How an Organic Disease Differs From a Functional Disorder

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A disease can sometimes be described as being organic or functional. By the broadest definition:

  • An organic disease is one in which there is a measurable change to tissues, organs, or body systems.
  • A functional disease is one in which there are abnormal symptoms or changes in function but with no measurable changes in tissues.

The key difference between an organic and functional disease is that the cause and pathophysiology (disease pathway) of an organic disease have largely been established, while the cause and pathophysiology of a functional disease have largely not.

Many diseases fall under the umbrella of being either organic or functional. The terms may not only be applied to physiological conditions (related to the body) but also to mental health conditions.

This article explains the difference between organic and functional diseases and why the distinction is important in medical practice.

Doctor checking X-Ray with patient
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Distinguishing Between an Organic and Functional Disease

The distinction between an organic or functional disease is central to the diagnosis of illnesses. The aim is to distinguish symptoms and syndromes (groups of symptoms) that can be explained by diagnosable biological changes and those that cannot.

For instance, persistent tiredness may be due to an organic cause like anemia, which can be definitively diagnosed with a red blood cell (RBC) count. Or, it could be due to a functional cause like chronic fatigue syndrome for which no measurable test is available.

This is not to suggest that the lack of testing means that a functional disease is "less real" than an organic disease. As the scientific understanding of a disease advances, the way in which that disease is categorized can change.

Epilepsy is one such example. While the seizure disorder was once considered functional, it is now regarded as an organic neurological condition.

Determining whether the cause is organic or functional is central to delivering the appropriate treatment. It is not always a straightforward process.

Organic Diseases

Organic diseases describe a broad category of conditions in which tissues or organs are affected, leading to symptoms and illness. These include infectious diseases, inflammatory diseases, metabolic disorders, neurological diseases, autoimmune disorders, and cancer.

With an organic disease, there will be tests and procedures that can detect or measure biological and/or anatomical changes that lead to illness or dysfunction. These may involve blood tests, imaging studies, medical instruments, or surgical procedures.

Examples include:

This is not to suggest that all organic diseases can be diagnosed in a straightforward and definitive manner. Some require multiple tests to support the diagnosis when a single test is unable to do so.

One example is celiac disease (CD). In addition to antibody tests, a small bowel biopsy, and a gluten-free diet, genetics tests called HLA-DQ2 and HLA-DQ8 may be used to see if you have a genetic predisposition for CD. While the tests are not diagnostic of CD (you can have these mutations and not have CD), the absence of mutations is uncommon in CD.

Other organic diseases may also require complicated testing to not only evidence the disease but also rule out other possible causes–including functional ones.

Functional Diseases

Broadly speaking, a functional disease is one in which there is no diagnosable pathophysiology to account for your symptoms. It is unclear where the problem is originating and how exactly it causes symptoms.

A functional disease may occur when an organ doesn't function as it should or there is some sort of miscommunication between systems (such as the brain and the gut). The symptoms are generally persistent and cause significant impairment or disability. While the symptoms may be disruptive, they are generally not life-threatening.

Because there are no tests to definitively diagnose a functional disease, the diagnosis is usually based on a set of criteria issued by a health authority. The criteria can change and evolve as the understanding of the disease expands.

The term "functional disorder" is generally taken to mean a functional somatic disorder (meaning one involving the body). The term can also be applied to many, but not all, psychiatric illnesses.

Functional Somatic Disorders

There are many conditions classified as functional somatic disorders (FSDs). These can cause symptoms limited to certain organs or affect the body as a whole.

Functional somatic disorders should not be confused with somatic symptom disorder (SSD). SSD is a condition in which persistent physical symptoms are due to psychological problems.

Examples of conditions classified as FSDs include:

It is not uncommon for some of these disorders to overlap. Fibromyalgia and ME/CFS frequently co-occur, suggesting that the two disorders have similar disease origins or pathways.

However, the classification of both ME/CFS and fibromyalgia as FSDs is controversial. Some researchers consider them to be organic diseases and argue that viewing them as psychosomatic can be harmful to those with the conditions.

Debating Psychiatric Illness as a Functional Condition

While psychiatric illnesses meet the strict definition of a functional disorder, there is considerable debate as to whether the term is appropriate in this setting.

Psychiatric conditions—ranging from depression and bipolar disorder to schizophrenia and post-traumatic distress syndrome (PTSD)—remain poorly understood and are diagnosed based on criteria issued by the American Psychiatric Association (APA).

However, many mental health experts argue that classifying them as "functional" suggests that the causes are inherently unknown and the conditions "appear out of nowhere."

Certainly, with conditions like depression and PTSD, scientists recognize that environmental, social, emotional, and neurological factors contribute to the onset of these conditions. Moreover, the drugs used to treat depression and attention-deficit hyperactivity disorder (ADHD) have specific neuropsychiatric effects that are no more or less consistent than with any other drug.

Even with respect to functional disorders, experts question whether they should be strictly classified as "somatic" and "mental" given that FSD and SSD frequently overlap. In 2020, experts in Europe recommended that "functional somatic disorders" neither be classified as purely somatic nor purely mental, but ones that occupy a neutral space in between.

Summary

Strictly speaking, an organic disease is one in which there are measurable biological changes in tissues and organs that lead to symptoms. Examples include infections, autoimmune diseases, and cancer.

A functional disease is one in which there are symptoms but with no measurable changes in tissues. Examples include fibromyalgia, chronic fatigue syndrome, and irritable bowel syndrome.

With an organic disease, the cause and disease pathway have largely been established, while the cause and disease pathway of a functional disease is often less understood.

With an organic disease, the diagnosis can be confirmed with specific tests like blood tests and medical imaging. With a functional disease, there are no available tests and the diagnosis is based largely on diagnostic criteria issued by an established health authority.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Barbara Bolen, PhD
Barbara Bolen, PhD, is a licensed clinical psychologist and health coach. She has written multiple books focused on living with irritable bowel syndrome.