The left side of the hypothalamus was revealed to be 5% larger in a new study of people with affective disorders, such as depression and bipolar.

Over 16 million adults in the United States have had at least one episode of major depression during the past year.

In fact, depression is one of the most prevalent mental health problems in the US.

At least 9 million more people aged 18 and over are living with bipolar disorder, according to the National Institute of Mental Health.

The precise underlying causes for both of these conditions remain unknown, but neuroscientists are unraveling more and more aspects that can affect a person's chances of developing these disorders.

The neurobiological roots of depression

Functional MRI and other new imaging techniques have allowed researchers to locate several brain areas and networks that are at play in affective disorders. For instance, in the case of depression, studies have found treatment-resistant depression is associated with decreased hippocampal volume and a smaller amygdala.

The hippocampus is a region heavily involved in memory formation and learning, whereas the amygdala is a small brain structure involved in emotional processing and the response to what is perceived as danger or stress.

In fact, the entire endocrine stress response system, or "HPA-axis," is disrupted in people who have depression. The axis between the hypothalamus, the pituitary gland, and the adrenal glands is overactive, which previous research established as a "core neurobiological feature of major depression."

Normally, the HPA-axis raises levels of the stress hormone cortisol to ensure that the body is better equipped to handle a situation perceived as dangerous or threatening.

After the threat passes, the HPA-axis is also responsible for lowering cortisol and diminishing other stress responses back to normal levels.

However, in the case of individuals with depression, these structures are in "overdrive" even in the absence of objective danger. New research aimed to clarify the role of the hypothalamus in this hyperactive chain reaction.

A team led by Stephanie Schindler, a doctoral researcher working in the Department of Psychiatry and Psychotherapy at University Hospital Leipzig in Germany, studied the volume of the hypothalamus in affective disorders such as bipolar disorder and depression.

The findings were published in the journal Acta Psychiatrica Scandinavica.

Left hypothalamus 5% larger

Schindler and her team examined the brains of 84 people, of whom 20 had depression but did not take any medication for it, 20 had depression but also took medication, 21 lived with bipolar, and 23 were controls who did not have an affective disorder.

They used MRI to study the participants' brains and high-precision volumetry to determine the size of their hypothalami.

Overall, they saw that people with an affective disorder had a 5% increase in the volume of the left side of their hypothalamus, on average.

"We observed that this brain region [the hypothalamus] is enlarged in people with depression as well as in those with bipolar disorder, two types of affective disorders."

–Stephanie Schindler

Importantly, in the case of people with depression, hypothalamus size correlated directly with the severity of the condition.

Medication did not affect the size of the brain region. The researchers warn that beyond the links they found, not much can be inferred on the causality underlying the findings.

"Higher activity could lead to structural changes and thus to a larger volume of the hypothalamus normally the size of a one-cent coin," explains study coauthor Stefan Geyer.

The authors write, "Supported by emerging evidence that the stress response may be related to structural and functional asymmetry in the brain, our finding suggests a crucial role of the hypothalamus in mood disorders."