Hyperthyroidism is a condition in which the thyroid gland produces too much thyroid hormone, thyroxin. This overabundance of thyroxin causes irregular and rapid heartbeat, irritability, weight loss, nervousness, anxiety and racing thoughts.

The anxiety and inability to focus is very common in hyperthyroidism and leads to racing thoughts, as well as panic attacks and difficulty concentrating.

Racing thoughts may be experienced as background or take over a person's consciousness. Thoughts, music, and voices might be zooming through one's mind as they jump tangentially from one to the next. There also might be a repetitive pattern of voice or of pressure without any associated "sound". It is a very overwhelming and irritating feeling, and can result in losing track of time. In some cases, it may also be frightening to the person experiencing it, as there is a loss of control. If one is experiencing these thoughts at night when going to sleep, they may suddenly awaken, startled and confused by the very random and sudden nature of the thoughts.

Racing thoughts differs in manifestation according to the individual's perspective. These manifestations can vary from unnoticed or minor distractions to debilitating stress, preventing the sufferer from maintaining a thought.

Generally, racing thoughts are described by an individual who has had an episode where the mind uncontrollably brings up random thoughts and memories and switches between them very quickly. Sometimes they are related, as one thought leads to another; other times they seem completely random. A person suffering from an episode of racing thoughts has no control over his or her train of thought, and it stops them from focusing on one topic or prevents sleeping.

Many people experience the type of bad or unwanted thoughts that people with more troubling intrusive thoughts have, but most people can dismiss these thoughts. For most people, intrusive thoughts are a "fleeting annoyance". Psychologist Stanley Rachman presented a questionnaire to healthy college students and found that virtually all said they had these thoughts from time to time, including thoughts of sexual violence, sexual punishment, "unnatural" sex acts, painful sexual practices, blasphemous or obscene images, thoughts of harming elderly people or someone close to them, violence against animals or towards children, and impulsive or abusive outbursts or utterances. Such bad thoughts are universal among humans, and have "almost certainly always been a part of the human condition".

When intrusive thoughts occur with obsessive-compulsive disorder (OCD), patients are less able to ignore the unpleasant thoughts and may pay undue attention to them, causing the thoughts to become more frequent and distressing. The thoughts may become obsessions which are paralyzing, severe, and constantly present, and can range from thoughts of violence or sex to religious blasphemy. Distinguishing them from normal intrusive thoughts experienced by many people, the intrusive thoughts associated with OCD may be anxiety provoking, irrepressible, and persistent.

How people react to intrusive thoughts may determine whether these thoughts will become severe, turn into obsessions, or require treatment. Intrusive thoughts can occur with or without compulsions. Carrying out the compulsion reduces the anxiety, but makes the urge to perform the compulsion stronger each time it recurs, reinforcing the intrusive thoughts. According to Lee Baer, suppressing the thoughts only makes them stronger, and recognizing that bad thoughts do not signify that one is truly evil is one of the steps to overcoming them. There is evidence of the benefit of acceptance as an alternative to suppression of intrusive thoughts. A study showed that those instructed to suppress intrusive thoughts experienced more distress after suppression, while patients instructed to accept the bad thoughts experienced decreased discomfort. These results may be related to underlying cognitive processes involved in OCD. However, accepting the thoughts can be more difficult for persons with OCD. In the 19th century, OCD was known as "the doubting sickness"; the "pathological doubt" that accompanies OCD can make it harder for a person with OCD to distinguish "normal" intrusive thoughts as experienced by most people, causing them to "suffer in silence, feeling too embarrassed or worried that they will be thought crazy".

The possibility that most patients suffering from intrusive thoughts will ever act on those thoughts is low. Patients who are experiencing intense guilt, anxiety, shame, and upset over these thoughts are different from those who actually act on them. The history of violent crime is dominated by those who feel no guilt or remorse; the very fact that someone is tormented by intrusive thoughts and has never acted on them before is an excellent predictor that they will not act upon the thoughts. Patients who are not troubled or shamed by their thoughts, do not find them distasteful, or who have actually taken action, might need to have more serious conditions such as psychosis or potentially criminal behaviors ruled out. According to Lee Baer, a patient should be concerned that intrusive thoughts are dangerous if the person does not feel upset by the thoughts, or rather finds them pleasurable; has ever acted on violent or sexual thoughts or urges; hears voices or sees things that others do not see; or feels uncontrollable irresistible anger.

