Cyclothymic Disorder 141
related disorder due to another medical condition and depressi ve disorder due to another
medical condition (e.g., multiple  sclerosis) before the cyclothymic disorder diagnosis is as-
signed. Among children with cyclothymic diso rder, the mean age at onset of symptoms is
6.5 years of age.
Risk and Prognostic Factors
Genetic and physiological. Major depressive disorder, bipolar I disorder, and bipolar II
disorder are more common among first-degree bi ological relatives of individuals with cyclo-
thymic disorder than in the general population. There may also be an increased familial risk of
substance-related disorders. Cyclothymic disord er may be more common in the first-degree
biological relatives of individuals with bipolar I disorder than in the general population. 
Differential Diagnosis
Bipolar and related disorder due to anothe r medical condition and depressive disorder
due to another medical condition. The diagnosis of bipolar and related disorder due to
another medical condition or depressive disorder due to another medical condition is
made when the mood disturbance is judged to be attributable to the physiological effect of
a specific, usually chronic medical condition (e.g., hyperthyroidism). This determination
is based on the history, physical examination, or laboratory findings. If it is judged that the
hypomanic and depressive symptoms are not the physiological consequence of the med-
ical condition, then the primary mental disorder (i.e., cyclothymi c disorder) and the med-
ical condition are coded. For example, this would be the case if the mood symptoms are
considered to be the psychological (not the physiological) consequence of having a chronic
medical condition, or if there is no etiological relationship between the hypomanic and de-
pressive symptoms and the medical condition.