Genetic Evidence Linking Thyroid Hormones to Dementia and Parkinson’s Disease
Endocrine regulation is increasingly recognised as a central determinant of brain ageing and vulnerability to neurodegenerative disease. Recent Mendelian randomisation analyses published in Scientific Reports have demonstrated that thyroid function exerts causal effects on both cognitive performance and neurodegenerative outcomes. The data revealed that elevated thyroid-stimulating hormone (TSH) levels reduce the risk of Parkinson’s disease, while genetic predisposition to hypothyroidism is linked with a lower risk of Alzheimer’s disease and dementia with Lewy bodies. These associations underscore the relevance of pituitary and thyroid signalling in modulating dopaminergic and cortical hippocampal circuits, pathways that are critically involved in memory and executive function. By contrast, no causal link was observed between thyroid cancer or radiotherapy exposure and dementia, despite clinical evidence of treatment-related cognitive decline.
From the perspective of ageing neuroscience, the implications are considerable. Prospective memory, supported by hippocampal and prefrontal networks, represents a sensitive early marker of cognitive decline. If thyroid status influences these networks, then hormones traditionally viewed as peripheral regulators of metabolism may in fact play an important role in shaping cognitive ageing trajectories. This aligns with evidence that thyroid hormones regulate neurogenesis, synaptic plasticity, and glucose metabolism within the hippocampus. Dysregulation of these mechanisms could accelerate decline or, conversely, contribute to protection depending on endocrine balance and feedback adaptation.
These endocrine findings intersect with broader advances in the genetics and epigenetics of ageing. Research on ageing plasticity has highlighted that lifespan and cognitive resilience are not fixed, but modifiable by both genetic variants and epigenetic programmes. Cellular senescence, telomere shortening, and genomic instability represent core mechanisms that progressively erode tissue repair and neural function. Epigenetic modifications, such as altered histone methylation, DNA methylation patterns and changes in chromatin structure, are now recognised as major drivers of ageing, shaping transcriptional programmes in ways that influence neurodegenerative risk. The concept of the epigenetic clock, based on DNA methylation signatures, provides a measurable index of biological age and can even be reversed under certain interventions including caloric restriction, epigenetic reprogramming and the use of extracellular vesicles.
Integrating these strands of evidence suggests that thyroid signalling should be considered alongside canonical hallmarks of ageing. The thyroid axis not only interacts with nutrient sensing pathways such as insulin and IGF-1 or mTOR, both of which are genetically linked to longevity, but may also modulate epigenetic stability in neuronal circuits. The identification of APOE as a key regulator of heterochromatin and cellular senescenceprovides a further example of how endocrine, genetic and epigenetic factors converge in shaping neurodegenerative risk.
Overall, the evidence indicates that the biology of ageing cannot be understood without considering the interaction between systemic endocrine signals and intrinsic cellular mechanisms. Thyroid hormones appear to be important modulators of neurocognitive resilience, while genetic and epigenetic research shows that ageing is a dynamic and plastic process rather than a fixed trajectory. The convergence of these perspectives opens opportunities for biomarker discovery and targeted intervention with the aim of delaying dementia onset, preserving memory and extending healthy lifespan.
References
Wang Y, Liu Y, Yang H, Mo Z, Liu H, Yu Q. Genetic evidence suggests a causal relationship linking thyroid function to prospective memory and dementia and Parkinson’s disease. Sci Rep. 2025;15:20425. doi: 10.1038/s41598-025-01596-w
Zhang J, Wang S, Liu B. New insights into the genetics and epigenetics of aging plasticity. Genes. 2023;14(2):329. doi: 10.3390/genes14020329