Generalized anxiety disorder (GAD) is a common disorder, characterized by long-lasting anxiety that is not focused on any one object or situation. Those suffering from generalized anxiety disorder experience non-specific persistent fear and worry, and become overly concerned with everyday matters. Generalized anxiety disorder is "characterized by chronic excessive worry accompanied by three or more of the following symptoms: restlessness, fatigue, concentration problems, irritability, muscle tension, and sleep disturbance". Generalized anxiety disorder is the most common anxiety disorder to affect older adults. Anxiety can be a symptom of a medical or substance abuse problem, and medical professionals must be aware of this. A diagnosis of GAD is made when a person has been excessively worried about an everyday problem for six months or more. A person may find that they have problems making daily decisions and remembering commitments as a result of lack of concentration/preoccupation with worry. Appearance looks strained, with increased sweating from the hands, feet, and axillae, and they may be tearful, which can suggest depression. Before a diagnosis of anxiety disorder is made, physicians must rule out drug-induced anxiety and other medical causes.

In children GAD may be associated with headaches, restlessness, abdominal pain, and heart palpitations. Typically it begins around 8 to 9 years of age.

The criteria below are based on the formal DSM-IV criteria for a major depressive episode. A diagnosis of major depressive episode requires that the patient has—over a two-week period—experienced five or more of the symptoms below, and these must be outside the patient's normal behaviour. Either depressed mood or decreased interest or pleasure must be one of the five (although both are frequently present).

The single largest category of anxiety disorders is that of specific phobias which includes all cases in which fear and anxiety are triggered by a specific stimulus or situation. Between 5% and 12% of the population worldwide suffer from specific phobias. Sufferers typically anticipate terrifying consequences from encountering the object of their fear, which can be anything from an animal to a location to a bodily fluid to a particular situation. Common phobias are flying, blood, water, highway driving, and tunnels. When people are exposed to their phobia, they may experience trembling, shortness of breath, or rapid heartbeat. People understand that their fear is not proportional to the actual potential danger but still are overwhelmed by it.

An intrusive thought is an unwelcome involuntary thought, image, or unpleasant idea that may become an obsession, is upsetting or distressing, and can feel difficult to manage or eliminate. When such thoughts are associated with obsessive-compulsive disorder (OCD), depression, body dysmorphic disorder (BDD), and sometimes attention-deficit hyperactivity disorder (ADHD), the thoughts may become paralyzing, anxiety-provoking, or persistent. Intrusive thoughts may also be associated with episodic memory, unwanted worries or memories from OCD, posttraumatic stress disorder, other anxiety disorders, eating disorders, or psychosis. Intrusive thoughts, urges, and images are of inappropriate things at inappropriate times, and generally have aggressive, sexual, or blasphemous themes.

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.

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.

Behaviors that are apparent in those with internalizing disorders include depression, withdrawal, anxiety, and loneliness. There are also behavioral characteristics involved with internalizing disorders. Some behavioral abnormalities include poor self-esteem, suicidal behaviors, decreased academic progress, and social withdrawal. Internalizing one's problems, like sadness, can cause the problems to grow into larger burdens such as social withdrawal, suicidal behaviors or thoughts, and other unexplained physical symptoms.

It is during depressive episodes that BP-II patients often seek help. Symptoms may be syndromal or subsyndromal. Depressive BP-II symptoms may include five or more of the below symptoms (at least one of them must be either depressed mood or loss of interest/pleasure). In order to be diagnosed, they need to be present only during the same two-week period, as a change from previous hypomanic functioning:

- Depressed mood most of the day, nearly every day, as indicated by either subjective report (e.g., feels sad, empty, or hopeless) or observation made by others (e.g., appears tearful). (Note: In children and adolescents, can be irritable mood.)

- Markedly diminished interest or pleasure in all, or almost all, activities most of the day, nearly every day (as indicated by either subjective account or observation).

- Significant weight loss when not dieting or weight gain (e.g., a change of more than 5% of body weight in a month), or decrease or increase in appetite nearly every day. (Note: In children, consider failure to make expected weight gain.)

- Insomnia or hypersomnia nearly every day.

- Psychomotor agitation or retardation nearly every day (observable by others; not merely subjective feelings of restlessness or being slowed down).

- Fatigue or loss of energy nearly every day.

- Feelings of worthlessness or excessive or inappropriate guilt nearly every day (not merely self-reproach or guilt about being sick).

- Diminished ability to think or concentrate, possible irritability or indecisiveness, nearly every day (either by subjective account or as observed by others).

- Recurrent thoughts of death (not just fear of dying), recurrent suicidal ideation without a specific plan, a suicide attempt, or a specific plan for committing suicide.

Evidence also suggests that BP-II is strongly associated with atypical depression. Essentially, this means that many BP-II patients exhibit reverse vegetative symptoms. BP-II patients may have a tendency to oversleep and overeat, while typically depressed patients sleep and eat less than usual.

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.

