Authors: Stephanie M. Engel; Gro D. Villanger; Amy Herring; Rachel C. Nethery; Samantha S. M. Drover; R. Thomas Zoeller; Helle M. Meltzer; Pal Zeiner; Gun Peggy Knudsen; Ted Reichborn-Kjennerud; Matthew P. Longnecker; Heidi Aase · Research
How Do Maternal Thyroid Hormone Levels During Pregnancy Affect a Child's Risk of ADHD?
Researchers found that both high and low maternal thyroid hormone levels during pregnancy may increase a child's risk of developing ADHD.
Source: Engel, S. M., Villanger, G. D., Herring, A., Nethery, R. C., Drover, S. S., Zoeller, R. T., Meltzer, H. M., Zeiner, P., Knudsen, G. P., Reichborn-Kjennerud, T., Longnecker, M. P., & Aase, H. (2022). Gestational thyroid hormone concentrations and risk of attention-deficit hyperactivity disorder in the Norwegian Mother, Father and Child Cohort Study. Paediatric and Perinatal Epidemiology, 00, 1– 11. https://doi.org/10.1111/ppe.12941
What you need to know
- Both high and low levels of maternal thyroid hormones during pregnancy were associated with increased risk of ADHD in children
- High levels of the thyroid hormone T3 were linked to higher ADHD risk
- Low iodine intake during pregnancy may increase the impact of thyroid hormone imbalances on ADHD risk
- The findings suggest that maintaining optimal thyroid function during pregnancy is important for child brain development
The importance of thyroid hormones during pregnancy
Thyroid hormones play a crucial role in fetal brain development, especially during the first half of pregnancy when the fetus relies entirely on maternal thyroid hormones. While severe thyroid dysfunction in pregnant women has been linked to adverse effects on child neurodevelopment, less is known about how more subtle variations in thyroid hormone levels may impact conditions like attention-deficit/hyperactivity disorder (ADHD).
ADHD is one of the most common neurodevelopmental disorders in children, affecting approximately 5% of children worldwide. While genetic factors play a major role, environmental influences during pregnancy may also contribute to ADHD risk. Researchers have been investigating whether maternal thyroid function during pregnancy could be one such factor.
How the study was conducted
To explore this question, researchers conducted a nested case-control study within the Norwegian Mother, Father and Child Cohort Study (MoBa). They examined 298 children diagnosed with ADHD and 554 control children without ADHD.
The researchers measured several markers of thyroid function in blood samples collected from mothers at around 17 weeks of pregnancy:
- Thyroid stimulating hormone (TSH)
- Thyroxine (T4)
- Triiodothyronine (T3)
- Free T4 index (FT4i)
- Free T3 index (FT3i)
- Thyroid peroxidase antibodies (TPO-Ab)
They then used statistical analyses to look for associations between these thyroid markers and the child’s risk of being diagnosed with ADHD later in life.
Key findings on thyroid hormones and ADHD risk
The study revealed several interesting relationships between maternal thyroid function during pregnancy and a child’s risk of developing ADHD:
High T3 levels linked to increased ADHD risk
Mothers with the highest levels of T3 (in the top 20% of measurements) had more than twice the odds of having a child diagnosed with ADHD compared to those with the lowest T3 levels. A similar pattern was seen for FT3i.
Both high and low T4/FT4i associated with ADHD
For T4 and FT4i, the researchers found a “U-shaped” relationship with ADHD risk. This means that both low and high levels were associated with increased odds of ADHD in children. Specifically, mothers with T4 or FT4i levels in the lowest 20% or highest 20% had about 1.6 times higher odds of having a child with ADHD compared to those with mid-range levels.
TSH and TPO-Ab not associated with ADHD
The study did not find significant associations between maternal TSH levels or TPO-Ab positivity and child ADHD risk.
Ratio of T3 to T4 may be important
The researchers also looked at the ratio of FT3i to FT4i, which can indicate how efficiently the body is converting T4 to the more active T3 form. They found that a higher FT3i/FT4i ratio was associated with increased ADHD risk, with the highest ratio group having about twice the odds of ADHD compared to the lowest.
The role of iodine intake
Iodine is an essential component of thyroid hormones, and the researchers found that maternal iodine intake from diet appeared to modify some of the relationships between thyroid hormones and ADHD risk.
For FT4i, the increased ADHD risk associated with both low and high levels was mainly seen in mothers with low iodine intake (less than 150 μg per day). This suggests that adequate iodine intake during pregnancy may help buffer against the effects of thyroid hormone imbalances on ADHD risk.
What do these findings mean?
This study provides evidence that both high and low maternal thyroid hormone levels during pregnancy may increase a child’s risk of developing ADHD, even when those levels fall within the range considered clinically normal.
Dr. Stephanie Engel, the study’s lead author, explains: “Our results suggest that optimal maternal thyroid function during pregnancy is important for fetal brain development and may play a role in ADHD risk. However, it’s important to note that the overall risk is still relatively small, and many other factors also contribute to ADHD.”
The findings highlight the potential importance of maintaining balanced thyroid hormone levels during pregnancy. They also suggest that ensuring adequate iodine intake may help reduce risks associated with thyroid imbalances.
Limitations and future research
While this study provides valuable insights, there are some limitations to consider:
- Thyroid hormones were only measured once during pregnancy, so changes over time couldn’t be assessed.
- The study can show associations but can’t prove that thyroid imbalances directly cause ADHD.
- The results may not apply to all populations, as iodine intake and other factors can vary between countries.
Future research could address these limitations by measuring thyroid function multiple times throughout pregnancy and in different populations. Studies that follow children from before birth through adolescence could also help clarify the long-term effects of maternal thyroid function on child neurodevelopment.
Conclusions
- Both high and low maternal thyroid hormone levels during pregnancy may increase a child’s risk of developing ADHD.
- Maintaining optimal thyroid function and adequate iodine intake during pregnancy could be important for reducing ADHD risk.
- More research is needed to fully understand the relationship between maternal thyroid function and child neurodevelopment.
This study adds to growing evidence that subtle variations in maternal thyroid function during pregnancy can have lasting effects on child brain development. While more research is needed, the findings underscore the importance of proper thyroid function and nutrition during pregnancy for supporting optimal child health and development.