ADHD And Serotonin: What Science Says About The Link

Typically characterized by symptoms of inattention, hyperactivity, and impulsivity, attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder affecting approximately 129 million children and adolescents and 366 million adults worldwide. 

While the exact cause of ADHD remains elusive, researchers speculate that it may be a combination of genetics, environmental factors, and brain physiology, including neurotransmitter function in the brain. Serotonin has emerged as a possible player in understanding the neurochemistry underlying ADHD.

What is serotonin and why it matters in ADHD

Often called the "feel-good" neurotransmitter, serotonin is primarily associated with mood control, sleep, and sexual desire. Serotonin may also play a key role in appetite and cognitive function. Differences in serotonin levels may contribute to ADHD symptoms, but common treatments like therapy and lifestyle changes are often effective in managing the effects of the disorder.

How serotonin is produced and regulated

Serotonin production is a biological process. It begins with an amino acid from the foods you eat and results in a neurotransmitter that helps regulate your mood, sleep, and emotional balance. 

First, the body obtains tryptophan, an essential amino acid that’s in protein-rich foods like eggs, poultry, dairy, nuts, and seeds. The body can’t produce tryptophan on its own, so it has to come from the foods you eat. Some of the tryptophan enters your brain, where it’s converted into 5-hydroxytryptophan (5-HTP) by an enzyme. Then, a different enzyme converts 5-HTP into serotonin. 

After your body creates the key neurotransmitter, it’s stored in small sacs called raphe nuclei inside neurons called raphe nuclei, then it’s released into the space between your nerve cells in order to send signals to your body. Once it’s released, most of the serotonin is reabsorbed, and either released again or broken down by an enzyme called monoamine oxidase.

What role does serotonin play in attention-deficit hyperactivity disorder (ADHD)?

Serotonin generally operates within a complex network of brain regions, including the frontal cortex, limbic system, basal ganglia, and reticular activating system, which are usually involved in attention and emotional control, as well as impulse control. These functions are frequently disrupted in individuals with ADHD. Research is ongoing, but older studies posit that serotonin deficit may mediate susceptibility to the development and severity of ADHD symptoms. 

What the research says about ADHD and serotonin

Here are some key findings that point to a relationship between serotonin and ADHD etiology throughout the last decade.

Genetics

Family, twin, and adoption studies have consistently demonstrated a heritable component to ADHD, with estimates of heritability ranging from 60-90%.

Genome-wide association studies have found serotonin transporter genes involved in serotonin signaling and encoding serotonin receptors, implicating them in ADHD susceptibility. Variations in these candidate genes may affect serotonin levels or receptor sensitivity, thereby influencing ADHD risk.

Neuroimaging findings

Functional magnetic resonance imaging (fMRI) studies have revealed aberrant activation patterns in brain regions associated with serotonin neurotransmission in individuals with ADHD. Specifically, reduced activity in the prefrontal cortex, a region that can be critical for executive functions and attentional control, has been observed. Dysfunctional serotonin signaling may contribute to these alterations in neural circuitry, potentially leading to ADHD symptoms.

Pharmacological interventions: ADHD and serotonin

The efficacy of certain medications in treating ADHD may further support the involvement of serotonin in the disorder. Stimulants targeting dopamine and norepinephrine transmission are typically the first-line treatments for ADHD. However, second-line treatments that enhance serotonin levels, like selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs), have also shown some effectiveness in alleviating ADHD symptoms, particularly in individuals with comorbid anxiety or depression.

Behavioral factors

Serotonin dysregulation may contribute to the emotional dysregulation and impulsive behavior often observed in individuals with ADHD. Serotonin usually controls the activity of brain regions involved in emotion processing and impulse control, influencing an individual's ability to control and modulate emotional responses. Dysfunctional serotonin signaling may disrupt these processes and exacerbate ADHD symptoms. 

Psychiatric and somatic comorbidities linked to serotonin

Recent research (Ward, C.L., et al) has found evidence that disruptions in serotonin signaling can contribute to both psychiatric and somatic comorbidities. According to a systematic review of almost 50,000 articles, altered serotonin activity is commonly associated with several mental health conditions such as:

• bipolar disorder
• hoarding disorder
• post-traumatic stress disorder
• dissociative disorders

Ultimately, the researchers were able to determine a serotonergic role in 91 psychiatric and 44 somatic conditions. This helps explain why people with ADHD are at an increased risk of overlapping mood and anxiety symptoms, as well as physical symptoms like disrupted sleep or digestive issues. 

How attention-deficit hyperactivity disorder (ADHD) and dopamine affects the brain

Neuroanatomical differences in the ADHD brain center may contribute to neurotransmitter function and differences in structure and functionality in certain areas. 

Attention-deficit hyperactivity disorder (ADHD) brain structure and function differences

For example, the frontal cortex, particularly the prefrontal cortex, often has reduced activity and volume in individuals with ADHD compared to those without the disorder. 

Disruptions in the default mode network (associated with mind-wandering and internal thoughts) and the task-positive network (related to goal-directed tasks) have also been observed in individuals with ADHD.

Neurotransmitter function in the attention deficit hyperactivity disorder (ADHD) brain: Dopamine, norepinephrine, and serotonin

Neurotransmitters typically allow different parts of the brain to communicate with each other and to other bodily systems. Recent research on the ADHD brain indicates that dopamine, norepinephrine, and serotonin are the three neurotransmitters that may affect the severity of symptoms.