As with other anxiety disorders, children with SAD face more obstacles at school than those without anxiety disorders. Adjustment and relating school functioning have been found to be much more difficult for anxious children. In some severe forms of SAD, children may act disruptively in class or may refuse to attend school altogether. It is estimated that nearly 75% of children with SAD exhibit some form of school refusal behavior.

This is a serious problem because, as children fall further behind in coursework, it becomes increasingly difficult for them to return to school.

Short-term problems resulting from academic refusal include poor academic performance or decline in performance, alienation from peers, and conflict within the family.

Although school refusal behavior is common among children with SAD, it is important to note that school refusal behavior is sometimes linked to generalized anxiety disorder or possibly a mood disorder. That being said, a majority of children with separation anxiety disorder have school refusal as a symptom. Up to 80% of children who refuse school qualify for a diagnosis of separation anxiety disorder.

The internalizing disorders, with high levels of negative affectivity, include depressive disorders, anxiety disorders, obsessive-compulsive and related disorders, trauma and stressor-related disorders, and dissociative disorders. Others like bulimia, and anorexia also come under this category.

Depression is a state of low mood and aversion to activity. It may be a normal reaction to occurring life events or circumstances, a symptom of a medical condition, a side effect of drugs or medical treatments, or a symptom of certain psychiatric syndromes, such as the mood disorders major depressive disorder and dysthymia. Depression in childhood and adolescence is similar to adult major depressive disorder, although young sufferers may exhibit increased irritability or aggressive and self-destructive behavior, rather than the all-encompassing sadness associated with adult forms of depression. Children who are under stress, experience loss, or have attention, learning, behavioral, or anxiety disorders are at a higher risk for depression. Childhood depression is often co-morbid with mental disorders outside of other mood disorders; most commonly anxiety disorder and conduct disorder. Depression also tends to run in families. Psychologists have developed different treatments to assist children and adolescents suffering from depression, though the legitimacy of the diagnosis of childhood depression as a psychiatric disorder, as well as the efficacy of various methods of assessment and treatment, remains controversial.

Just how SAD affects a child's attendance and participation in school, their avoidance behaviors stay with them as they grow and enter adulthood. Recently, "the effects of mental illness on workplace productivity have become a prominent concern on both the national and international fronts". In general, mental illness is a common health problem among working adults, 20 to 30% of adults will suffer from at least one psychiatric disorder. Mental illness is linked to decreased productivity, and with individuals diagnosed with SAD their levels at which they function decreases dramatically resulting in partial work-days, increase in number of total absences, and "holding back" when it comes to carrying out and completing tasks.

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.

Minor depressive disorder, also known as minor depression, is a mood disorder that does not meet the full criteria for major depressive disorder but at least two depressive symptoms are present for two weeks. These symptoms can be seen in many different psychiatric and mental disorders, which can lead to more specific diagnoses of an individual's condition. However, some of the situations might not fall under specific categories listed in the "Diagnostic and Statistical Manual of Mental Disorders". Minor depressive disorder is an example of one of these nonspecific diagnoses, as it is a disorder classified in the DSM-IV-TR under the category Depressive Disorder Not Otherwise Specified (DD-NOS). The classification of NOS depressive disorders is up for debate. Minor depressive disorder as a term was never an officially accepted term, but was listed in Appendix B of the DSM-IV-TR. This is the only version of the DSM that contains the term, as the prior versions and the most recent edition, DSM-5, does not mention it.

A person is considered to have minor depressive disorder if they experience 2 to 4 depressive symptoms, with one of them being either depressed mood or loss of interest or pleasure, during a 2-week period. The person must not have experienced the symptoms for 2 years and there must not have been one specific event that caused the symptoms to arise. Although not all cases of minor depressive disorder are deemed in need of treatment, some cases are treated similarly to major depressive disorder. This treatment includes cognitive behavioral therapy (CBT), anti-depressant medication, and combination therapy. A lot of research supports the notion that minor depressive disorder is an early stage of major depressive disorder, or that it is simply highly predictive of subsequent major depressive disorder.

OCD can present with a wide variety of symptoms. Certain groups of symptoms typically occur together. These groups are sometimes viewed as dimensions or clusters that may reflect an underlying process. The standard assessment tool for OCD, the Yale–Brown Obsessive Compulsive Scale (Y-BOCS), has 13 predefined categories of symptoms. These symptoms fit into three to five groupings. A meta analytic review of symptom structures found a four factor structure(grouping) to be most reliable. The observed groups included a "symmetry factor", a "forbidden thoughts factor", a "cleaning factor" and a "hoarding factor". The "symmetry factor" correlated highly with obsessions related to ordering, counting, symmetry as well as repeating compulsions. The "forbidden thoughts factor" correlated highly with intrusive and distressing thoughts of a violent, religious or sexual nature. The "cleaning factor" correlated highly with obsessions about contamination and compulsions related to cleaning. The "hoarding factor" only involved hoarding related obsessions and compulsions, and was identified as being distinct from other symptom groupings.

