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Before dysthymia is diagnosed, a physical examination needs to be performed by a physician to rule out other medical conditions. Many long-term medications (e.g., corticosteroids) or chronic medical conditions (e.g., hypothyroidism, anemia) can cause symptoms similar to those of dysthymia.

The diagnosis of dysthymia begins with a mental health evaluation performed either by a physician or after a referral to a non-physician mental health professional. This evaluation includes a complete history of symptoms, including when they started, how long they have lasted and how severe they are. It is also noted whether the patient has experienced these symptoms before and, if so, whether and how they were treated. The physician or other mental health professional will also inquire about alcohol and drug use and whether other family members have had a depressive illness, such as dysthymia or major depression. If there is a history of a depression in any family members, the treatment method and effectiveness will need to be reported.

The American Psychiatric Association identifies particular criteria for the diagnosis of dysthymia in the Diagnostic and Statistical Manual of Mental Disorders IV (DSM-IV). These criteria include:

• Quality and duration of mood. Adults must have a depressed mood for most of the day on the majority of days for at least two years. Children or adolescents must have a depressed or irritable mood for most of the day on the majority of days for at least one year.
  
  
• Symptoms. Two different, overlapping sets of symptoms may be used by different physicians or non-physician mental health professionals. In general, patients must display two or more of the following symptoms during the time that they have an altered mood:
  
  
  • Substantial change in appetite (overeating or reduced appetite)
  • Too much or too little sleep (hypersomnia or insomnia)
  • Reduced energy or fatigue
  • Reduced self-image or confidence (poor self-esteem)
  • Indecisiveness or problems with concentration
  • Hopelessness, despair or pessimism
    
    
• Consistency. Symptoms must not be absent for more than two consecutive months in a two-year period in adults or a one-year period in children.
  
  
• Impairment. The symptoms must be severe enough to cause significant distress (as per the DSM-IV) or impairment in function of work or school performance, socially or otherwise.

The diagnosis of dysthymia is only made if the patient has not been diagnosed with other particular mood disorders (e.g., major depression, bipolar disorder). However, there are certain circumstances where both dysthymia and major depression may be diagnosed (double depression). For instance, many patients with dysthymia will eventually develop major depression, and patients with major depression may eventually develop dysthymia. To be diagnosed with either of these conditions, the symptoms must be unrelated to a physical medical condition or use of any prescribed medications or recreational substances.