According to researchers from the National Institute of Mental Health (NIMH) there are twice as many Americans with bipolar disorder as previously thought, and many are untreated.
Researchers say what was once thought of as just one mental illness is in fact just a part of a spectrum of disorders carrying symptoms which range from less severe to devastating.
The NIMH report has found that people with the mildest form of the condition, often referred to as sub-threshold bipolar disorder, generally sought treatment for other mental health conditions such as depression or substance abuse.
Dr. Kathleen R. Merikangas, a senior investigator at the NIMH says many people diagnosed with major depression may actually have this form of bipolar disorder.
Dr. Merikangas says this is a worry as a misdiagnosis can mean the drugs prescribed to treat depression can actually trigger bipolar symptoms.
Bipolar disorder falls into two main categories, bipolar disorder I and bipolar disorder II.
The symptoms include dramatic moods swings between euphoria and severe depression and patients may have hallucinations or delusions.
Bipolar I, the classic form of the disorder, is the most severe, while bipolar II patients have more moderate symptoms.
The NIMH researchers however say health professionals should recognize a third and milder category, a sub-threshold bipolar disorder.
In 2006, the NIMH estimated that 2.6% of the U.S. population, around 5.7 million American adults, suffered from bipolar disorder in any given year.
However by including patients who meet the diagnostic criteria for the sub-threshold bipolar disorder the researchers say around 4.4% of U.S. adults have some degree of bipolar illness during some point in their lives.
The researchers reached this conclusion by evaluating data from a nationwide mental disorders survey conducted between February 2001 and April 2003, involving 9,282 adults living in the U.S.
They found the lifetime incidence of bipolar I and bipolar II was roughly 1% each in the surveyed population but was 2.4% for sub-threshold bipolar disorder.