The type 2 deiodinase Thr92Ala polymorphism and cognitive aging: current evidence and biological context

Abstract

Type 2 iodothyronine deiodinase (DIO2) plays a central role in regulating thyroid hormone (TH) metabolism in the brain by catalyzing the conversion of thyroxine (T4) to the active triiodothyronine (T3), thereby ensuring adequate local TH signaling in neural tissues. Among DIO2 genetic variants, the Thr92Ala (rs225014) polymorphism has been extensively investigated in relation to TH homeostasis and diverse clinical phenotypes. Accumulating evidence suggests a potential association between the Thr92Ala variant and cognitive aging; however, findings across studies remain inconsistent. Proposed mechanisms include altered local TH availability, mitochondrial dysfunction, oxidative stress, and cerebrovascular regulation, although the relative contribution of these pathways has not been fully clarified. Emerging data further indicate that the cognitive effects attributed to the Thr92Ala polymorphism may be context-dependent and influenced by factors such as thyroid functional status, sex, genetic background, and age. In this narrative review, we integrate molecular, physiological, and clinical evidence linking the DIO2 Thr92Ala polymorphism to cognitive aging, with particular emphasis on biological context and sources of heterogeneity that may underlie divergent findings across populations. Rather than providing a quantitative synthesis, we aim to contextualize existing data within the broader regulatory framework of TH signaling to better understand how DIO2-related variation may contribute to brain aging.

Plain language summary

The link between a DIO2 gene variant and cognitive aging

The enzyme Type 2 iodothyronine deiodinase (DIO2) is crucial for converting the inactive thyroid hormone (T4) into the active form (T3) required by brain cells. A common genetic variant in this enzyme, Thr92Ala, has been linked to cognitive decline in the elderly. This link is likely because the variant reduces DIO2 activity, leading to low local T3 levels in the brain. This could trigger mitochondrial dysfunction and increased oxidative stress, which may harm cognitive function. The influence of this variant may depend on genetic background, and more research is needed in Chinese populations. A deeper understanding of this mechanism could open new paths for early risk assessment and personalized interventions for age-related cognitive impairment.

Introduction

Cognitive dysfunction has emerged as a major challenge in aging populations worldwide. Cognitive impairment is defined as a decline in one or more cognitive domains, which, when progressive, can lead to dementia. ¹ Dementia is a heterogeneous neurological syndrome that can arise from various direct or indirect factors, ² with clinical manifestations including memory loss, language deficits, psychological and behavioral changes, executive dysfunction, and a progressive decline in the ability to perform activities of daily living. ³ Alzheimer’s disease (AD) represents the most common cause of dementia, accounting for approximately 50%–70% of cases. ³ Dementia predominantly affects older adults and poses a substantial global public health burden. It is estimated that approximately 50 million people worldwide are currently living with dementia, and this number is projected to nearly triple by 2050, according to the World Health Organization. ¹ Population-based studies from different regions have highlighted marked variation in dementia prevalence, disease subtypes, and economic burden, underscoring the influence of demographic, environmental, and biological factors on cognitive aging. For example, data from China indicate a large and rapidly growing dementia population, with substantial associated healthcare costs, reflecting broader global trends observed in aging societies.^2,4,5 Cognitive impairment imposes a significant economic and societal burden on patients, caregivers, and healthcare systems. Identifying biological pathways that contribute to individual vulnerability to cognitive aging is therefore of considerable clinical and scientific interest. Thyroid hormone (TH) signaling has emerged as one such pathway, given its essential role in brain development, metabolism, and neural function. In this context, the present narrative review aims to integrate molecular, physiological, and clinical evidence concerning the DIO2 Thr92Ala polymorphism and its potential relevance to cognitive aging. Particular attention is given to biological context and methodological heterogeneity that may underlie inconsistent findings in the literature.

