Scientists warn persistent thyroid imbalance during pregnancy raises autism risk
Two large studies suggest that untreated or persistent maternal thyroid imbalance during pregnancy is linked to higher autism risk in children, underscoring the importance of monitoring thyroid health in expectant mothers.
As more children are diagnosed with autism, many people continue to ask what causes it.
Autism, also known as autism spectrum disorder (ASD), affects how a person communicates, interacts with others and behaves, often involving repetitive actions or particular interests. Research indicates that a wide range of factors may contribute to autism.
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A recent study suggests that untreated thyroid hormone imbalance during pregnancy may increase the likelihood of a child developing autism.
Thyroid dysfunction — a condition in which the thyroid gland produces either too little or too much hormone — may raise the chances of giving birth to a child with autism.
Thyroid hormone imbalance in pregnancy occurs when a woman’s thyroid gland does not produce the correct levels of hormones (T3 and T4). These hormones are essential because they support the mother’s metabolism and the baby’s growth and brain development.
When thyroid hormone levels are too low (hypothyroidism) or too high (hyperthyroidism) during pregnancy, complications may arise, including miscarriage, preterm birth, low birth weight and developmental challenges for the baby.
Untreated hypothyroidism, in particular, has been linked in some studies to neurodevelopmental conditions in children, including an increased risk of autism.
Subtle symptoms
Because symptoms of thyroid imbalance can be subtle — such as fatigue, weight changes, mood swings or changes in heart rate — many cases may go unnoticed without routine blood tests.
The recent study found that the longer a mother’s thyroid problems continued, the higher the child’s risk became. For example, children born to mothers with low thyroid hormone levels throughout all three trimesters had more than three times the likelihood of being diagnosed with autism. However, when thyroid imbalances were effectively managed during pregnancy, the increased risk was not significant.
The study, published in The Journal of Clinical Endocrinology & Metabolism, examined more than 51,000 singleton pregnancies between 2011 and 2017 and tracked the children’s health until 2021.
Researchers focused on how thyroid hormone levels changed over time during pregnancy and whether prolonged imbalance increased the chance of a child later being diagnosed with autism.
According to the findings, around 8.6 per cent of the mothers had some type of thyroid hormone abnormality either before or during pregnancy. When a woman already had a diagnosed thyroid condition before conception — meaning it was typically monitored and treated — the researchers found no significant rise in autism risk for her child.
Abnormal thyroid hormone levels
However, the situation differed when thyroid hormone levels remained abnormal during pregnancy. When pre-existing thyroid dysfunction continued into pregnancy, or when thyroid imbalance persisted across multiple trimesters, the risk of autism increased considerably. In these cases, the adjusted hazard ratio reached 2.61, indicating more than double the risk compared with pregnancies in which thyroid levels stayed normal.
The researchers noted a stepwise rise in risk: a modest increase when the imbalance occurred in one trimester, a greater increase when it continued into two trimesters, and the highest risk when thyroid levels were abnormal throughout all three trimesters, with an adjusted hazard ratio of 3.25. This pattern suggests that the duration of imbalance plays an important role in neurodevelopmental outcomes.
These findings highlight that the key issue is not simply having a thyroid disorder, but whether thyroid hormone levels remain unbalanced during pregnancy. Well-managed thyroid conditions before pregnancy do not appear to increase autism risk, but unrecognised, untreated or poorly controlled thyroid dysfunction during gestation may carry higher risks.
Another study, titled “Maternal Thyroid Disorders and Risk of Autism Spectrum Disorder in Progeny” and published in the Epidemiology journal, analysed data from 437,222 singleton births between 1999 and 2013. The children were followed through 2016 to identify cases of ASD.
Researchers examined whether mothers had documented thyroid conditions, such as hypothyroidism or hyperthyroidism, at any time, and whether these conditions were linked to a higher risk of ASD in their children. When available, they also reviewed gestational thyroid hormone measurements and maternal use of thyroid medication during pregnancy.
The study found that children of mothers who had ever experienced hypothyroidism — before or during pregnancy — had a significantly higher risk of ASD compared with children of mothers without hypothyroidism. The adjusted odds ratio was 1.26, with a 95 per cent confidence interval of 1.12 to 1.42. The association with maternal hyperthyroidism was less consistent but still suggested an elevated risk.
Interestingly, these associations were not explained by measured thyroid hormone levels in pregnancy, nor reduced by the use of thyroid medications. In other words, simply having a thyroid condition at any point in life was linked to increased autism risk, regardless of monitoring or treatment during gestation.
Other factors
These findings imply that the increased risk of ASD may not be caused solely by hormone levels during pregnancy.
Other factors affecting maternal thyroid function, such as autoimmune processes or broader biological pathways, may influence neurodevelopment in ways that standard thyroid treatments cannot fully address.
This suggests a more complex relationship between maternal thyroid health and autism risk than a straightforward “low thyroid hormone → developmental risk” explanation.
Overall, the study shows that maternal thyroid disorders, even when occurring outside pregnancy or treated, are associated with a modest but meaningful increase in autism risk.
It also raises important questions about the biological mechanisms linking thyroid health and foetal brain development, highlighting the need for further research into immune and other systemic pathways.
Exact cause remains unclear
The exact cause of autism remains unclear. While genetics play a major role, environmental factors may also contribute. This combination makes autism a complex condition, and ongoing uncertainty has fuelled public debate.
The rising number of autism diagnoses has added to the discussion. Some people interpret the increase as evidence of a growing “epidemic”, while experts argue that better awareness, wider diagnostic criteria and improved screening account for much of the rise. This has led to disagreements about whether autism is genuinely becoming more common or simply more frequently recognised.
According to the World Health Organisation (WHO), around one in every 127 people worldwide was estimated to have autism in 2021. A global study in 2024 estimated that approximately 61.8 million people — about 0.79 per cent of the world’s population — were living with ASD. The study also assessed the overall health impact of autism, showing that it remains a significant public health issue.
In Kenya, a study of young people aged 15 to 25 found that around 0.63 per cent had high autistic traits based on a screening tool, while broader community surveys placed the figure slightly higher at 0.98 per cent.
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