Just the thought of retirement is fraught with what-ifs. What if I live 30 or 40 years after I retire? What if I run out of money? What if I need long term care?

But the retirement issue almost no one talks about is Senior depression. It’s usually underdiagnosed, seldom recognized, and almost never planned for even though it affects millions of Americans.

Here are some excerpts from an article written by Jamie Hopkins, director or retirement research and vice president of client services at Carson Group.

Happiness in retirement is complex. Some research shows that many retirees are happier than they were before retirement, and, as a whole, are happier than any other group. But at the same time, retirees can suffer from depression at rates higher than the average person. It becomes a tale of two cities: increased happiness for some retirees and increased depression among others.

In general, depression has long been a misdiagnosed, misunderstood and misreported illness impacting Americans. Deaths of high-profile and respected celebrities over the years caused by depression have brought the mental illness into the broader public consciousness, but senior depression remains behind the curtain.

Depression can be hard to diagnose and track, as many people don’t seek professional help — so the number of people suffering from depression is fuzzy. Some estimates say it’s less than 1% or up to 5%. The National Alliance on Mental Illness says depression impacts roughly 6.5 million seniors, more than double the number in other reports.

Retirement includes many factors that trigger depression. Research from the Institute of Economic Affairs shows that the risk of depression increases 40% after an individual stops working. Factors such as isolation, caregiving responsibilities, financial stress and health issues can cause or impact depression. According to the Centers for Disease Control and Prevention, depression rates can triple for older people receiving home health care, to 13.5%.

Isolation in retirement is a huge risk for women, as nearly 37% of senior women live alone, as opposed to 19% of men. Over age 75, that percentage jumps to almost half of women. The poverty rate is two-thirds higher for senior women than men — 12% as opposed to 7% — according to the Social Security Administration. Unmarried elderly people are disproportionally poorer than married couples.

Depression is not and should not be viewed as just a normal part of aging. For some, depression is an illness they struggle with throughout life as biological factors can play a significant role. In these cases, depression can be diagnosed, treated and managed like other mental illnesses. But for others, it’s important to identify factors that can exacerbate or cause depression. Isolation, health-care concerns and wealth issues can cause stress and ultimately lead to depression.

There is still a lot to learn about depression, but acknowledging the devastating impact the illness has on seniors is important. Retirement is not just the “best of times,” with golfing, sunsets, travel and happy seniors. The “worst of times” side of the retirement tale needs to be addressed as well.