The central importance of DIO2 in brain TH homeostasis

Physiological functions of DIO2

Iodothyronine deiodinases (DIOs) are essential for regulating local TH bioavailability by controlling the intracellular concentration of the active triiodothyronine (T3) hormone within specific tissues and cell types. The precise control of THs is paramount, given the fundamental role of T3 in regulating essential physiological processes throughout life, ranging from growth and development to overall metabolic rate. The deiodinase enzymes comprise three isoforms: type 1 iodothyronine deiodinase (DIO1), type 2 iodothyronine deiodinase (DIO2), and type 3 iodothyronine deiodinase (DIO3). They serve distinct functions in terms of tissue distribution, catalytic properties, and substrate specificity. ⁶ Both DIO1 and DIO2 can activate thyroxine (T4) to T3. The difference lies in the fact that DIO1 is involved in iodine recycling, whereas DIO2 provides T3 for the cell nucleus. This distinction is most applicable under euthyroid conditions; in hyperthyroidism DIO1 contributes more to circulating T3 production, whereas in hypothyroidism DIO2 assumes a relatively greater role. The primary role of DIO3 is to catalyze the inactivation of THs, converting T4 to reverse triiodothyronine (rT3) or T3 to diiodothyronine (T2). These isoforms exhibit distinct, tissue-specific expression patterns in fetuses and adults. By virtue of this spatial and temporal regulation, they selectively activate or inactivate THs, thereby fine-tuning hormonal balance in both the circulation and local tissues. ⁷

DIO2 serves as the key contributor to local T3 levels in tissues, ensuring the precise regulation of TH signaling that is critical for development, metabolism, and thermogenesis. ⁸ DIO2 acts as a central modulator of adaptive thermogenic responses in brown fat, directly linking local TH activation to the regulation of human metabolic rate. ⁹ As an endoplasmic reticulum (ER)-resident enzyme, DIO2 generates T3 in close proximity to thyroid hormone receptors (TRs), facilitating receptor activation. ¹⁰

Implications of the Thr92Ala polymorphism

Among the polymorphisms of the type 2 deiodinase gene, the DIO2 Thr92Ala (rs225014) has been relatively well-studied. This single-nucleotide polymorphism involves the substitution of threonine (Thr) by alanine (Ala) at position 92 of DIO2. Approximately 12%–36% of the Caucasian population is homozygous for this variant. ¹¹ The relatively high frequency of this polymorphism in the population underscores its significant research relevance. The Thr92Ala polymorphism in DIO2 has been clinically linked to various neuropsychiatric and cognitive phenotypes, such as bipolar disorder, intellectual disability, and an elevated susceptibility to AD. ¹⁰ Studies in mice have shown that FVB-Ala92-DIO2 mice exhibit altered thyroid phenotypes, whereas B6-Ala92-DIO2 mice do not. Furthermore, female mice demonstrated more pronounced thyroid phenotypes compared to males, highlighting the crucial role of genetic background in modulating the phenotypic outcomes of the Thr92Ala-DIO2 polymorphism. This provides important insights for understanding differences in disease susceptibility in human populations. ¹² Studies show that the DIO2 Thr92Ala polymorphism confers a disproportionately higher risk for AD and related cognitive decline in African Americans relative to their European American counterparts. ¹³ Nevertheless, a significant knowledge gap persists regarding East Asian populations, making the investigation into the population-specific effects of the DIO2 Thr92Ala polymorphism on cognitive health an urgent research priority.

The cerebral cortex of DIO2 Thr92Ala carriers exhibits a distinct transcriptional profile linked to neurological pathogenesis. This profile is characterized by alterations in gene sets involved in ubiquitination, mitochondrial dysfunction, inflammation, apoptosis, DNA repair, and growth factor signaling. Similarly, HEK-293 cells expressing Ala92-DIO2 reproduced this transcriptional fingerprint, yet in both models, the expression of T3-responsive genes remained unchanged, providing no evidence for compromised TH signaling. ¹⁴

In line with this, another study reported comparable T3 levels between case and control groups carrying the heterozygous DIO2 Thr92Ala polymorphism. This finding underscores a critical limitation: circulating T3 levels are a poor reflector of cerebral thyroid status, positioning DIO2 enzyme activity as a more precise indicator of brain hypothyroidism. ¹⁵

