Cyclothymia is characterized by short cycles of baseline, stable periods of not over two months and numerous swings between depression and hypomania that fail to meet the severity of sustained duration criterion for major affective syndromes for at least two years.

Depressive/dysthymic episodes. Symptoms of the "depressive/dysthymic phase" may include any of the following conditions:difficulty making decisions, problems concentrating, poor memory recall, guilt, self-criticism, low self-esteem, pessimism, self-destructive thinking, constant sadness, apathy, hopelessness, helplessness and irritability. Also common are quick temper, poor judgment, lack of motivation, social withdrawal, appetite change, lack of sexual desire, self-neglect, fatigue, insomnia and sleepiness.

Hypomanic episodes. Symptoms of the "hypomanic episode" may include any of the following conditions: unusually good mood or cheerfulness (euphoria), extreme optimism, inflated self-esteem, rapid speech, racing thoughts, aggressive or hostile behavior, lack of consideration for others, agitation, massively increased physical activity, risky behavior, spending sprees, increased drive to perform or achieve goals, increased sexual drive, decreased need for sleep, tendency to be easily distracted, and inability to concentrate.

Depressive mixed states occur when patients experience depression and non-euphoric, usually subsyndromal, hypomania at the same time. As mentioned previously, it is particularly difficult to diagnose BP-II when a patient is in this state.

In a mixed state, mood is depressed, but the following symptoms of hypomania present as well:

- Irritability

- Mental overactivity

- Behavioral overactivity

Mixed states are associated with greater levels of suicidality than non-mixed depression. Antidepressants may increase this risk.

Hypomania is the signature characteristic of Bipolar II disorder. It is a state characterized by euphoria and/or an irritable mood. In order for an episode to qualify as hypomanic, the individual must also present three or more of the below symptoms, and last at least four consecutive days and be present most of the day, nearly every day

- Inflated self-esteem or grandiosity.

- Decreased need for sleep (e.g., feels rested after only 3 hours of sleep).

- More talkative than usual or pressure to keep talking.

- Flight of ideas or subjective experience that thoughts are racing.

- Distractibility (i.e., attention too easily drawn to unimportant or irrelevant external stimuli), as reported or observed.

- Increase in goal-directed activity (either socially, at work or school, or sexually) or psychomotor agitation.

- Excessive involvement in activities that have a high potential for painful consequences (e.g., engaging in unrestrained buying sprees, sexual indiscretions, or foolish business investments).

It is important to distinguish between hypomania and mania. Mania is generally greater in severity and impairs function, sometimes leading to hospitalization and in the most severe cases, psychosis. In contrast, hypomania usually increases functioning. For this reason, it is not uncommon for hypomania to go unnoticed. Often it is not until individuals are in a depressive episode that they seek treatment, and even then their history of hypomania may go undiagnosed. Even though hypomania may increase functioning, episodes need to be treated because they may precipitate a depressive episode.

Because the nature of acute schizophrenia is similar to depression, it is difficult to differentiate normal levels of depression in patients with schizophrenia from depressive levels in post-schizophrenic depression. "Prominent subjectively low mood, suggesting depression, and prominent blunting of affect, suggesting negative symptoms, are the two features which are most helpful in differentiating [schizophrenia and depression]." A number of researchers believe that depression is actually a symptom of schizophrenia that has been hidden by the psychosis. However, symptoms usually arise after the first psychotic episodes if they will arise at all. Officially, diagnosing post-schizophrenia depression in a patient requires for the patient to be experiencing a depressive episode of either short or long term following the overcoming of schizophrenia. The patient must still demonstrate some schizophrenic symptoms but those symptoms must no longer be the focus of the illness. Typically, the depressive symptoms are not severe enough to be classified as a severe depressive episode. Formally, diagnosis entails the patient having had schizophrenia within the past year, a number of schizophrenic symptoms, and depression being present for two weeks or more. Mild schizophrenic signs may be withdrawing socially, agitation or hostility, and irregular sleep such as in the case of insomnia and hypersomnia.

Melancholic depression, or depression with melancholic features, is a DSM-IV subtype of clinical depression requiring at least one of the following symptoms:

- Anhedonia (the inability to find pleasure in positive things)

- Lack of mood reactivity (i.e. mood does not improve in response to positive events)

And at least three of the following:

- Depression that is subjectively different from grief or loss

- Severe weight loss or loss of appetite

- Psychomotor agitation or retardation

- Early morning awakening

- Guilt that is excessive

- Worse mood in the morning

Melancholic features apply to an episode of depression that occurs as part of either major depressive disorder or bipolar disorder I or II.

