We all have someone who'd explain their situation this way - I feel ill and my doctor can't find out what's wrong. All my tests look okay. Is this some kind of a disease that does not have any diagnosis? If you are looking for an answer then perhaps this read could help! Sometimes, people have symptoms deemed medically unexplained, which is typically known as Psychosomatic Disorder. You only realise that you have a problem when you suffer from significant physical symptoms disturbing your lifestyle or making it difficult for you to function. Just to clarify that these issues are sometimes not neurological disorders. They belong to the fields of psychology and psychiatry.
Disease and Illness The Difference Yes, emotional distress can cause Psychosomatic Disorders and the two most common psychosomatic symptoms are chronic fatigue and pain. These are difficult symptoms to assess because they cannot be objectively measured, they can only be described. As per the findings, up to one-third of people seen in an average general neurology clinic have neurological symptoms that can't be explained and, in those people, an emotional cause is often suspected. To understand this better it's important to know the difference between disease and illness. The disease is an organic issue which is diagnosed and illness is produced by the symptoms it causes. Now, how can we identify if the issue is more than an organic issue? Here is an example. Generally, disease and illness come together. You have a Gut issue and you will have an illness (symptoms of issues with your digestion etc). Someone could have a disease but due to medication might suffer mind or no illness. In some cases, you could have a disease and illness (symptoms) both, but in case the illness goes beyond the diagnosed scope and medication, then this is something to think about. Imagine someone who has an underactive thyroid gland that causes fatigue. 
They are on medication and blood tests show that the treatment has returned their thyroid hormone level to normal. We might expect this person to have minimal fatigue symptoms of thyroid disease. However, if that person has crippling tiredness which the thyroid disease does not fully explain, then that tiredness might be called psychosomatic even though there is a known underlying medical condition. Take a note of this - when you suppress stress, it leaks out in the form of physical symptoms. Stress or allostatic load can impair your cognitive ability? Yes! Lomg term stress can result in structural changes in your brain? Yes! Another example that is becoming quite common is that people with Epilepsy sometimes go through the wrong diagnosis and pills fail to work on them. Why? The seizures could be disassociative seizures which are more psychological, not necessarily due to the electrical activity issues with the brain. Yes, your mind has the ability to create seizures! Your body can respond to upset producing blackouts and convulsions. This sort of convulsion is known as a disassociative seizure (just an example!). Often the physical symptoms are there in place of the emotional upset. So for eg, if there is a memory or emotion that is too painful for a person to experience or recall, that emotion is converted into a physical disability as a sort of protective mechanism. In a convulsion, it is as if your brain is shutting down for a minute to keep you safe. If you did read the above paragraph carefully, you must have noticed some ear-prick up words - too painful to recall, protective, safe. Psychosomatic disorders are psychological and they exist in someone's life with a purpose or a role. And if you are one of them, you may want to think about the 3Ps involved in WHY you may have those symptoms, without a diagnosed disease? There are only 3Ps (roles) psychosomatic disorders play Protect (safe), Punish or Prioritise (attention). For eg for a person with OCD, the purpose is mainly to either protect them or get them attention. Also, the tendency to somatize often begins in childhood. Recurrent abdominal pain is common in children but the organised cause is found in fewer than ten per cent of those affected. Conversion Disorders occur when distressing emotions or traumatic events cant be voiced, and some treatments (therapies) focus on finding a way to release that painful voice or emotions. We have seen similar cases with IBS and also chronic fatigue syndrome (CFS) where especially in the latter, it's mainly associated with Psychosomatic Disorder. What's the cure? Usually, people who live longer with psychosomatic disorders will have less percentage of recovery as the body goes into homeostasis for a more extended period of time (repeating or reliving the traumatic experience of the past-producing chemical reactions of homeostasis). Plus, most of the patients are totally in denial and refuse to get help from a psychiatrist or psychologist. If we have a disease it's easier to convince people (and ourselves!) about our dysfunctionality, but the same is problematic if that's arising due to psychological issues in our minds. Plus stigmatizing the whole condition doesn't help the process of seeking help.