Scientists uncover key role of thyroid hormones in fear memory formation

A new study published in Molecular Psychiatry suggests that the thyroid hormone system in the brain may be a powerful driver of how fear memories are formed. Thyroid hormone signaling in the amygdala—the part of the brain involved in processing emotions—was not only activated by fear learning, but also necessary for storing fear memories. Boosting thyroid hormone activity strengthened fear memories, while blocking it impaired them. These results may help uncover new treatment pathways for trauma-related disorders such as post-traumatic stress disorder (PTSD).

The amygdala is known to be essential for learning to associate danger with a particular stimulus—such as a tone paired with a shock in laboratory settings. This process, known as Pavlovian fear conditioning, has long been used in animal research to study the brain’s response to threat. Meanwhile, the thyroid hormone system has traditionally been associated with metabolism and early brain development. But it is increasingly being linked to mood, anxiety, and memory. Still, researchers have had limited understanding of how thyroid hormones influence the adult brain’s ability to store emotionally significant memories—especially in brain regions like the amygdala.

Thyroid hormones such as triiodothyronine (T3) interact with specific receptors in the brain called thyroid hormone receptors (TRs). These receptors act as transcriptional regulators: when they bind to T3, they turn on genes that help regulate brain plasticity—the brain’s ability to adapt based on experience. In their unbound state, these receptors suppress gene activity. This dual function makes them a promising target for exploring how hormones can shape emotional learning at the molecular level.

To investigate this, the research team focused on mice undergoing fear conditioning. The study used a widely accepted experimental procedure: mice were exposed to a tone followed by a mild foot shock, creating an association between the two. Researchers examined whether genes involved in the thyroid hormone pathway were activated in the amygdala following this learning event. They also tested whether directly manipulating thyroid hormone activity in the amygdala would influence fear memory formation.

The study involved several sets of experiments. In one, the researchers surgically implanted tiny cannulas into the amygdala of adult male mice. These cannulas allowed them to precisely deliver thyroid hormone T3 or a TR antagonist—either before or after the mice experienced tone-shock pairings. The goal was to observe whether adding or blocking hormone signaling in this brain region would enhance or impair the consolidation of fear memories, which is the process by which short-term memories become stable over time.

They found that administering T3 directly into the amygdala led to stronger fear memories, as measured by increased freezing behavior when the tone was later played. In contrast, blocking thyroid receptors with an antagonist drug weakened fear memory. These changes were not due to differences in how mice learned during the training session itself, but rather reflected how well they retained the memory afterward.

In additional experiments, the researchers created a hypothyroid state in mice by feeding them a low-iodine diet combined with a chemical that blocks thyroid hormone production. As expected, these mice showed lower levels of thyroid hormones in the blood. They also had impaired fear memory. Remarkably, the researchers were able to reverse this memory deficit by injecting T3 into the amygdala, confirming that local hormone signaling in this specific brain region was enough to rescue the behavioral impairment caused by systemic hormone deficiency.

The team also examined which specific genes in the amygdala were regulated by thyroid hormones during fear learning. Using a technique called quantitative PCR, they identified several genes whose activity levels changed following fear conditioning. These included both genes that are usually activated by T3 (such as Dio2 and Reln) and genes that are typically suppressed (such as Trh and Aldh1a3). These transcriptional changes were consistent with the idea that thyroid hormones help coordinate the brain’s molecular response to threat, priming it for storing emotional memories.

The researchers used a type of imaging called RNAscope to validate that some of these genes were being turned on in the amygdala itself, not in nearby regions. They also confirmed that these changes happened within hours of fear learning, suggesting a direct link between hormone signaling and the early phases of memory formation.

Beyond memory, the researchers also tested how amygdala thyroid hormone activity influenced anxiety-related behaviors. In a separate open field test, mice given T3 in the amygdala spent less time exploring the center of the arena and showed less movement overall. These behaviors suggest an increase in anxiety-like responses, aligning with the idea that thyroid hormones can influence emotional arousal beyond memory alone.

Overall, the study provides strong evidence that the thyroid hormone system is a dynamic and influential player in the adult brain’s emotional memory network. While past research has linked thyroid dysfunction to mood disorders like depression and anxiety, this study offers a direct mechanism by which thyroid hormones affect how emotionally significant events are encoded and remembered.

The findings also raise intriguing possibilities for clinical research. Thyroid hormone levels are not routinely measured in patients with PTSD or trauma-related symptoms, despite known associations between stress and thyroid function. If future research supports these findings in humans, thyroid hormone regulation could become a target for novel therapies aimed at preventing or modifying fear-related memory formation.

But the study has limitations. All of the experiments were conducted in male mice, leaving open questions about whether the same results would apply to females. The researchers also focused on relatively short time frames, mainly testing memory 24 hours after learning. It remains unclear how long the observed effects of T3 or its antagonists last, or how these hormone pathways interact with other known regulators of memory such as serotonin, dopamine, or stress hormones.

Future work could explore whether the same hormone pathways are involved in more chronic or generalized anxiety, and whether targeting these pathways could improve treatment outcomes in individuals with trauma-related disorders. Researchers may also investigate how early life stress interacts with thyroid signaling in the brain, potentially helping to explain why some individuals are more vulnerable to the long-term effects of trauma than others.

The study, “Evidence for thyroid hormone regulation of amygdala dependent fear-relevant memory and plasticity,” was authored by Stephanie A. Maddox, Olga Y. Ponomareva, Cole E. Zaleski, Michelle X. Chen, Kristen R. Vella, Anthony N. Hollenberg, Claudia Klengel, and Kerry J. Ressler.