Molecular characteristics of the Thr92Ala polymorphism

Enzyme activity and structural stability

DIO2 localizes to the ER-Golgi apparatus intermediary compartment (ERGIC) and engages in retrograde transport, functioning as a cargo protein that shuttles between the ER and the Golgi apparatus. The Ala92-DIO2 variant is prone to misfolding, which triggers a cascade that includes ER stress and the activation of the unfolded protein response. Unlike its variant, Thr92-DIO2 is predominantly resident in the ER but undergoes continuous cycling to the cis-Golgi through the ERGIC, maintaining a low steady-state level in the Golgi compartment. Consequently, the misfolded Ala92-DIO2 variant accumulates in the trans-Golgi, leading to diminished T3 generation. Intervention with the chemical chaperone 4-phenylbutyric acid alleviates ER stress, thereby restoring T3 generation to normal levels. ¹⁰ It is hypothesized that type 2 deiodinase polymorphisms contribute to subtle cerebral alterations, potentially underlying variations in local TH metabolism and related brain functions. These variations are associated with related conditions such as metabolic abnormalities and neurodevelopmental disorders, thereby offering new potential therapeutic targets for TH-related diseases.

Mitochondrial dysfunction drives a pathogenic cascade in AD, characterized by bioenergetic failure and oxidative stress that cause synaptic and neuronal loss. These alterations, evident already in mild cognitive impairment (MCI), intensify as MCI progresses, culminating in overt AD. ¹⁶ Ala92-DIO2 induces ER stress and can be readily observed in the trans-Golgi apparatus. ¹⁰ Experiments in human brain tissue and cellular models have revealed that the expression of Ala92-DIO2 disrupts fundamental cellular processes. A key manifestation of this disruption is an 81-gene transcriptional fingerprint, encompassing pathways linked to central nervous system (CNS) disorders, ubiquitination, ER stress, mitochondrial dysfunction, inflammation, cell death, DNA repair, and growth factor signaling. ¹⁴ A single amino acid substitution can produce a misfolded protein that disrupts essential ER functions, thereby triggering ER stress and subsequently altering the expression of genes within the ubiquitination pathway. Following its initial mislocalization, Ala92-DIO2 ultimately escapes to the Golgi apparatus, where its accumulation may consequently disrupt the normal ribbon-like morphology of this organelle. Mitochondrial dysfunction, inflammation, and cell death can be conceptualized as downstream consequences of primary ER/Golgi disruptions, ¹⁴ with the Ala92-DIO2 variant serving as a key initiating factor.

Physiological impact of the DIO2 Thr92Ala polymorphism on TH homeostasis

Beyond its established role in local T3 generation, DIO2 contributes to the central regulation of the hypothalamic–pituitary–thyroid axis. In the pituitary, DIO2-mediated conversion of T4 to T3 is critical for sensing circulating T4 concentrations and for fine-tuning thyroid stimulating hormone (TSH) secretion, thereby participating in the establishment of the individual TSH–T4 set point.^9,17 Alterations in DIO2 activity may therefore influence TH feedback regulation even in the absence of overt changes in circulating hormone levels.

Emerging human data suggest that the Thr92Ala polymorphism may subtly modify pituitary sensitivity to TH. In particular, aging carriers of the variant have been reported to exhibit reduced pituitary responsiveness to circulating T4, reflected by modest shifts in the TSH–T4 relationship despite biochemically euthyroid profiles. ¹⁸ Although these changes are generally small and clinically unapparent, they indicate altered central feedback dynamics and suggest that the Thr92Ala variant may influence thyroid homeostasis at the regulatory set-point level rather than through gross hormonal abnormalities.

At the systemic level, most population-based studies do not demonstrate consistent differences in circulating T3 or T4 concentrations associated with the Thr92Ala polymorphism. ⁶ However, serum hormone concentrations primarily reflect endocrine output and may not fully capture tissue-specific TH action. In tissues that rely heavily on intracellular T4-to-T3 conversion, including the brain, reduced DIO2 efficiency may result in altered local TH signaling even in the context of normal circulating hormone levels. Experimental and translational studies further support a dissociation between systemic euthyroidism and impaired intracellular TH signaling in carriers of the variant. ¹⁰ Collectively, these observations provide a physiological framework linking DIO2 Thr92Ala to subtle modifications of TH homeostasis at both central and tissue-specific levels.

