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The thyroid hormone, involved in the regulation of metabolism, temperature, heart rate, and growth, also likely controls behaviour, according to a new study.
Mice with high levels of the thyroid hormone were more likely to show exploratory behaviour by acting on the brain, the study published in the journal Cell highlighted.
“It’s well known that thyroid hormone modulates metabolism, and now we’ve shown that it also modulates exploratory behaviours through direct action on the brain,” Daniel Hochbaum, research fellow in neurobiology at the Blavatnik Institute at Harvard Medical School, and lead author of the study, said in a statement.
Low levels of the hormone could lead to depressive states marked by a low desire to explore. Very high levels could trigger manic states due to an extreme desire for exploration.
The thyroid gland — a butterfly-shaped gland in the front of the human neck — produces two hormones: triiodothyronine (T3) and thyroxine (T4). Both are collectively referred to as ‘thyroid hormone’ as T4 is relatively inactive, while T3 is active.
Many body functions slow down in response to low thyroid hormone levels in the blood. Individuals with hypo- or hyperthyroidism tend to face psychiatric symptoms, including depression or mania. However, Hochbaum did not find any satisfactory scientific explanation for this connection.
The thyroid hormone affects the heart, central nervous system, autonomic nervous system, bones, and gastrointestinal tract.
Cells in the brain host receptors for thyroid hormone. These binds to the hormone, according to the researchers. “What is quite remarkable is that in the adult brain, the thyroid hormone receptor is not only in the hypothalamus, but is basically everywhere,” Hochbaum said.
He and the others sequenced individual cells in mice’s cortex. They found that the hormone likely switches on various genes, essentially changing the wiring of brain cells, the study highlighted.
The researchers exposed the cortex to higher levels of thyroid hormone and found that mice became more willing to explore the environment and take risks. When the team blocked the hormone’s action only in the cortex, the animals no longer changed how much they explored. “This told us that thyroid hormone is doing important things directly in the cortex,” Hochbaum said.
The team turned to previous studies to understand if there are any benefits to the hormone controlling both metabolism and behaviour.
The published field studies conducted in lemurs, squirrel monkeys, and other mammals found that hormone levels and metabolic rates were higher in warmer seasons when food and resources were in abundance. During this time, animals tend to explore more.
“T3 has been used as an effective therapy for depression in humans, independent of thyroid status, supporting our results that thyroid hormone levels regulate exploratory drive,” the study read.
Hochbaum speculates that the thyroid hormone acts directly on brain circuits to coordinate exploratory behaviours with metabolic rate. “It’s syncing up your brain and body for the current environment,” he added.
Going forward, the team plans to investigate the basic biology powering the exploratory brain circuits activated by increased thyroid hormone. The researchers hope these findings could help them better understand psychiatric conditions such as depression and bipolar disorder.
“The thought is that these conditions are also shaping exploratory activity; so perhaps manipulating thyroid hormone to change brain circuits will reveal relevant points of entry for treatment,” Hochbaum said.