Melancholic depression is often considered to be a biologically based and particularly severe form of depression. Treatment involves antidepressants, electroconvulsive therapy, or other empirically supported treatments such as cognitive behavioral therapy and interpersonal therapy for depression. A 2008 analysis of a large study of patients with unipolar major depression found a rate of 23.5% for melancholic features. It was the first form of depression extensively studied, and many of the early symptom checklists for depression reflect this.

The incidence of melancholic depression has been found to increase when the temperature and/or sunlight are low.

According to the DSM-IV, the "melancholic features" specifier may be applied to the following only:

1. Major depressive episode, single episode

2. Major depressive episode, recurrent episode

3. Bipolar I disorder, most recent episode depressed

4. Bipolar II disorder, most recent episode depressed

As affirmed by the "Diagnostic and Statistical Manual of Mental Disorders", fifth edition (DSM-V), the symptomology specifier "with mixed features" can be applied to manic episodes of bipolar I disorder, hypomanic episodes of either bipolar I disorder or bipolar II disorder, and depressive episodes of either bipolar disorder or major depressive disorder, with at least three concurrent features of the opposite polarity being present. As a result, the presence of "mixed features" are now recognized in patients with bipolar II disorder and major depression; as earlier noted, however, although it is customary to withhold a diagnosis of a bipolar disorder until a manic or hypomanic episode appears, the presence of such features in a depressed patient even with no history of discrete mania or hypomania is strongly suggestive of the disorder.

Two features of both mania or hypomania and depression may superficially overlap and even resemble each other, namely "an increase in goal-directed activity" (psychomotor acceleration) vs. psychomotor agitation and "flight of ideas" and "racing thoughts" vs. depressive rumination. Attending to the patient's experiences is very important. In the psychomotor agitation commonly seen in depression, the "nervous energy" is always overshadowed by a strong sense of exhaustion and manifests as purposeless movements (e.g., pacing, hand-wringing); in psychomotor acceleration, however, the excess in movement stems from an abundance of energy and is often channelled and purposeful. Likewise, in depressive rumination, the patient experiences the repetitive thoughts as heavy, leaden, and plodding; in psychic acceleration, however, (as seen in mania or hypomania) the thoughts move in a rapid progression, with many themes, rather than a singular one, being touched upon. Even when such experiences are accounted for on the basis of depression, the possibility does still exist, however, that the depressive episode may be complicated by other manic or hypomanic symptoms, in which case it is often prudent to attend to the patient's personal and family history (e.g., family history of bipolar disorder, early age of onset) to determine whether or not the patient has bipolar disorder.

Mania is a distinct period of at least one week of elevated or irritable mood, which can range from euphoria to delirium, and those experiencing hypo- or mania may exhibit three or more of the following behaviors: speak in a rapid, uninterruptible manner, short attention span, racing thoughts, increased goal-oriented activities, agitation, or they may exhibit behaviors characterized as impulsive or high-risk, such as hypersexuality or excessive spending. To meet the definition for a manic episode, these behaviors must impair the individual's ability to socialize or work. If untreated, a manic episode usually lasts three to six months.

People with hypomania or mania may experience a decreased need of sleep, impaired judgment, and speak excessively and very rapidly. Manic individuals often have a history of substance abuse developed over years as a form of "self-medication". At the more extreme, a person in a full blown manic state can experience psychosis; a break with reality, a state in which thinking is affected along with mood. They may feel unstoppable, or as if they have been "chosen" and are on a "special mission", or have other grandiose or delusional ideas. This may lead to violent behavior and, sometimes, hospitalization in an inpatient psychiatric hospital. The severity of manic symptoms can be measured by rating scales such as the Young Mania Rating Scale, though questions remain about the reliability of these scales.

The onset of a manic or depressive episode is often foreshadowed by sleep disturbances. Mood changes, psychomotor and appetite changes, and an increase in anxiety can also occur up to three weeks before a manic episode develops.

