Developmental Disorders Caused By Thyroid Dysfunction
There are a
number of developmental disorders that are caused by thyroid
dysfunction. Listed below are a number of scientific papers with
quotes, indicating the range of effects that can occur.
Interactions
of Persistent Environmental Organohalogens With The Thyroid Hormone
System: Mechanisms And Possible Consequences For Animal And Human
Health.
Brouwer A.,
Morse D. C., Lans M. C., Schur A. G., Murk A. J., Klasson_Wehler E.,
Bergman A., Visser T. J.
Toxicology
and Industrial Health 14 (1/2) 1998, 59-84.
Abstract
Several
classes of environmental contaminants have been claimed or suggested to
possess endocrine-disrupting potency, which may result in reproductive
problems and developmental disorders. In this paper the focus is on the
multiple and interactive mechanisms of interference of persistent
polyhalogenated aromatic hydrocarbons (PHAHs) and their metabolites
with the thyroid hormone system. Evidence suggests that pure congeners
or mixtures of PHAHs directly interfere with the thyroid gland; with
thyroid hormone metabolising enzymes, such as
uridine-diphosphate-glucuronyl transferases (UGTs), iodothyronine
deiodinases (IDs), and sulfotransferases (SULTs) in liver and brain;
and with the plasma transport system of thyroid hormones and
experimental animals and their offspring. Changes in thyroid hormone
levels in conjunction with high PHAH exposure was also observed in
captive as well as free ranging wildlife species and in humans.
Maternal exposure to PHAHs during pregnancy resulted in considerable
fetal transfer of hydroxylated PHAHs, which are known to compete with
thyroxine (T4) for plasma transthyretin (TTR) binding sites, and thus
may be transported to the fetus with those carrier proteins that
normally mediate the delivery of T4 to the fetus. Concomitant changes
in thyroid hormone concentrations in plasma and in brain tissue were
observed in fetal and neonatal stages of development, when sufficient
thyroid hormone levels are esential for normal brain development.
Alterations in structural and functional neurochemical parameters, such
as glial fibrillary acidic protein (GFAP), synaptophysin, calcineurin,
and serotonergic neurotransmitters, were observed in the same offspring
up to postnatal day 90. In addition, some changes in locomotor and
cognitive indices of behaviour were observed in rat offspring,
following i utero and lactational exposure to PHAHs. Alterations in
thyroid hormone levels and subtle changes in neurobehavioural
performance were also observed in human infants exposed in utero and
through lactation to relatively high levels of PHAHs. Overall these
studies indicate that persistent PHAHs can disrupt the thyroid hormone
system at a multitude of interation sites, which may have a profound
impact on normal brain development in experimental animals, wildlife
species, and human infants.
Quotes
Many
classes of chemicals, both natural and of anthropogenic origin, have
been claimed to possess endocrine-disrupting potential, in particular
on the sex steroid systems. In addition, these classes of "endocrine
disruptors" have been linked with abnormalities in sexual development,
gonadal functions, and reproduction in wildlife and in humans.
...there
also exists a wide body of evidence for interations of similar classes
of chemicals with the thyroid hormone system. Thyroid hormones, as well
as sex steroids, are very important in normal structural and functional
development of sexual organs and the brain. ...Therefore it is
important to include potential disruptions of the thyroid system in the
evaluations of the possible impact of chemicals in our food and
environment on health and normal development.
Hypothyroidism
and Brain Development
Thyroid
hormones are known to be very important for normal development.
Disruption of the thyroid hormone system during pregnancy may have
serious consequences for structural and functional aspects of normal
development of the brain and sexual organs, among other things.
Hypothyroidism is known to lead to disorders of neuronal process growth
in the developing brain
Another
aspect of hypothyroidism may be an interference or disruption of
neurotransmitter systems. Thyroid hormones are shown to be modulators
of nonadrenergic, serotonergic, and dopaminergic receptor function.
...hypothyroidism
is associated with emotional instability, which may actually be a
consequence of changes in the serotonergic neurotransmitter system.
Other behavioural and functional consequences of developmental
hypothyroidism involve retarded development of cognitive and neuromotor
funtions.
Resistance
To Thyroid Hormone: Implications For Neurodevelopmental Research On The
Effects Of Thyroid Hormone Disruptors.
Hauser P.,
McMillin J. M., Bhatara V. S.
Toxicology
and Industrial Health 14 (1/2) 1998, 85-101.
Abstract
Thyroid
hormones are essential for normal behavioural, intellectual, and
neurological development. Congenital hypothyroidism, if not treated,
can result in irreversible mental retardation, whereas thyroid diseases
with more moderate impairment of thyroid function, such as resistance
to thyroid hormone, cause less severe intellectual and behavioural
abnormalities, including attention deficit hyperactivity disorder.
There is increasing evidence the exposure to certain synthetic
compounds, including dioxins and polychlorinated biphenyls (PCBs),
during the perinatal period can also impair learning, memory, and
attentional processes in offspring. Animal and human studies suggest
that exposure to these environmental toxicants impair normal thyroid
function. Although the precise mechanisms of action of the adverse
effects these toxicants have on neurodevelopment have not yet been
elucidated, it is possible that they are partially or predominantly
mediated by the alterations in hormone binding to the thyroid hormone
receptor. The convergence of studies that examine neurodevelopmental
consequences of moderate impairment of thyroid function, such as is
found in resistance to thyroid hormone, with those studies that
demonstrate the adverse behavioural and cognitive effects.
The
Importance Of Thyroid Hormone For Auditory Development In The Fetus And
Neonate.
Sohmer H.,
Freeman S.
Audiology
and Neuro-Otology 1 1996, 137-147.
Abstract
It seems
that many auditory maturational events are regulated by thyroid hormone
since elevation in thyroid hormone level always precedes the onset of
hearing in the fetus-neonate; low thyroid activity in the developing
human fetus or rat neonate leads to hearing loss; earlier, elevated
thyroid levels in rat neonate lead to earlier onset of hearing. The
hormone, bound to its receptors in the nucleus, acts as a transcription
factor activating genes which lead to the synthesis of several proteins
and enzymes involved in the structural and functional development of
many tissues (e.g. brain, heart, kidney, skeletal muscle) including the
ear. Several types of congenital hearing loss of unexplained etiology
may be due to abnormalities in one or more stages of this gene cascade
since several types of congenital hearing loss have been shown to
involve defects in genes related to these events.
Quotes
There is
accumulating evidence that an important factor in the programmed
development of auditory function in the fetus in utero is the presence
and level of thyroid hormone.
A hearing
loss may also be associated with a defect in the production of thyroid
hormone.
...it has
been suggested that this hormone functions as a biological timing
mechanism for the normal sequence of brain maturation. It has also been
shown that the rate of transcription ofthe myelin basic protein gene in
rat brain is regulated by thyroid hormone.
It is
likely that thyroid hormone is also involved in the regulation of the
structural development of the brain and the ear.
Hearing
Loss Related To Disorders In The Thyroid Hormone Cascade.
The overall
findings presented here show that just as thyroid hormone regulates and
is responsible for several aspects of the structural and functional
development of several tissues and organs including skeletal muscle,
heart, kidney and brain, it is likely that the hormone is also a key
factor in the regulation of development of the ear.
|
HOME
