Diagnosed cases of depression have skyrocketed, especially among adolescents and millennials, according to a new study by one of the nation's largest commercial insurers.

Nationally, depression diagnoses surged by 33 percent from 2013 to 2016, says a report on medical claims released Thursday by the Blue Cross Blue Shield Association, a federation of 36 Blues that provide insurance for one in three Americans.

During that period, depression among the network's adolescents rose 63 percent; among millennials (ages 18 to 34), 47 percent.

At all ages, women were twice as likely to be diagnosed as men, and depression was second only to high blood pressure in the Blue Cross Blue Shield Health Index, an analysis of claims information for more than 41 million Americans.

Some diagnoses may be a reflection of greater awareness and better detection. "The fact that the number of diagnoses has gone up is important because now we are looking for it, we are screening for it, and people are getting treatment to help them," said Ginny Calega, vice president for medical management and policy at Independence Blue Cross, Southeastern Pennsylvania's largest health insurance organization.

At the same time, the trend is cause for concern. According to the U.S. Centers for Disease Control and Prevention, suicide is the second-highest cause of death for Americans ages 10 to 34.

"The increase in depression among adolescents and millennials is particularly concerning, as this could have a significant health effect on this population in the years to come," said Richard Snyder, chief medical officer for Independence Blue Cross.

Shelley Leaphart-Williams, a board member for the American Foundation for Suicide Prevention and founder of Lifesaver U, a local training organization, says many factors are contributing to youthful despair.

"Mental health is like an onion; there are layers upon layers," Leaphart-Williams said. "Our children are suffering from neglect, bullying, abuse, low self-esteem — but the root cause [of depression] is a lack of identity. When we build their confidence and self-esteem, the layers of the onion are thin and don't bother them as much."

The Blues study's findings can be a cue for schools and youth organizations to encourage discussion groups or invite mental health experts to reach out to young people before depression becomes a serious problem, suggested Thea Gallagher, clinic director of the Center for the Treatment and Study of Anxiety at the University of Pennsylvania.

"A lot of people suffer, and they suffer in silence," Gallagher said.

In the Philadelphia area, the depression diagnosis rate for IBC members of all ages and both genders was 4.2 percent, somewhat lower than the 4.4 percent average among all Blue Cross Blue Shield members.

Locally, the rates for major depression, which affects about nine million nationwide, were 5.3 percent for women and 2.9 percent for men of all ages. The rate was 2.5 percent for adolescents ages 12 to 17 and 4.3 percent for millennials ages 18 to 34.

The study also linked depression to other health disorders. Eighty-five percent of people diagnosed with major depression suffered from one or more additional health concerns, and almost 30 percent had four or more other conditions. People diagnosed with depression were twice as likely to suffer from one or more other chronic illnesses and seven times as likely to have alcohol or substance use disorder compared with those without the diagnosis, according to the report.

Consequently, those with depression average more than $10,000 per person in health-care spending a year, as opposed to about $4,300 for those without major depression, the study found.

The report does not identify whether depression usually precedes other health conditions or is a consequence of them. However, other health problems may be a signal to health-care providers to inquire about depression as well.

"People going through life-changing illness can find it changes their self-image," Calega said. "They're no longer as invulnerable as they thought they were, and it's important they work with their primary care provider … to get screened and get the appropriate help."

On June 16, Philadelphia will host an Out of the Darkness Overnight Walk, a decade-old event organized by the American Foundation for Suicide Prevention to raise funds and awareness. For details: afsp.org.