Thyroid hormones are essential for proper brain development and function. A new study in mice showed that the absence of the deiodinase type 2 protein, essential for having correct levels of thyroid hormone in the brain, triggers emotional disorders probably derived from a state of cerebral hypothyroidism.

A deficiency of thyroid hormones in the brain can lead to serious neurological disorders and psychiatric disorders, particularly mood disorders. This is because thyroid hormones play an essential role in the development and function of the central nervous system.

The thyroid gland mainly synthesizes the T4 prohormone, but most of the thyroid hormones’ actions are mediated by T3, the main active form genomically. In the brain, type 2 deiodinase (D2) is of great importance, and its activity depends on the local formation of T3 from T4.

Now, a group of scientists from the Biomedical Research Institute ‘Alberto Sols’, a joint center of the Autonomous University of Madrid (UAM), the Superior Council of Scientific Investigations (CSIC), and the Department of Psychobiology at the National University for Distance Education (UNED), conducted a study to analyze the consequences of D2 deficiency.

Specifically, the team of researchers assessed anxiety behaviors and learning and memory capabilities in D2-deficient adult mice, using behavioral testing. They also studied the thyroid status in two brain regions involved in these behavioral processes: the amygdala and hippocampus.

The findings, published in Psychoneuroendocrinology, revealed that D2-deficient mice did not showed alterations in learning and spatial memory, which depend from the hippocampus. However, the mice did show emotional dysfunctions: they showed an enhanced fear memory, plus an increase in anxiety behavior. All these processes depend on a proper function of the amygdala, a region of the limbic system.

The assessment of thyroid status of the amygdala and hippocampus revealed that both brain structures have a state of hypothyroidism in D2-deficient mice. In the amygdala, this state of hypothyroidism appears to be associated with decreased expression of a calcium binding protein, Calb2, which could be affecting neuronal activity in the structure; and thus, it may contribute to the observed emotional disorders, explained the researchers.

These results highlight the importance of maintaining appropriate levels of thyroid hormone in the brain, particularly through local generation of T3, to avoid emotional disorders.

Furthermore, this research may help to understand the possible emotional disturbances present in humans with polymorphisms in the gene encoding D2 (DIO2).

 

Source: SINC