Biology Reference
In-Depth Information
time, it soon became evident that to be effective thyroid extract had to be
given to young children, as the effects of treating older cretins were delete-
rious. In the past few decades, the roles of TH in brain maturation have been
documented in detail. Most research concentrates on the most rapid period
of brain growth, that is, in mammals, the perinatal period (
Bernal, 2007;
Nunez, Celi, Ng, & Forrest, 2008
). However, data from numerous
epidemiological and clinical studies point to the possibility that earlier stages
of neuronal development might also be TH dependent (
de Escobar,
Obregon, & del Rey, 2004; Gyamfi, Wapner, & D'Alton, 2009;
Pharoah, Ellis, Ekins, &Williams, 1976; Pop et al., 1999
). For instance, both
radial glial and microglial cells are negatively affected by hypothyroid
conditions, indicating that TH may influence very early stages of brain de-
velopment (
Howdeshell, 2002
). Measuring maternal TH levels in humans
and assessing the neurodevelopmental scores of infants show that the most
marked neurological deficits are associated with low maternal TH levels
in the first trimester of pregnancy as opposed to the later stages (
Kooistra,
Crawford, van Baar, Brouwers, & Pop, 2006; Pop et al., 1999
). One of
the most important ideas to emerge is that the mother's TH levels prior
to the development of the fetal thyroid (
Obregon et al., 2007
) are critical
in determining her child's postnatal neuromotor development. Most
significantly, data from different groups examining maternal TH levels dur-
ing pregnancy show a child's intellectual quotient to better correlate with its
mother's thyroid status during the first trimester of pregnancy as opposed to
maternal levels in the second or third trimester (
de Escobar et al., 2004;
Henrichs et al., 2010; Li et al., 2011; Pop et al., 1999; Zimmermann,
2011
).
Henrichs et al. (2010)
reported that even slight maternal hypo-
thyroxinemia at 13 weeks of gestation was associated with an increased risk
of certain forms of
language delay, with greater
risk for
severe
hypothyroxinemia.
After birth, TH is equally (if not more) important, hence the current
testing for TH levels in newborns and, in many countries, the use of
iodine-supplemented salt. However, it should be noted that despite such
governmental actions, epidemiological studies show that in many countries
pregnant women and young children suffer from lack of iodine with poten-
tially deleterious effects on their TH levels (
Kutlu & Kara, 2012; Szybinski
et al., 2010
).
Experimentally addressing the question of the significance of TH signal-
ing in early mammalian embryos is experimentally challenging, implicating
surgery of the dam. In contrast, using aquatic embryos such as fish and
