Millions of children in the United States live with depression, anxiety, ADHD, autism spectrum disorders and countless other mental health issues with varying levels of severity. As a pediatric and adolescent psychiatrist explains some signs and symptoms of mental health issues in children and teenagers.

"Mental health can impact how children and teenagers learn and interact with the world," said Dr. Andrew Harper, child and adolescent psychiatrist with the Texas A&M Outpatient Psychiatry Clinic and clinical professor at the Texas A&M College of Medicine.

"Parents and teachers alike need to be aware of the major warning signs that a child might be struggling with mental illness. The sooner a mental illness is diagnosed and treated, then the sooner the child can get back on track to develop into a healthy adult."

Mental health disorders in children and teenagers

"The most common mood problems in children are depression and anxiety," Harper said. "Every child will feel depressed and anxious at times. However, if their feelings last longer than a few weeks or interfere with their life, then you may need to seek help from your health care provider."

Anxiety disorders in children can range from post-traumatic stress disorder to separation anxiety. Disorders such as depression and anxiety present differently in different children. It's important to be aware of what is normal for your child and what isn't.

Many children and teenagers also struggle with behavioral mental health conditions such as attention deficient/hyperactivity disorder (ADHD). "Most often, behavioral problems are first noticed around the time the child starts going to school," Harper said. "Yet, it is not uncommon for older children -- or even sometimes teenagers and adults -- to be diagnosed with a behavioral disorder."

Most of these mental health issues come to fruition as the child enters school. However, some children struggle with mental health issues such as autism spectrum disorder since birth. If your young child is not connecting with you or does not respond appropriately to stimuli, then he or she may need to be evaluated by a professional.

Sudden mood changes

Children with a mood disorder such as depression may feel very sad for extended periods of time without a clear precipitant or event.

"If they lose a beloved pet or experience a breakup, then those feelings of sadness are normal," Harper said. "If the child does not bounce back from a sad event or if the change in mood seemingly happens for no reason, then speak to your child to see what is wrong."

The difference between a normal teenage mood swing and something more serious like depression. The difference lies in the severity and duration of the feelings. If the feelings impact the child's thoughts, social behaviors and feelings for more than two weeks, then it is not a mood swing.

Declining grades

"Another sign of a mental illness in a child or teenager is a sudden change in performance at school," Harper said. If this happens, Harper also recommends you speak with your child.

"There are many reasons your child acts the way they act, and the only way you can know for sure is if you ask." A bully could be preventing him or her from focusing in class. Or, your child's grades may drop because he or she no longer can see the board and needs an eyeglass prescription adjusted. However, if there isn't some simple explanation, it might be time for a visit to a primary care provider to see if there is a physical or mental condition that might be affecting his or her school performance.

Changes in behavior

There are many behavioral changes that can signify your child is suffering from a mental health issue. A few examples are:

• Inability to concentrate

• Aggressive or disruptive behavior

• Suddenly socially withdrawn, when they were previously very outgoing

• Defiant or oppositional behavior, when they were previously compliant with the rules

"Parents and teachers may notice these changes in behavior. Children and teenagers may also change friend groups or start to abuse substances," Harper said. "They may also notice a change in sleep and eating behaviors."

Does my child need help?

If you suspect your child is suffering from a mental health issue, Harper recommends the first thing you do is create an open dialogue with your child.

"Depending on the issue, they may be hesitant to speak with a parent. If you try and do not succeed, then do not force the conversation," Harper said. "At this point, perhaps setting your child up with a health care provider may be the best course of action. They may feel more comfortable."

All mental illnesses are treatable, whether through psychotherapy, medication or a combination of the two treatments. If you have questions, then use your available resources.

"The school counselor or your primary care provider are great first steps to getting help," Harper said. "You have a team of people ready to help your child with his or her mental health."

Mary Leigh Meyer writes for the Texas A&M University Health Science Center.