The DSM-IV-TR defines a hypomanic episode as including, over the course of at least four days, elevated mood plus three of the following symptoms OR irritable mood plus four of the following symptoms:

- pressured speech

- inflated self-esteem or grandiosity

- decreased need for sleep

- flight of ideas or the subjective experience that thoughts are racing

- easily distracted and attention-deficit; the inability to 'follow-through' with complete tasks, as similar to attention deficit hyperactivity disorder

- increase in psychomotor agitation, or occasionally in some, increased irritability

- hypersexuality

- involvement in pleasurable activities that may have a high potential for negative psycho-social or physical consequences (e.g., the person engages in unrestrained buying sprees, sexual indiscretions, reckless driving, physical and verbal conflicts, foolish business investments, quitting a job to pursue some grandiose goal, etc).

Individuals with psychotic depression experience the symptoms of a major depressive episode, along with one or more psychotic symptoms, including delusions and/or hallucinations. Delusions can be classified as mood congruent or incongruent, depending on whether or not the nature of the delusions is in keeping with the individual's mood state. Common themes of mood congruent delusions include guilt, persecution, punishment, personal inadequacy, or disease. Half of patients experience more than one kind of delusion. Delusions occur without hallucinations in about one-half to two-thirds of patients with psychotic depression. Hallucinations can be auditory, visual, olfactory (smell), or haptic (touch), and are congruent with delusional material. Affect is sad, not flat. Severe anhedonia, loss of interest, and psychomotor retardation are typically present.

Hypomania is the milder form of mania, defined as at least four days of the same criteria as mania, but does not cause a significant decrease in the individual's ability to socialize or work, lacks psychotic features such as delusions or hallucinations, and does not require psychiatric hospitalization. Overall functioning may actually increase during episodes of hypomania and is thought to serve as a defense mechanism against depression by some. Hypomanic episodes rarely progress to full blown manic episodes. Some people who experience hypomania show increased creativity while others are irritable or demonstrate poor judgment.

Hypomania may feel good to some persons who experience it, though most people who experience hypomania state that the stress of the experience is very painful. Bipolar people who experience hypomania, however, tend to forget the effects of their actions on those around them. Even when family and friends recognize mood swings, the individual will often deny that anything is wrong. What might be called a "hypomanic event", if not accompanied by depressive episodes, is often not deemed problematic, unless the mood changes are uncontrollable, volatile, or mercurial. Most commonly, symptoms continue for a few weeks to a few months.

The following are the revised criteria for a diagnosis of cyclothymic disorder (DSM-IV-TR 301.13) from the "Diagnostic and Statistical Manual of Mental Disorders" (DSM-IV-TR):

- A. For at least 2 years (1 year in children and adolescents), the presence of numerous periods with hypomanic symptoms and numerous periods with depressive symptoms that do not meet criteria for a Major Depressive Episode

- B. During the above period, the person has not been without the symptoms in A for more than 2 months in the 2-year period

- C. No Major Depressive Episode, Manic Episode, or Mixed Episode has been present during the first 2 years of the disturbance.

- D. The symptoms in Criterion A are not better accounted for by Schizoaffective Disorder and are not superimposed on Schizophrenia, Schizophreniform Disorder, Delusional Disorder, or Psychotic Disorder Not Otherwise Specified.

- E. The symptoms are not due to the direct physiological effects of a substance (e.g. drug of abuse, a medication) or a general medical condition (e.g., hyperthyroidism).

- F. The symptoms cause clinically significant distress or impairment in social, occupational, or other important areas of functioning.

The DSM-IV-TR notes that the mood disturbance must be not severe enough to constitute a full-fledged manic or major depressive episode. The diagnosis requires that there be periods of both hypomania and depression and that periods of normal mood not last longer than 2 months.

The DSM-IV-TR also notes that Cyclothymia and borderline personality disorder share similar features and that individuals can be diagnosed with both BPD and Cyclothymia depending on the symptoms they present.

Psychotic symptoms are often missed in psychotic depression, either because patients do not think their symptoms are abnormal or they attempt to conceal their symptoms from others. On the other hand, psychotic depression may be confused with schizoaffective disorder. Due to overlapping symptoms, differential diagnosis includes also dissociative disorders.

A person experiencing a depressive episode may have a marked loss or gain of weight (such as 5% of their body weight in one month) or a change in appetite.

Changes in appetite take on two manifestations: under- or over-eating.

In the first instance, some people never feel hungry, can go long periods without wanting to eat, or may forget to eat; if they do eat, a small amount of food may be sufficient. In children, failure to make expected weight gains may be counted towards this criteria. Under-eating is often associated with a melancholic type of depression.

