory
function. Alternatively, there is a link between the hypothyroidism and the
vulnerability of the CNS to inflammatory diseases by the thymus or spleen (cells
of the immune system). In maternal hypothyroidism, the dysfunction in the
thymic selection increases the accumulation of autoimmune T cells and the risk
of autoimmune-inflammatory disorders in the offspring such as multiple
sclerosis.
Multiple
sclerosis is a long-lasting, neuroinflammatory demyelinating dysfunction of the
CNS that mostly disturbs young adults. The etiology of multiple sclerosis might
be due to a disturbance in the genetic process, immunological (autoimmune
disorders), infectious, or environmental factors. Symptoms of multiple
sclerosis during pregnancy are urinary insistence, fatigue, lower extremities
paresthesias and gait difficulties. On the other hand, deficiency in the levels
of maternal THs during the gestation can decrease the growth and
differentiation of myelinated axons and cause permanent defects in the
developing CNS including a mental retardation and cognitive disturbances. In
mild-moderate iodine insufficiency, maternal and neonatal hypothyroxinemia, a
low circulating free T4 with no change in free T3 or thyroid stimulating
hormone (TSH), can disrupt the levels of nuclear Myelin Binding Protein (MBP)
and increase the apoptosis causing a reduction in the cellular survival. More
importantly, Wei et al. reported that hypothyroxinemia due to the maternal mild
iodine decreases the expression of myelin-related proteins and delays the
growth of neonatal myelination. Notably, a reduction in the levels of
gestational THs can increase the severity of multiple sclerosis.
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