Thyroid Hormone: Implications For Neurodevelopment
by Patricia Lemer, M. Ed.
The effects of thyroid hormone on the growing brain and on later cognitive functioning are well-documented. It is clear that thyroid hormone is essential to normal behavioral and intellectual development.
Thyroid hormone inﬂuences the development of synapses, dendrites and myelination of the central nervous system (CNS). Doctors know that congenital hypothyroidism (low thyroid), if not treated, can result in mental retardation. They are only now recognizing, however, that mild to moderate impairment of thyroid function in a pregnant mother may adversely affect brain development, as well as cause subtle to severe intellectual and behavioral abnormalities, such as learning disabilities, attention deﬁcits and possibly certain pervasive developmental disorders such as autism.
Precise amounts of the hormone appear to be of vital importance for CNS development during three critical periods. In phase 1 (conception to 12 weeks) the source of thyroid hormones is maternal and is transported through the placenta. If the supply is insufﬁcient, the brainstem and brain development are affected. In phase 2 (12 to 40 weeks gestation), neurodevelopment is inﬂuenced by both maternal and fetal thyroid hormone. It is during this exquisitely sensitive period that nerves and their dendrites and synapses form. Even the smallest degree of insufﬁciency could cause subtle problems. During phase 3 (birth to 2 years), the child‘s own thyroid gland takes over, as the CNS matures.
Complicating this scenario is increasing evidence that exposure to environmental toxins can impair normal thyroid function. Certain synthetic compounds including polychlorinated biphenyls (PCBs), dioxins and dioxin-like compounds (DLCs) have been proven to be damaging. It is well-documented in both Japan and Taiwan that exposing children to these toxins during critical developmental stages impairs learning, memory and attention. Other studies suggest that exposing pregnant mothers to these synthetic chemicals during pregnancy may adversely affect their offspring‘s intelligence and behavior.
While some neurodevelopmental effects are temporary, others persist into adulthood. Reported long-term effects include impairment of cognitive functioning, altered neuromotor and sexual behavior, and reduced thyroid hormone levels.
A parent who learned about thyroid ﬁrst hand is Audrey McMahon, a founder of the Learning Disabilities Association (LDA) and the past chairperson of its Research Committee. She knew something was wrong with her son, born in 1951, long before a great deal was known about developmental problems. Audrey had been taking thyroid supplements on and off, according to medical direction, since the age of 13. After researching her family‘s history, Audrey deduced that her brother‘s delays were related to her mother‘s hypothyroidism, and that her great uncle was also probably affected. Her son is thus the third generation of offspring from a family of women with underactive thyroids.
McMahon‘s indefatigable search for answers ﬁnally led to the diagnosis of her son‘s Asperger‘s syndrome in the early ‘80s. She discovered that this syndrome had only been recognized in Europe, and not yet in the USA. She was later instrumental in the inclusion of Asperger‘s syndrome as a classiﬁcation in the DSM-IV. However, this did not take place until 1994! In 1995, she arranged a pre- conference symposium for the LDA on "Thyroid Function and Learning Disabilities." Speakers included Drs. Hauser, Patricia Rodier, Reed Larsen and Joanne Rovet.
Audrey‘s family and others are now part of a research study. Strong evidence suggests that hypothyroidism plays an important role in the birth of their ﬁrst-born sons with delays. Research is expanding on ﬁnding markers for Asperger‘s and other developmental disorders. A few years ago, the American Thyroid Association had only a handful of researchers interested in pre-natal brain development. Last year 18 of their grant proposals mentioned fetal brain development. In 1998, ﬁve grants on CNS topics were funded. We are making progress to learn about the thyroid/ neurodevelopment correction.
Many of tomorrow‘s offspring are at risk. It is important for a woman to have her thyroid tested at least six months before getting pregnant. Make sure that levels are well into the normal range, and establish therapy, if needed, prior to conception. "An ounce of prevention is worth a pound of cure."
[Excerpted from "Resistance to Thyroid Hormone: Implications for Neurodevelopmental Research on the Effects of Thyroid Hormone Disrupters, " by Peter Hauser, J. Michael McMillin, and Vinod S. Bhatara, and from personal communication with Audrey McMahon. Initially published in New Developments: Volume 4, Number 1 - Summer, 1998]