In the second instance, some people tend toward an increase in appetite and may gain significant amounts of weight. They may crave certain types of food, such as sweets or carbohydrates. People with seasonal affective disorder (SAD) often crave foods high in carbohydrates. Over-eating is often associated with a type of depression called atypical depression.

A person experiencing a major depressive episode may report depressed mood or may appear depressed to others. Often, interest or pleasure in everyday activities is decreased; this is referred to as anhedonia. These feelings must be present on an everyday basis for two weeks or longer to meet DSM-IV criteria for a major depressive episode.

Depressed people may have feelings of guilt that go beyond a normal level or are delusional. Depressed people may think of themselves in very negative, unrealistic ways, such as manifesting a preoccupation with past failures, personalisation of trivial events, or believing that minor mistakes prove their inadequacy. They also may have an unrealistic sense of personal responsibility and see things beyond their control as being their fault. Additionally, self-loathing is common in clinical depression, and can lead to a downward spiral when combined with other symptoms.

Minor depressive disorder is very similar to major depressive disorder in the symptoms present. Generally, a person's mood is affected by thoughts and feelings of being sad or down on themself or by a loss of interest in nearly all activities. People can experience ups and downs in their life everyday where an event, action, stress or many other factors can affect their feelings on that day. However, depression occurs when those feelings of sadness persist for longer than a few weeks.

A person is considered to have minor depressive disorder if they experience 2 to 4 depressive symptoms during a 2-week period. The Diagnostic and Statistical Manual of Mental Disorders lists the major depressive symptoms. Depressed mood most of the day and/or loss of interest or pleasure in normal activities must be experienced by the individual to be considered to have minor depressive disorder. Without either of these two symptoms, the disorder is not classified as minor depressive disorder. Other depressive symptoms include significant weight loss or weight gain without trying to diet (an increase/decrease in appetite can provide clues as well), insomnia or hypersomnia, psychomotor agitation or psychomotor retardation, fatigue or loss of energy, and feelings of worthlessness or excessive guilt.

All of these signs can compound on each other to create the last major symptom group of minor depressive disorder: thoughts of death, suicidal thoughts, plans to commit suicide, or a suicide attempt.

Minor depressive disorder differs from major depressive disorder in the number of symptoms present with 5 or more symptoms necessary for a diagnosis of major depressive disorder. Both disorders require either depressed mood or loss of interest or pleasure in normal activities to be one of the symptoms and the symptoms need to be present for two weeks or longer. Symptoms also must be present for the majority of the length of a day and present for a majority of the days in the two-week period. Diagnosis can only occur if the symptoms cause "clinically significant distress or impairment". Dysthymia consists of the same depressive symptoms, but its main differentiable feature is its longer-lasting nature as compared to minor depressive disorder. Dysthymia was replaced in the DSM-5 by persistent depressive disorder, which combined dysthymia with chronic major depressive disorder.

"Mood disorder due to a general medical condition" is used to describe manic or depressive episodes which occur secondary to a medical condition. There are many medical conditions that can trigger mood episodes, including neurological disorders (e.g. dementias), metabolic disorders (e.g. electrolyte disturbances), gastrointestinal diseases (e.g. cirrhosis), endocrine disease (e.g. thyroid abnormalities), cardiovascular disease (e.g. heart attack), pulmonary disease (e.g. chronic obstructive pulmonary disease), cancer, and autoimmune diseases (e.g. rheumatoid arthritis).

The forefront indication a depressive episode is manifesting is the sudden loss of energy or motivation in daily routines. When this occurs, it is not uncommon for individuals to seek medical attention with excessive worrying or anxiety that a more severe, physiological disease may be the underlying issue. However, without an actual disease present, this neurotic thinking often results in severe anxiety, sleep disturbance, and mood swings which may hinder social relationships. Individuals with endogenous depression may experience inconsistencies in symptom severity which is often the reason for delayed treatment. If left untreated, symptoms may progress to a major depressive episode.

Involutional melancholy's 'course was chronic, with agitation, depersonalization and delusions of bodily change and guilt' featuring strongly, but 'without manic features'. Symptoms of fear are also considered to occur, as well as despondency and hypochondriacal delusions. The late onset of the disorder was matched with a prolonged course with poor prognosis and/or deterioration, in the absence of treatment.