While OCD has been considered a homogenous disorder from a neuropsychological perspective, many of the putative neuropsychological deficits may be due to comorbid disorders. Furthermore, some subtypes have been associated with improvement in performance on certain tasks such as pattern recognition(washing subtype) and spatial working memory(obsessive thought subtype). Subgroups have also been distinguished by neuroimaging findings and treatment response. Neuroimaging studies on this have been too few, and the subtypes examined have differed too much to draw any conclusions. On the other hand, subtype dependent treatment response has been studied, and the hoarding subtype has consistently responded least to treatment.

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.

Whereas vanity involves a quest to aggrandize the appearance, BDD is experienced as a quest to merely normalize the appearance. Although delusional in about one of three cases, the appearance concern is usually nondelusional, an overvalued idea.

The bodily area of focus can be nearly any, yet is commonly face, hair, stomach, thighs, or hips. Some half dozen areas can be a roughly simultaneous focus. Many seek dermatological treatment or cosmetic surgery, which typically do not resolve the distress. On the other hand, attempts at self-treatment, as by skin picking, can create lesions where none previously existed.

BDD shares features with obsessive-compulsive disorder, but involves more depression and social avoidance. BDD often associates with social anxiety disorder. Some experience delusions that others are covertly pointing out their flaws. Cognitive testing and neuroimaging suggest both a bias toward detailed visual analysis and a tendency toward emotional hyperarousal.

Most generally, one experiencing BDD ruminates over the perceived bodily defect up to several hours daily, uses either social avoidance or camouflaging with cosmetics or apparel, repetitively checks the appearance, compares it to that of other persons, and might often seek verbal reassurances. One might sometimes avoid mirrors, repetitively change outfits, groom excessively, or restrict eating.

BDD's severity can wax and wane, and flareups tend to yield absences from school, work, or socializing, sometimes leading to protracted social isolation, with some becoming housebound for extended periods. Social impairment is usually greatest, sometimes approaching avoidance of all social activities. Poor concentration and motivation impair academic and occupational performance. The distress of BDD tends to exceed that of either major depressive disorder or type-2 diabetes, and rates of suicidal ideation and attempts are especially high.

In psychiatry, thought withdrawal is the delusional belief that thoughts have been 'taken out' of the patient's mind, and the patient has no power over this. It often accompanies thought blocking. The patient may experience a break in the flow of their thoughts, believing that the missing thoughts have been withdrawn from their mind by some outside agency. This delusion is one of Schneider's first rank symptoms for schizophrenia. Because thought withdrawal is characterized as a delusion, according to the DSM-IV TR it represents a positive symptom of schizophrenia.

OCD is often confused with the separate condition obsessive–compulsive personality disorder (OCPD). OCD is egodystonic, meaning that the disorder is incompatible with the sufferer's self-concept. Because ego dystonic disorders go against a person's self-concept, they tend to cause much distress. OCPD, on the other hand, is egosyntonic—marked by the person's acceptance that the characteristics and behaviours displayed as a result are compatible with their self-image, or are otherwise appropriate, correct or reasonable.

As a result, people with OCD are often aware that their behavior is not rational, are unhappy about their obsessions but nevertheless feel compelled by them. By contrast people with OCPD are not aware of anything abnormal; they will readily explain why their actions are rational, it is usually impossible to convince them otherwise, and they tend to derive pleasure from their obsessions or compulsions.

In ICD-10, the latest version of the International Statistical Classification of Diseases and Related Health Problems, somatization syndrome is described as:

ICD-10 also includes the following subgroups of somatization syndrome:

- Undifferentiated somatoform disorder

- Hypochondriasis

- Somatoform autonomic dysfunction

- Persistent somatoform pain disorder

- Other somatoform disorders, such ones predominated by dysmenorrhoea, dysphagia, pruritus and torticollis

- Somatoform disorder, unspecified

Primarily cognitive obsessive-compulsive disorder (also commonly called "primarily obsessional OCD", purely obsessional OCD, Pure-O, OCD without overt compulsions or with covert compulsions) is a lesser-known form or manifestation of OCD. For people with primarily obsessional OCD, there are fewer observable compulsions, compared to those commonly seen with the typical form of OCD (checking, counting, hand-washing, etc.). While ritualizing and neutralizing behaviors do take place, they are mostly cognitive in nature, involving mental avoidance and excessiverumination. Primarily obsessional OCD often takes the form of intrusive thoughts of a distressing or violent nature.