Autism oldie: transient in utero hypothyroxinemia: environmental factors - critical periods

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Notice the categories of environmental factors, including cruciferous

vegetables:

Autism: transient in utero hypothyroxinemia related to maternal

flavonoid ingestion during pregnancy and to other environmental

antithyroid agents. <http://www.ncbi.nlm.nih.gov/pubmed/17651757>

Román GC.

J Neurol Sci. 2007 Nov 15;262(1-2):15-26.

The incidence and prevalence of autism have increased during the past

two decades. Despite comprehensive genetic studies the cause of autism

remains unknown. This review emphasizes the potential importance of

environmental factors in its causation. Alterations of cortical neuronal

migration and cerebellar Purkinje cells have been observed in autism.

Neuronal migration, via reelin regulation, requires triiodothyronine

(T3) produced by deiodination of thyroxine (T4) by fetal brain

deiodinases. Experimental animal models have shown that */transient

intrauterine deficits of thyroid hormones (as brief as 3 days) result in

permanent alterations of cerebral cortical architecture reminiscent of

those observed in brains of patients with autism/*. I postulate that

early maternal hypothyroxinemia resulting in low T3 in the fetal brain

during the period of neuronal cell migration (weeks 8-12 of pregnancy)

may produce morphological brain changes leading to autism. Insufficient

dietary iodine intake and a number of environmental antithyroid and

goitrogenic agents can affect maternal thyroid function during pregnancy.

The most common causes could include inhibition of deiodinases D2 or D3

from maternal ingestion of dietary flavonoids or from antithyroid

environmental contaminants.

Some plant isoflavonoids have profound effects on thyroid hormones and

on the hypothalamus-pituitary axis. Genistein and daidzein from soy

(Glycine max) inhibit thyroperoxidase that catalyzes iodination and

thyroid hormone biosynthesis. Other plants with hypothyroid effects

include pearl millet (Pennisetum glaucum) and fonio millet (Digitaria

exilis); thiocyanate is found in Brassicae plants including cabbage,

cauliflower, kale, rutabaga, and kohlrabi, as well as in tropical plants

such as cassava, lima beans, linseed, bamboo shoots, and sweet potatoes.

Tobacco smoke is also a source of thiocyanate. Environmental

contaminants interfere with thyroid function including 60% of all

herbicides, in particular 2,4-dichlorophenoxyacetic acid (2,4-D),

acetochlor, aminotriazole, amitrole, bromoxynil, pendamethalin,

mancozeb, and thioureas.

Other antithyroid agents include polychlorinated biphenyls (PCBs),

perchlorates, mercury, and coal derivatives such as resorcinol,

phthalates, and anthracenes.

A leading ecological study in Texas has correlated higher rates of

autism in school districts affected by large environmental releases of

mercury from industrial sources. Mercury is a well known antithyroid

substance causing inhibition of deiodinases and thyroid peroxidase.

*/The current surge of autism could be related to transient maternal

hypothyroxinemia resulting from dietary and/or environmental exposure to

antithyroid agents/*.

Additional multidisciplinary epidemiological studies will be required to

confirm this environmental hypothesis of autism.