The difference between the two is a matter of degree. Cyclothymia is a milder version of bipolar disorder.

There are no sharp lines dividing the different categories of mood disorders. These labels are a starting point for understanding a person's problem.

When trying to diagnose a person, mental health professionals consider a short list of probable or possible problems. This creates a framework for treatment. Refining the diagnosis is a gradual process. As the patient and doctor learn more about the problem, they modify the treatment based on what helps and what doesn't.

Here are short definitions for bipolar disorder and cyclothymia:

Bipolar Disorder
Doctors make this diagnosis if a person has had at least one manic or mixed episode. This is a period (of at least one week) when the person is in a high, expansive, or elated mood. People with bipolar disorder often also have episodes of depression. Sometimes these are severe enough to be called "major depression." Think of a manic episode as the polar opposite of a major depressive episode. A person in a manic state feels energetic and active, has little need for sleep, and may behave recklessly and overoptimistically. In a mixed episode, symptoms of both depression and mania alternate. Sometimes the symptoms overlap in confusing ways.

Cyclothymia
This is a less intense but often longer lasting version of bipolar disorder. A person with cyclothymia has both high and low mood, but never as severe as either mania or major depression. To make this diagnosis, the person usually has symptoms that last for at least two years.

I would not be too concerned with these labels. The treatment for both can be very similar. A person with cyclothymia, for example, may be helped by the same mood stabilizing medications that help a person with bipolar disorder.

By the way, the reverse can also be true: two people with the same diagnosis (whether it be bipolar disorder or cyclothymia) may receive very different treatments!

It's confusing, I know. Fortunately, these disorders are often treated successfully with a combination of psychotherapy and medication. The key is to find a doctor who can help you with your situation without getting too hung up on the label.