• Dopamine plays a role in reward processing, motivation, and attention. Individuals with ADHD often have lower levels of dopamine or impaired dopamine signaling, which can contribute to difficulty focusing attention and an inability to control behavioral domains effectively.
• Norepinephrine may also be implicated in ADHD. Dysfunctions in this system can affect attention, arousal, and response to stimuli, contributing to symptoms like distractibility and hyperactivity.
• Serotonin generally helps with emotion and mood stabilization and controls some of the body's automatic functions, like digestion and sleeping. Problems in the serotonin system can contribute to impulsive behaviors and emotional dysregulation, which can both be common symptoms of ADHD.

While there may be a connection, neurotransmitter dysfunction is not necessarily an indicator of ADHD in and of itself. There are many other mental health conditions that can be associated with abnormal levels of neurotransmitters.

Since ADHD is generally considered a developmental disorder, its effects on the brain can vary depending on factors like age and developmental stage. Affected brain regions and networks may show different patterns of development and maturation in ADHD children and ADHD adults compared to typically developing individuals.

How serotonin affects motivation and long-term rewards

Serotonin helps regulate how the brain evaluates effort, patience, and long-term rewards, which is why it can influence motivation and goal-directed behavior. 

Serotonin supports the brain’s ability to pause, plan, and tolerate delayed gratification. When your serotonin levels are balanced, it’s usually easier to stick with the activities that don’t give you an immediate reward, like studying, getting further on a work project, or completing small tasks that contribute to your long-term goals. 

While dopamine pushes you to pursue rewards and novelty, serotonin helps you stay steady and focused on future outcomes. When these systems are out of balance, tasks that require sustained effort and delayed gratification can feel especially difficult. 

ADHD symptoms and diagnosis

To better understand the relationship between neurological function and ADHD, it may be helpful to understand its types and their symptoms. ADHD is usually categorized into three main types according to their predominant symptom presentation. 

Predominantly inattentive presentation (previously referred to as attention deficit disorder)

Difficulties with inattention and organization tend to characterize this type of ADHD. People with this type of ADHD may have problems focusing attention on a single task and being easily distracted. They may have trouble following directions, organizing tasks, and performing activities that require sustained mental effort. Individuals with the predominantly inattentive presentation of ADHD may also appear to be detached or preoccupied with internal thoughts when engaged with others.

Predominantly hyperactive/impulsive presentation

People with a predominantly hyperactive/impulsive presentation of ADHD may present behaviors that create difficulties in school, work, and social interactions due to hyperactivity and impulsivity. Restlessness, difficulties waiting in line or taking turns, and impatience can be typical hallmarks of this type of ADHD. Individuals presenting this type of ADHD may have difficulties with impulsivity, such as interrupting others during conversations, making impulsive or inappropriate comments, or engaging in hasty decision-making.

Combined presentation

People with combined ADHD often display a combination of hyperactive/impulsive and inattentive behaviors. 

Symptoms like emotional dysregulation and executive dysfunction, such as difficulty with time management and problems with working memory, can also be common in all types of ADHD.

Treatment implications for ADHD and serotonin

ADHD is typically managed with a combination of therapy, lifestyle changes, and medication. Individuals with ADHD can work closely with healthcare professionals to develop personalized treatment plans that address their specific needs and preferences. Treatment may need to be adjusted over time based on the individual's response and changing circumstances.

Common ADHD therapy approaches include the following:

Psychotherapeutic interventions

Cognitive behavioral therapy (CBT) and other forms of behavioral therapy can help individuals develop coping strategies, identify and change unhealthy thoughts and behaviors, and improve executive functioning skills, such as organizing tasks and managing time.

Medication

Medication can be a common form of treatment for ADHD patients. Please speak to your doctor or psychiatrist regarding any medication questions, and always consult a licensed medical professional before starting, stopping, or changing the way you take prescription drugs. According to a recent systematic literature review, stimulants are usually the most effective medication option, but non-stimulant drugs may be a preferred treatment course for patients with a history of substance use disorders.

Psychoeducation

Psychoeducational programs that teach individuals about ADHD, how it affects behavior, and the impact it can have on relationships may help individuals manage symptoms and teach their families how to support their treatment.

Group support

Connecting with others with ADHD can provide individuals with understanding, validation, and practical tips for everyday coping.

Parenting and school interventions

For inattentive or hyperactive children, implementing accommodations and modifications in the classroom, such as preferential seating and extra time on tests, can support academic success. Psychoeducational programs for parents can give them strategies to help their child manage challenges effectively at home and in school.

Lifestyle changes

Modifications to eating habits, exercise regimens, and sleep routines, as well as organizational and time management tools, can help individuals manage ADHD symptoms in daily life.

Reducing sugary and processed foods while prioritizing nutritious foods can support brain health. Meanwhile, regular physical activity can improve mood, focus, and overall well-being. Remember to consult your physician when adopting any new eating habits or exercise routines. 

ADHD has been linked to sleep disorders like:

• sleep apnea
• restless leg syndrome (RLS)
• delayed sleep phase syndrome (DSPS)

Because of this, maintaining sleep hygiene and a regular sleep schedule may be especially helpful for those with ADHD.

Establishing consistent daily routines and using organizational tools, such as planners and timers, may help individuals with ADHD manage their time more effectively, reduce stress, and increase productivity. 

Getting support through BetterHelp

Some people with ADHD may experience difficulties that present barriers to treatment. For example, challenges with time management and organization can make scheduling and attending appointments in an office difficult.

To alleviate some of these barriers, some individuals find that online therapy provides a convenient alternative to traditional in-office treatment. Individuals can attend sessions from the comfort of their homes, potentially reducing distractions associated with traveling to and attending sessions in an office. Online therapy often offers more flexible scheduling options, and platforms like BetterHelp tend to eliminate geographical barriers, providing patients with a broader pool of mental health professionals experienced in treating ADHD.

Studies suggest that online therapy can be as effective at treating ADHD symptoms as in-person treatment.