Involutional melancholia or involutional depression is a traditional name for a psychiatric disorder affecting mainly elderly or late middle-aged people, usually accompanied with paranoia. It is classically defined as "depression of gradual onset occurring during the involutional years (40-55 in women and 50-65 in men), with symptoms of marked anxiety, agitation, restlessness, somatic concerns, hypochondriasis, occasional somatic or nihilistic delusions, insomnia, anorexia, and weight loss." Involutional melancholia is not recognized as a psychiatric disorder by the DSM-5, the American Psychiatric Association's (APA) classification and diagnostic tool.

SAD is a type of major depressive disorder, and sufferers may exhibit any of the associated symptoms, such as feelings of hopelessness and worthlessness, thoughts of suicide, loss of interest in activities, withdrawal from social interaction, sleep and appetite problems, difficulty with concentrating and making decisions, decreased sex drive, a lack of energy, or agitation. Symptoms of winter SAD often include oversleeping or difficulty waking up in the morning, nausea, and a tendency to over eat, often with a craving for carbohydrates, which leads to weight gain. SAD is typically associated with winter depression, but springtime lethargy or other seasonal mood patterns are not uncommon. Although each individual case is different, in contrast to winter SAD, people who experience spring and summer depression may be more likely to show symptoms such as insomnia, decreased appetite and weight loss, and agitation or anxiety.

A minority of people with bipolar disorder have high creativity, artistry or a particular gifted talent. Before the mania phase becomes too extreme, its energy, ambition, enthusiasm and grandiosity often bring people with this type of mood disorder life's masterpieces.

Post-schizophrenic depression is a "depressive episode arising in the aftermath of a schizophrenic illness where some low-level schizophrenic symptoms may still be present." Someone that suffers from post-schizophrenic depression experiences both symptoms of depression and can also continue showing mild symptoms of schizophrenia. Unfortunately, depression is a common symptom found in patients with schizophrenia and can fly under the radar for years before others become aware of its presence in a patient. However, very little research has been done on the subject, meaning there are few answers to how it should be systematically diagnosed, treated, or what course the illness will take. Some scientists would entirely deny the existence of post-schizophrenic depression, insisting it is a phase in schizophrenia as a whole. As of late, post-schizophrenic depression has become officially recognized as a syndrome and is considered a sub-type of schizophrenia.

Often in those who have experienced their first episode of hypomania – generally without psychotic features – there may be a long or recent history of depression or a mix of hypomania combined with depression (known as mixed-state) prior to the emergence of manic symptoms. This commonly surfaces in the mid to late teens. Because the teenage years are typically an emotionally charged time of life, it is not unusual for mood swings to be passed off as normal hormonal teen behavior and for a diagnosis of bipolar disorder to be missed until there is evidence of an obvious manic or hypomanic phase.

Hypomania may also occur as a side effect of pharmaceuticals prescribed for conditions or diseases other than psychological states or mood disorders. In those instances, as in cases of drug-induced hypomanic episodes in unipolar depressives, the hypomania can almost invariably be eliminated by lowering medication dosage, withdrawing the drug entirely, or changing to a different medication if discontinuation of treatment is not possible.

Hypomania may also be triggered by the occurrence of a highly exciting event in the patient's situation, such as a substantial financial gain or recognition.

Hypomania can be associated with narcissistic personality disorder.

To meet criteria for a major depressive episode, a patient must have 5 of these 9 symptoms nearly every day for at least 2 weeks.

1. Depressed or sad mood

2. Anhedonia (loss of interest in pleasurable activities)

3. Sleep disturbance (increased or decreased sleep)

4. Appetite disturbance (increased or decreased appetite) typically with weight change

5. Energy disturbance (increased or decreased energy/activity level), usually fatigue

6. Poor memory and/or concentration

7. Feelings of guilt or worthlessness

8. Psychomotor retardation or agitation (a change in mental and physical speed perceived by other people)

9. Thoughts of wishing you were dead; suicidal ideation or suicide attempts

Endogenous depression was initially considered valuable as a means of diagnostic differentiation with reactive depression. While the latter's onset could be attributed to adverse life events and treated with talk therapy, the former would indicate treatment with antidepressants. Indeed, this view of endogenous depression is at the root of the popular view that mood disorders are a reflection of a 'chemical imbalance' in the brain. More recent research has shown that the probability of an endogenous depression patient experiencing an adverse life event prior to a depressive episode is roughly the same as for a reactive depression patient and the efficacy of antidepressant therapy bears no statistical correlation with the patient's diagnostic classification along this axis.