The child diagnosed with ADHD has got quite a lot to deal with, so uncertainty over abilities, moodiness, and sadness go hand in hand with the impulsivity, distractibility, inattention, and hyperactivity.
Some children can develop both disorders at the same time–this is called “ADHD comorbid with __________." Treatment of a child who has two mental disorders differs from that of a child diagnosed solely with ADHD.
ADHD or Bipolar Disorder
Diagnosing ADHD can be difficult because there are no reliable blood tests or X-rays that point out the disorder. Imagine the doctor’s quandary when he is presented with a child who appears to have ADHD mood disorders. The psychiatrist must now differentiate between between consistency and intensity of the symptoms in this child.
Children with ADHD are diagnosed with the disorder only after several tests, including psychological testing. The child’s symptoms must be present for at least six months and they must be present to the extent that the child’s ability to function in more than one environment is impaired.
ADHD and bipolar disorder share common symptoms–energy bursts, inattention, talkativeness and restlessness. For the child who suffers from bipolar disorder, what makes these “in-common" symptoms different is the intensity of the symptoms. In addition, the periods of mania and depression can last for several weeks at a time; conversely, the child may experience symptom-free periods, writes ADDitudeMag. 
Description of the Symptoms of Both Disorders
Symptoms of bipolar mood disorder (BMD: high-energy episodes, or mania, interspersed with periods of low energy or depression; intense feelings of happiness or sadness; child’s mood changes are unrelated to life events.
Symptoms of ADHD: Distractibility, inattention, impulsivity and physical restlessness. Children who are diagnosed with ADHD must show symptoms by the age of seven; ADHD is chronic (present every day of the child’s life); moods are influenced by triggers in the child’s life; ADHD mood shifts happen quickly, feeling instantaneous; in the child with ADHD, mood shifts are measured by hours, not days or weeks, states ADDitudeMag. 
ADHD or Major Depressive Disorder
Children with ADHD have moods that run the gamut from happy to irritable to sad. The child with major depressive disorder (depression) is nearly always irritable or depressed, expressing little to no interest or pleasure in almost any activity.
The child with MDD may cry every day, becomes very self-critical, withdraws from others and talks of dying. When the child suffers from MDD, the ability to concentrate differs from the child with ADHD because it is accompanied by other symptoms of depression–lack of appetite or loss of interest in previously favored activities.
MDD occurs “more frequently" in children diagnosed with ADHD. However, it usually develops several years after the child has been diagnosed with ADHD. Because children with ADHD struggle to complete tasks or assignments, they may exhibit sadness or discouragement more frequently than children who do not have ADHD, advises Healthy Children. 
ADHD or Dysthymic Disorder
The child who appears irritable or sad for several hours during the day, and whose sadness is apparent on more days than it is not, should be evaluated for an underlying dysthymic disorder. Dysthymic disorder is a “low-grade" chronic depression, marked by feelings of low self esteem and demoralization. As with MDD, dysthymic disorder is usually diagnosed several years after the child receives a diagnosis of ADHD.
A child with dysthymic disorder must experience at least two of the following symptoms: feelings of hopelessness, poor appetite/overeating, poor concentration/inability to make decisions, excessive sleeping or insomnia, low self-esteem and low energy or fatigue. The doctor needs to determine that the child’s symptoms are not related to ADHD, according to Healthy Child. 
Children with ADHD mood disorders have additional challenges to confront and overcome–the parents need to be attentive to their child’s moods and the consistency of these moods. ADHD can exist in the child alongside mood disorders, which makes the correct treatment an imperative.
 Dr. William Dodson. Is It Bipolar Disorder or ADHD? ADDitudeMag. August/September 2007, retrieved at https://www.additudemag.com/adhd/article/2511.html
 Dr. William Dodson. Bipolar Mood Disorder and ADHD. ADDitudeMag, retrieved at https://www.additudemag.com/adhd-web/article/4861.html
 Mood Disorders. Healthy Children, retrieved at https://www.healthychildren.org/English/health-issues/conditions/emotional-problems/pages/Mood-Disorders.aspx