December 8, 2023
Hyperthyroidism, an overactive thyroid gland, occurs in about one in five hundred women. It has been tied to adverse effects in both mother and fetus, including pre-eclampsia (a condition in pregnancy characterized by high blood pressure, sometimes with fluid retention and excessive protein in the urine, which can indicate kidney damage), preterm delivery, heart failure, and in uteri retardation of growth.
In hypothyroidism, on the other hand, thyroid activity is abnormally low, which retards growth and mental development. It is particularly common in regions with widespread iodine deficiency. Depending on the region, it affects one in three hundred to one in thirty women. Maternal hypothyroidism is associated with an increased risk of pre-eclampsia, premature separation of the placenta from the wall of the uterus, miscarriage, in uteri growth retardation, and fetal death.
The fetus relies on maternal thyroid hormones until its own thyroid function initiates halfway through pregnancy. As we have just seen, this direct link in the early stages of pregnancy has serious consequences described above. Does it also affect the risk of developing ADHD in offspring?
A team of researchers based in Hong Kong reformed a comprehensive search of the peer-reviewed medical literature on this subject. It then conducted two meta-analyses, one examining maternal hyperthyroidism during pregnancy, the other on maternal hypothyroidism.
The meta-analysis for maternal hyperthyroidism during pregnancy combined two nationwide cohort studies with a total of over 3.1 million persons, using the Danish and Norwegian medical registries. It found a slight but significant association with ADHD in offspring.
The meta-analysis for maternal hypothyroidism during pregnancy included the same two nationwide cohort studies, plus an Israeli nationwide cohort study (along with a tiny U.S. cohort study), with a total of over 3.4 million persons. It likewise found a slight but significant association with ADHD in offspring.
Though the component studies did some assessment of confounders, the authors of the meta-analyses noted, "By including a more comprehensive range of confounding factors and biologically relevant covariate (e.g. thyroxine treatment), future studies are warranted to re-visit the association between maternal thyroid dysfunction and various health outcomes in offspring."
Grace Mengqin Ge, Miriam T. Y. Leung, Kenneth K. C. Man, Wing Cheong Leung, Patrick Ip, GloriaH.Y. Li, Ian C. K. Wong, Annie W.C. Kung, Ching-Lung Cheung, "Maternal thyroid dysfunction during pregnancy and the risk of adverse outcomes in the offspring: a systematic review and meta-analysis," The Journal of Clinical Endocrinology& Metabolism (2020), https://doi.org/10.1210/clinem/dgaa555.