Association between the Thr92Ala polymorphism and cognitive function

The role of TH in cognitive function

Population-based analyses establish AD as the primary etiological driver of both MCI and dementia. ¹⁹ According to standard laboratory criteria, thyroid dysfunction is classified into two categories: overt (characterized by abnormal TSH and free thyroxine (FT4) levels) and subclinical (defined by abnormal TSH with normal FT4 levels). Several studies suggest a potential association between thyroid dysfunction and cognitive impairment; however, the direction and magnitude of these associations remain inconsistent across cohorts and meta-analyses.

A 2023 meta-analysis of 15 studies found that hyperthyroidism (both clinical and subclinical) is associated with a higher incidence of dementia. Conversely, the analysis linked hypothyroidism to a reduced risk. ²⁰ However, a separate 2023 meta-analysis of 7 studies, involving 1189 AD subjects and 72,717 controls, revealed a link between hypothyroidism and AD but found no such significant association with hyperthyroidism. ²¹ In contrast, an individual participant data analysis of 23 cohorts (74,565 subjects) suggested that subclinical thyroid dysfunction is associated with neither cognitive decline nor incident dementia, while the link with overt dysfunction remains unclear. ²² In an elderly Peruvian cohort, the lowest serum TSH levels were identified as a factor associated with increased dementia risk, implicating hyperthyroidism as a potential contributor to cognitive decline. ²³

Importantly, these apparently discrepant findings are likely driven by context-dependent heterogeneity, including differences in TH status and pharmacological treatment, sex-specific and hormonal influences, variation in genetic background beyond a single locus, and heterogeneity in age range and clinical characteristics (e.g., baseline cognitive status, comorbidity burden, and follow-up duration), all of which can substantially affect both exposure classification and cognitive endpoints. These sources of heterogeneity are discussed in detail in a dedicated section below.

In a meta-analysis of 11 studies, elevated dementia risk in community-dwelling older adults was associated with both higher FT4 levels and lower TSH levels, with the latter correlation being significant only for subnormal TSH values rather than variations within the normal range. ²⁴ Serum thyroid profile alterations, including elevated FT4 and a reduced T3/T4 ratio, were identified in patients with mild AD compared to healthy controls. This pattern, suggestive of reduced peripheral deiodination, was not accompanied by changes in cerebrospinal fluid TH levels. ²⁵ Higher TSH levels were associated with a lower risk of dementia across the full spectrum of thyroid function, according to a prospective study with 8 years of follow-up in a cohort of nearly 9500 older adults (mean age 65). Independently of cardiovascular risk factors, higher FT4 levels were also separately associated with an increased risk of dementia. As TSH levels rose in older women, the 10-year dementia risk decreased from 15% to 10%; correspondingly, higher levels were also linked to improved global cognitive function. ²⁶ These recent studies highlight the association between thyroid-related hormones and cognitive function, though the underlying mechanisms require further investigation. The impact of both overt and subclinical thyroid diseases on cognitive function remains incompletely understood.

Given that circulating thyroid indices may not fully capture tissue-specific TH action, particularly within the brain, variability in local regulatory pathways (including deiodinase-dependent mechanisms) provides a plausible biological context for the population-dependent and sometimes inconsistent associations observed across clinical studies.

Impact of type 2 deiodinase on thyroid function and cognition

In the CNS, DIO2 is predominantly expressed in astrocytes, whereas neurons express TRs and DIO3, which inactivates T3, forming a coordinated glia–neuron regulatory axis. ²⁷ Through this spatial organization, astrocytic DIO2 catalyzes the local conversion of T4 to T3, and the generated T3 acts in a paracrine manner to activate T3-responsive genes in adjacent neurons. ²⁸ Neuronal DIO3 activity further modulates this signaling by terminating intracellular T3 action, positioning deiodinases as key regulatory nodes of TH signaling in the brain.

As illustrated in Figure 1, DIO2-mediated T3 production in astrocytes ensures adequate neuronal TH receptor activation. The Thr92Ala substitution has been associated with altered intracellular processing of DIO2, including enhanced ubiquitination, which may reduce effective T3 generation. Because neuronal TH signaling depends largely on astrocyte-derived T3, such alterations may induce a state of local cerebral hypothyroidism despite normal circulating hormone levels. It should be noted that while astrocytic DIO2-mediated T3 production and its regulation by ubiquitination are supported by experimental evidence, several downstream pathways depicted in Figure 1 represent proposed or context-dependent mechanisms rather than fully established causal links. In particular, alterations in neurotrophic signaling, glial activation markers, and vascular-related pathways have been associated with changes in TH signaling in experimental models, but direct mechanistic evidence specifically linking the DIO2 Thr92Ala polymorphism to these downstream molecular events in humans remains limited. The figure therefore integrates current molecular and experimental findings to provide a conceptual framework, rather than a definitive linear pathway, through which DIO2-related variation may influence cognitive vulnerability.

Figure 1.

Conceptual framework illustrating proposed pathways linking the DIO2 Thr92Ala polymorphism to altered cerebral thyroid hormone signaling.

DIO2 expressed in astrocytes converts T4 to T3, enabling activation of TRs in adjacent neurons. The Thr92Ala substitution has been associated with altered intracellular processing of DIO2, including increased ubiquitination and reduced functional stability, potentially decreasing effective T3 availability. Disruption of local TH signaling may influence neurotrophic pathways, mitochondrial function, and glia–neuron communication, thereby contributing to cognitive vulnerability.

DIO2 is characterized by a relatively short half-life due to ubiquitination and proteasomal degradation. Notably, its turnover is tightly regulated by substrate availability: T4 binding accelerates DIO2 degradation, reducing its half-life to approximately 20 min, whereas T4-deficient conditions prolong DIO2 stability to several hours. ¹⁷ This dynamic regulation allows fine-tuning of intracellular T3 levels and underscores the sensitivity of cerebral TH signaling to subtle changes in DIO2 activity.

Emerging evidence also suggests that DIO2-mediated TH signaling may influence cerebrovascular integrity. Single-cell analyses of human cavernous and arteriovenous malformations demonstrate activation of TH pathways and upregulation of DIO2 within fibroblast populations. ²⁹ Experimental models further indicate that modulation of DIO2 or T3 levels can alter vascular remodeling and hemorrhagic susceptibility. Although these findings extend beyond cognition per se, they highlight the broader neurovascular context in which DIO2-dependent TH signaling may operate.

The DIO2 Thr92Ala polymorphism: Linking thyroid dysfunction to cognitive impairment

Evidence linking the DIO2 Thr92Ala polymorphism to cognitive impairment remains heterogeneous across clinical and experimental contexts, suggesting that its functional impact may depend on biological background and thyroid status.

Human observational studies in euthyroid populations

In a cohort of euthyroid older adults without prior cognitive impairment, no significant association was observed between the Thr92Ala variant and age-related cognitive decline. ³⁰ This finding suggests that, in individuals with preserved systemic thyroid homeostasis, the phenotypic effect of the polymorphism may be subtle or clinically unapparent. Such results underscore that the impact of DIO2 variation may not be detectable in populations without altered thyroid reserve or additional vulnerability factors.

Experimental models and context-dependent effects

In contrast, experimental data from murine models indicate that aging exacerbates cognitive deficits in mice carrying the Thr92Ala-DIO2 variant. ³¹ Importantly, short-term LT3 administration improved cognitive performance in male Thr92Ala mice, supporting the hypothesis that reduced local T3 signaling contributes to the observed phenotype. The divergence between human observational findings and animal model data likely reflects contextual differences, including controlled genetic background, species-specific TH regulation, and the heightened sensitivity of experimental behavioral paradigms to intracellular T3 alterations.

Hypothyroidism, LT4 treatment, and persistent symptoms

Clinical heterogeneity is particularly evident among hypothyroid patients treated with levothyroxine (LT4). A subset of patients continues to experience cognitive complaints and fatigue despite biochemical euthyroidism.^32–35 Although most large population-based studies do not demonstrate a consistent association between Thr92Ala genotype and cognitive performance in LT4-treated individuals, ³⁶ other investigations report that carriers of the polymorphism may experience lower well-being scores and greater perceived benefit from LT3 + LT4 combination therapy, despite comparable serum T3 and T4 levels.^37,38 These findings suggest that systemic euthyroidism does not necessarily equate to optimal tissue-level TH signaling. The variability observed across studies may reflect differences in thyroid functional status, treatment exposure, duration of disease, and cognitive assessment methods, as well as possible interactions with other genetic components of TH transport and receptor signaling.

Neuropsychiatric and neurodegenerative associations

Genomic analyses of human brain tissue have shown that the transcriptional signature associated with the Ala92-DIO2 variant overlaps with pathways implicated in neurodegenerative disorders, including Huntington’s disease. ¹⁴ In psychiatric populations, associations between Thr92Ala and schizophrenia have been reported in some cohorts, accompanied by altered TH parameters.^15,39 However, other neurodevelopmental studies, such as those in autism spectrum disorder, did not demonstrate increased allele frequency and even suggested higher adaptive functioning in carriers. ⁴⁰ Similarly, case-control studies in MCI populations have yielded mixed results, with genotype-related differences in serum TH levels but no consistent differences in allele distribution between cases and controls. ⁴¹

Collectively, these divergent findings suggest that the Thr92Ala polymorphism is unlikely to act as a strong independent determinant of cognitive impairment. Rather, its influence may be modulated by age, thyroid functional status, sex, comorbid conditions, and population-specific genetic backgrounds.

Integration with pituitary and systemic physiology

Physiological data provide further context for these observations. Aging and hypothyroidism are associated with alterations in pituitary DIO2 activity and feedback regulation. Notably, carriers of the homozygous Ala/Ala genotype exhibit a blunted TSH secretory response, reflecting reduced pituitary sensitivity to circulating T4. ¹⁸ In experimental FVB-Ala92-DIO2 mice, the polymorphism produces profound thyroid dysfunction characterized by impaired hormonogenesis, elevated TSH, reduced serum T4, and transcriptomic signatures of ER stress. ¹² These findings demonstrate that the phenotypic consequences of the variant can become pronounced under specific genetic or physiological conditions.

Taken together, current evidence indicates that the relationship between the DIO2 Thr92Ala polymorphism and cognitive outcomes is context-dependent rather than uniformly pathogenic. Apparent inconsistencies across studies likely reflect differences in thyroid status, age distribution, treatment exposure, genetic background, and methodological design. A more integrative framework that considers both systemic thyroid homeostasis and tissue-specific TH signaling is necessary to clarify the circumstances under which DIO2-related variation may meaningfully influence cognitive function.

Sources of heterogeneity in studies linking DIO2 Thr92Ala to cognitive outcomes

Despite growing interest in the role of TH metabolism in cognitive aging, studies examining the association between the DIO2 Thr92Ala polymorphism and cognitive outcomes have yielded heterogeneous and sometimes conflicting results. These discrepancies are unlikely to reflect a lack of biological relevance. Instead, they underscore the context-dependent nature of TH signaling and the substantial methodological variability across clinical studies. A systematic consideration of these factors is essential for a balanced and critical interpretation of the existing evidence.

Pharmacological treatment and TH status

One major and often underappreciated source of heterogeneity is exposure to pharmacological treatments that alter TH homeostasis, particularly LT4 replacement therapy. Because DIO2 contributes to intracellular T3 availability, its functional relevance may be amplified under conditions of altered TH supply, such as in individuals receiving LT4 therapy.^9,42,43 In treated hypothyroid individuals, normalization of circulating TSH and free T4 concentrations does not necessarily ensure adequate tissue-specific or cerebral T3 availability.^38,42 As cognitive function is likely more sensitive to local TH signaling than to systemic hormone levels, failure to stratify analyses by treatment status may obscure genotype-dependent effects of the DIO2 Thr92Ala polymorphism. Variability in baseline thyroid status, treatment exposure, and duration of therapy may therefore contribute substantially to inconsistent findings across studies.

Sex-specific effects and hormonal context

Sex-specific biological differences represent another potential contributor to heterogeneity. Epidemiological and clinical evidence indicates that thyroid function and TH sensitivity differ between men and women, likely reflecting interactions between the hypothalamic–pituitary–thyroid axis and sex hormones.^44,45 Although direct evidence linking DIO2 Thr92Ala to sex-specific regulatory mechanisms remains limited, failure to perform sex-stratified analyses may obscure genotype-associated associations. Pooling male and female participants without adequate adjustment for hormonal status could dilute modest genotype effects or generate divergent results, particularly in cohorts with imbalanced sex distributions. Such differences may partially explain why associations are observed in certain populations but not replicated in others.

Genetic background beyond a single SNP

Most human studies evaluate the DIO2 Thr92Ala polymorphism in isolation, although TH action depends on coordinated activity among multiple interacting components, including transporters and nuclear receptors.^9,46 Variability in transporters such as MCT8 and OATP1C1, which are critical for cerebral TH uptake, may independently influence intracellular T3 availability.^47,48 Likewise, polymorphisms in TR genes (THRA and THRB) may modify downstream transcriptional responses. Importantly, the phenotypic impact of DIO2 Thr92Ala cannot be interpreted independently of this broader signaling context. Failure to account for interacting genetic components may result in population-specific or context-dependent associations that are not reproducible across studies, thereby limiting the interpretability and generalizability of single-SNP analyses.

Age range and clinical characteristics of study populations

Differences in age range, baseline cognitive status, and clinical characteristics further complicate interpretation. Aging is associated with alterations in thyroid axis regulation and tissue sensitivity, which may modify the functional impact of DIO2-mediated T3 production. ⁴⁹ The relevance of local TH activation may therefore increase in contexts of declining thyroid reserve or heightened neurodegenerative vulnerability. Variation in cognitive endpoints and study design across published cohorts may influence both detection and magnitude of genotype effects, as reflected in differences among population-based studies.^50,51 Differences in sample size, statistical power, and residual confounding may further contribute to divergent findings.

Taken together, these sources of heterogeneity suggest that inconsistent findings in the literature do not necessarily refute a role for the DIO2 Thr92Ala polymorphism in cognitive aging. Rather, they underscore the context-dependent nature of TH signaling and highlight the importance of considering treatment status, sex, genetic background, age, and methodological differences when interpreting existing evidence.

Importantly, while DIO2 plays a central role in regulating intracellular T3 availability, TH action is orchestrated by a broader regulatory network that includes transporters, nuclear receptors, co-regulatory proteins, and other deiodinases. The functional impact of a single genetic variant such as Thr92Ala cannot be fully understood in isolation from this integrated signaling framework. Failure to account for variability in these interacting components may contribute to population-specific associations and inconsistent replication across studies. Recognizing this complexity is essential for achieving a balanced interpretation of the current evidence and for refining future research strategies.

Future studies incorporating harmonized cognitive phenotyping, careful control of thyroid functional status, and integrative genetic approaches will be essential to clarify under which conditions DIO2-related variation exerts clinically meaningful effects on cognitive outcomes.

Conclusion

In summary, available evidence suggests that the DIO2 Thr92Ala polymorphism may be associated with interindividual variability in cognitive aging through its influence on local TH signaling and neuronal function. However, current findings across molecular, physiological, and clinical studies remain heterogeneous and context-dependent, precluding definitive conclusions regarding its impact on cognitive outcomes in older adults.

Importantly, the effects attributed to the DIO2 Thr92Ala variant should be interpreted within the broader regulatory network of TH action and in conjunction with factors such as age, sex, pharmacological treatment, genetic background, and study design. At present, the existing literature does not support the use of this polymorphism as a standalone marker for cognitive decline. Future well-designed studies integrating genetic, physiological, and clinical data, with appropriate stratification and longitudinal follow-up, will be required to clarify under which biological and clinical conditions DIO2-related variation may meaningfully influence cognitive trajectories in aging populations.