Thyroid Hormone, Endocrine Disruptors, & Fetal Grey Matter Volume

A long-term study out of the Netherlands found fetal brain development was most highly linked to the mother’s levels of thyroid stimulating hormone (TSH) early in pregnancy.¹

During the first half of pregnancy, a mother’s thyroid levels are crucial to her baby’s brain development.² Thyroid hormones are important to every cell in the body, regulating how they develop and metabolize proteins, fats, and carbs. During the first trimester a developing baby is wholly depending on their mother as a source of thyroid hormones, which are transferred via the placenta.

Starting in the pituitary gland in the brain, the thyroid-stimulating hormone (TSH) does exactly what its name suggests: it stimulates the production of the two thyroid hormones, triiodothyronine (T₃) and it’s prohormone thyroxine (T₄). Besides metabolism, thyroid hormones are necessary for normal brain development, where it is instrumental in creation and migration of neurons, differentiation of neurons and glial cells (the most abundant cell type in your nervous system).³ These hormones are so important that even moderate dysfunction of maternal thyroid levels can affect neurodevelopment of a child.²

While various studies have shown a child’s brain is susceptible to thyroid dysregulation during the first half of pregnancy, the exact window of time where they were most vulnerable was previously unknown.

The researchers in Rotterdam, Netherlands looked at nearly 2000 mother-child pairs, comparing maternal gestational TSH and free thyroxine (FT₄) samples taking at 18 weeks or earlier and MRI data of the child at 10 years old.

They found that the mother’s TSH levels were associated with both the volume of total grey matter and the volume of cortical grey matter. This association was the strongest at 8 weeks, but after about 14 weeks the maternal TSH level did not really impact the child’s brain morphology any more.

Also, they found this was not just a matter of thyroid hormone insufficiency that resulted in reduced grey matter volume; both high and low maternal thyroid function negatively affected total volume in the child.

In the brain there are two types of tissues – grey matter and white matter, generally the former on top of the latter – so named because of their colors when looked at with the naked eye. A neuron looks like a bunch of trees (dendrites) all sprouting from the same spot, with one very long tail (axon) attached. Grey matter is made up of the actual cell bodies of the neurons where all the trees are, while the white matter is generally composed of the myelinated axon that makes up the tail protruding the cell body. While the white matter is responsible for relaying the electrical messages from one neuron to another, grey matter is important in processing the input from our senses, memory, intellect, and emotional control.

Knowing the time point during which maternal thyroid hormones can affect fetal brain development is important in risk mitigation as we learn more and more about how endocrine-disrupting compounds (EDCs) disrupt our body’s hormone levels.

Thyroid hormones are particularly susceptible to EDCs because they have iodine in their molecular structure. Iodine belongs to a family of elements called halogens, which also includes fluorine (F), chlorine (Cl), and bromine (Br). Because families of elements share similar chemical and physical properties, EDCs that include members of the halogen family are able to “mimic natural [thyroid hormones] and thereby interact with multiple aspects of hormone production, feedback, distribution, entry into cells, intracellular metabolism…” and consequently affect your thyroid hormone levels their roles in the body.²

Here is a list of EDCs that can affect your thyroid hormones:

• Perchlorate
• Phenols
  • Bisphenol A
  • Triclosan (TCS, 2,4,4-trichloro-hydroxy diphenyl ether)
• Flavanoids
• Pesticides
  • DDT (Dichlorodiphenyltrichloroethane)
  • HCB (Hexachlorobenzene)
  • Chlorpyrifos (CBF)
• Polychlorinated biphenyls (PCBs)
• Polybrominated flame retardants
• Phthalates (DEHP, DBP, MBP, MEHP, BBP, DIDP)
• Perfluorinated Compounds (PFCs)

Sources of these EDCs and how they disrupt thyroid hormone and fetal brain development: