Relationship: 444



T4 in neuronal tissue, Decreased leads to BDNF, Reduced

Upstream event


T4 in neuronal tissue, Decreased

Downstream event


BDNF, Reduced

Key Event Relationship Overview


AOPs Referencing Relationship


AOP Name Adjacency Weight of Evidence Quantitative Understanding
Inhibition of Na+/I- symporter (NIS) leads to learning and memory impairment adjacent Low Low

Taxonomic Applicability


Term Scientific Term Evidence Link
rat Rattus norvegicus High NCBI
mouse Mus musculus High NCBI

Sex Applicability


Sex Evidence
Mixed High

Life Stage Applicability


Term Evidence
Birth to < 1 month High
Adult Moderate
During brain development High

Key Event Relationship Description


It is widely accepted that the thyroid hormones (TH) have a prominent role in the development and function of the Central Nervous System (CNS) and their action has been closely linked to the cognitive function because of their importance in the neocortical development (Gilbert et al., 2012). During the early cortical network development TH has been shown to influence the number of cholinergic neurons and the degree of innervation of hippocampal CA3 and CA1 regions (Oh et al., 1991; Thompson and Potter 2000), and to regulate the morphology and function of GABAergic neurons (Westerholz et al., 2010).

One of the mediators of this regulation has been suggested to be the brain derived neurotrophic factor (BDNF), whose role in brain development and function has been very well-documented (Binder and Scharfman, 2004) and which function has been associated with TH levels in the brain (Gilbert and Lasley, 2013). Several studies have shown that TH can regulate BDNF expression in the brain (Koibuchi et al., 1999; Koibuchi and Chin, 2000; Sui and Li, 2010), with the subsequent neurodevelopmental consequences.

In view of the above evidence, it has been shown that the thyroid insufficiency (lower TH levels) results in reduction of BDNF levels (mRNA or protein) in the developmental brain.

Evidence Supporting this KER


Biological Plausibility


The importance of thyroid hormones (TH) in brain development has been recognised and investigated for many decades (Bernal, 2011). Several human studies have shown that low levels of circulating maternal TH, even in the modest degree, can lead to neurophysiological deficits in the offspring, including learning and memory deficits, or even cretinism in most severe cases (Zoeller and Rovet, 2004; Henrichs et al., 2010). The levels of serum TH at birth are not always informative, as most of the neurological deficits are present despite the normal thyroid status of the newborn. That means that the cause of these impairments is rooted in the early stages of the neuronal development during the gestational period. The nature and the temporal occurrence of these defects suggest that TH may exert their effects through the neurotrophins, as they are the main regulators of neuronal system development (Lu and Figurov, 1997). Among them, BDNF represents the prime candidate because of its critical role in CNS development and its ability to regulate synaptic transmission, dendritic structure and synaptic plasticity in adulthood (Binder and Scharfman, 2004). Additionally, hippocampus and neocortex are two of the regions characterized by the highest BDNF expression (Kawamoto et al., 1996), and are also key brain areas for learning and memory functions.

Empirical Evidence


The empirical support for this direct KER is weak. There are a limited number of studies investigating TH concentrations in the brain and BDNF levels. This is due to the fact that TH is difficult to measure in the brain and TH-induced changes in BDNF expression (mRNA or protein levels) can be subtle.

For the current AOP the temporal nature of this KER is described in the context of different developmental stages (Seed et al., 2005). The impact of brain TH concentrations on regulation of TH receptor (TR) regulated genes is age-dependent for a number of genes critical for normal hippocampal development. It is widely accepted that different genes, including BDNF, are downregulated under conditions of TH insufficiency (Pathak et al, 2011). TH supplementation has been shown to reverse some of the effects on gene expression (Liu et al., 2010; Pathak et al., 2011), including also BDNF expression (Wang et al., 2012).

Many in vivo studies have focused on the determination of the relationship between TH-mediated effects and BDNF expression in the brain. The majority of the work has been performed by evaluating the effects of TH insufficiency on BDNF developmental expression profile. The results, despite some differences, are showing a trend toward BDNF down-regulation.

Reductions in BDNF mRNA and protein were observed in hypothyroid rat models, created by exposing the animals to the TPO inhibitors methimazole (MMI) (Sinha et al., 2009) or propylthiouracil (PTU) (Neveu and Arenas, 1996; Lasley and Gilbert, 2011), and perchlorate (NIS inhibitor) (Koibuchi et al., 1999; 2001). These studies supported direct associations between lower level of TH and lower BDNF expression in the developmental cerebellum, hippocampus and cortex. The dose-response relationship could not be evaluated in these studies, as they were conducted in conditions of severe maternal hypothyroidism, namely after exposure to very high doses of the chemicals.

Additionally, in more complex models of maternal hypothyroidism in rats a reduction of hippocampal and cortex BDNF expression was observed (Wang et al., 2012; Liu et al., 2010). In these latter cases, hypothyroidism was developed via thyroidectomies, and T4 supplementation was performed at specific stages during gestation.

Indirect evidences supporting this KER:

- Koibuchi et al., 2001 Here newborn mice were rendered hypothyroid by administering MMI (TPO inhibitor) and perchlorate (NIS inhibitor) in drinking water to their mothers. Neurotrophin-3 (NT-3) and BDNF gene expression was depressed in the perinatal hypothyroid cerebellum. Since TH levels in the brain were not measured, the same study was also included in the indirect KER "TH synthesis decrease leads to BDFN reduction".

- Morte et al., 2010 induced maternal and fetal hypothyroidism by maternal thyroidectomy (Tx) followed by the antithyroid drug MMI (TPO inhibitor) treatment. Tx dams treated with MMI showed increased TSH (~10 fold increase) and decreased serum T4 (~95%) and serum T3 (~90%). Whilst fetuses from Tx rats had normal serum TSH and cerebral cortex T4 and T3, pups born from dams treated with MMI showed increase of TSH (~8 fold), decreased cortex T4 (~75%) and cortex T3 (~95%). Additionally, analysis of gene expression in the fetal cerebral cortex showed a reduction of Camk4 (Ca(2+) and calmodulin-activated kinase) gene expression (~70%) induced by maternal and fetal hypothyroidism. As Camk4 during development is known to induce BDNF expression (Shieh et al., 1998), a reduction of cortical TH levels (leading to Camk4 mRNA downregulation) may lead to reduction of BDNF expression and/or signaling; however, BDNF expression was not measured in Morte et al. 2010.

- da Conceição et al., 2016 Thyroidectomized (i.e,. hypothyroid) adult Wistar rats showed significant increase of serum TSH (~ 750% increase vs control rats), decrease of T4 (~ 80% decrease vs control) and T3 serum levels (~ 45% decrease vs control), together with a reduced hippocampal expression of MCT8 (~ 83% decrease vs control rats), TH receptor alfa (TRα1) (~ 77 % decrease vs control), deiodinase type 2 (DIO2) (~ 90% decrease vs control), and BDNF mRNA expression in hippocampus (~ 75% decrease vs control). The reduced gene expression of TRα1 (expressed in nearly all neurons in the brain (Schwartz et al., 1992)), MCT8 and DIO2 (both essential for the regulation of glia-neuron cell interaction in TH metabolism in the brain (Remaud et al., 2014)), indicate the presence of low TH levels in the hippocampus (hippocampal hypothyroidism) in thyroidectomized rats. However, direct measures of TH (T3 or T4) brain levels were not assessed in this study.

 - Pathak et al, 2011 Here the effect of maternal TH deficiency on neocortical development was investigated. Rat dams were exposed to MMI (TPO inhibitor) from GD6 until sacrifice. Decreased number and length of radial glia, loss of neuronal bipolarity, and impaired neuronal migration were recovered with early TH replacement (at E13-15). BDNF mRNA resulted downregulated (80% decrease at E14) while trkB expression was increased (2-fold) in hypothyroid fetuses at E14 stage. However, TH levels in the brain were not measured in this study.

Uncertainties and Inconsistencies


Hypothyroidism (i.e., induced by chemicals known to inhibit TPO or NIS, or by thyroidectomy, leading to low TH serum levels) is generally associated with lower levels of BDNF in brain tissues. As described in Conceição et al., 2016, thyroidectomized adult rats, apart from showing reduced TH levels, also presented reduced hippocampal gene expression of MCT8, TRα1, DIO2 and BDNF, which support a link between hippocampal hypothyroidism and reduced BDNF levels.

However, despite the fact that many in vivo studies have shown a correlation between hypothyroidism and BDNF expression in the brain, there are no studies simultaneously measuring the levels of both TH and BDNF in the brain. Therefore, no clear consensus can be reached by the overall evaluation of the existing data. There are numerous conflicting studies showing no significant decrease in BDNF mRNA or protein levels under hypothyroid conditions (Alvarez-Dolado et al., 1994; Bastian et al., 2010; 2012; Royland et al., 2008; Lasley and Gilbert, 2011). However, the results of these studies cannot exclude the possibility of temporal- or region-specific decreased BDNF effects as a consequence of foetal hypothyroidism. A transient TH-dependent BDNF reduction in early postnatal life can be followed by a period of normal BDNF levels or, on the contrary, normal BDNF expression in the early developmental stages is not predictive of equally normal BDNF expression throughout development. Moreover, significant differences in study design, the assessed brain regions, the age and the method of assessment in the existing studies, further complicate result interpretation.

While PTU (TPO inhibitor) has been shown to decrease serum TH levels and brain BDNF protein levels and mRNA expression in offspring born from PTU-treated rat dams (Shafiee et al. 2016; Chakraborty et al., 2012; Gilbert et al. 2016), in Cortés et al., 2012 study, treatment of adult male Sprague-Dawley rats with PTU induced an increase in the amount of BDNF mRNA in the hippocampus, while the content of TrkB, the receptor for BDNF, resulted reduced at the postsynaptic density (PSD) of the CA3 region compared with controls. Treated rats presented also thinner PSD than control rats, and a reduced content of NMDAr subunits (NR1 and NR2A/B subunits) at the PSD in hypothyroid animals. While these data indicate differential effects elicited by PTU (i.e., TPO inhibition) on BDNF expression/regulation comparing the adult vs foetal brain, downregulation of TrkB receptors still leads to decrease signalling pathways regulated by BDNF.

Quantitative Understanding of the Linkage


There are no consistent quantitative data linking TH levels in the brain and the level of BDNF expression (mRNA or protein), due to differences in study designs, dose regimes and the methods used to assess the endpoints.

As discussed in the Empirical support section, there are some quantitative data in support of the indirect KER “Decreased of TH synthesis leads to reduced BDNF release”, but not to this direct KER.

Response-response Relationship




Known modulating factors


Known Feedforward/Feedback loops influencing this KER


Domain of Applicability


The connection between TH levels and BDNF expression has been studied only in rodent models up to date (see above studies).



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