2026 ICD-10-CM Diagnosis Code T43.016AUnderdosing of tricyclic antidepressants, initial encounter
T43.016A is a billable ICD-10-CM diagnosis code for underdosing of tricyclic antidepressants, initial encounter. The 7th character A marks it as an initial encounter code, used while the patient is receiving active treatment. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026). The code is not accepted as a principal diagnosis by the Medicare Code Editor. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Underdosing of drugs and medicaments, initial encounter.
Code Identity
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for T43.016A.
Coding GuidelinesGuidance
Underdosing refers to taking less of a medication than is prescribed by a provider or a manufacturer's instruction. Codes for underdosing should never be assigned as principal or first-listed codes. If a patient has a relapse or exacerbation of the medical condition for which the drug is prescribed because of the reduction in dose, then the medical condition itself should be coded.
The appropriate 7th character is to be added to each code from block Poisoning by, adverse effect of and underdosing of psychotropic drugs, not elsewhere classified (T43). Use the following options for the applicable episode of care:
- A - initial encounter
- D - subsequent encounter
- S - sequela
Source: ICD-10-CM Official Guidelines for Coding and Reporting, FY 2026, published by CMS and the National Center for Health Statistics.
Clinical ClassificationClinical
AHRQ’s CCSR groups this code into broader clinical categories.
Clinical InformationClinical
Amitriptyline
tricyclic antidepressant with anticholinergic and sedative properties. it appears to prevent the re-uptake of norepinephrine and serotonin at nerve terminals, thus potentiating the action of these neurotransmitters. amitriptyline also appears to antagonize cholinergic and alpha-1 adrenergic responses to bioactive amines.Amoxapine
the n-demethylated derivative of the antipsychotic agent loxapine that works by blocking the reuptake of norepinephrine, serotonin, or both; it also blocks dopamine receptors. amoxapine is used for the treatment of depression.Clomipramine
a tricyclic antidepressant similar to imipramine that selectively inhibits the uptake of serotonin in the brain. it is readily absorbed from the gastrointestinal tract and demethylated in the liver to form its primary active metabolite, desmethylclomipramine.Desipramine
a tricyclic dibenzazepine compound that potentiates neurotransmission. desipramine selectively blocks reuptake of norepinephrine from the neural synapse, and also appears to impair serotonin transport. this compound also possesses minor anticholinergic activity, through its affinity to muscarinic receptors.Dothiepin
a tricyclic antidepressant with some tranquilizing action.Doxepin
a dibenzoxepin tricyclic compound. it displays a range of pharmacological actions including maintaining adrenergic innervation. its mechanism of action is not fully understood, but it appears to block reuptake of monoaminergic neurotransmitters into presynaptic terminals. it also possesses anticholinergic activity and modulates antagonism of histamine h(1)- and h(2)-receptors.Cytochrome P-450 CYP2D6
a cytochrome p450 enzyme that catalyzes the hydroxylation of many drugs and environmental chemicals, such as debrisoquine; adrenergic receptor antagonists; and tricyclic antidepressants. this enzyme is deficient in up to 10 percent of the caucasian population.Imipramine
the prototypical tricyclic antidepressant. it has been used in major depression, dysthymia, bipolar depression, attention-deficit disorders, agoraphobia, and panic disorders. it has less sedative effect than some other members of this therapeutic group.Iprindole
a tricyclic antidepressant that has actions and uses similar to those of amitriptyline, but has only weak antimuscarinic and sedative effects. (from martindale, the extra pharmacopoeia, 30th ed, p257)Lofepramine
a psychotropic imipramine derivative that acts as a tricyclic antidepressant and possesses few anticholinergic properties. it is metabolized to desipramine.Nortriptyline
a metabolite of amitriptyline that is also used as an antidepressive agent. nortriptyline is used in major depression, dysthymia, and atypical depressions.Opipramol
a tricyclic antidepressant with actions similar to amitriptyline.Protriptyline
tricyclic antidepressant similar in action and side effects to imipramine. it may produce excitation.Trimipramine
tricyclic antidepressant similar to imipramine, but with more antihistaminic and sedative properties.
Table of Drugs and ChemicalsClinical
Substances in the Table of Drugs and Chemicals that reference this code family. Always confirm in the Tabular List before coding.
Patient EducationClinical
Antidepressants
Antidepressants are prescription medicines to treat depression. Depression is more than feeling a little sad or "blue" for a few days. It's a very common, serious medical illness that affects your mood and general mental health It can make you feel tired, hopeless, worried, or fearful. It can change your thinking, sleeping, and eating.
The full article covers:
- What are antidepressants?
- Are antidepressants used for other conditions?
- What are the different types of antidepressants?
- Which type of antidepressant is right for me?
- How long do antidepressants take to work?
- How long will I need to take an antidepressant?
- What are the side effects of antidepressants?
- What can I do to take antidepressants safely?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T43.016A to ICD-9-CMHistory
Code HistoryHistory
Questions About T43.016AOverview
Is T43.016A (Underdosing of tricyclic antidepressants) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of tricyclic antidepressants, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T43.016A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of tricyclic antidepressants, such as an emergency visit or first evaluation.
Can T43.016A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of tricyclic antidepressants, initial encounter describes a circumstance that influences health status rather than a current illness. Report it as a secondary diagnosis.
Footnotes
[1] Not chronic - A diagnosis code that does not fit the criteria for chronic condition (duration, ongoing medical treatment, and limitations) is considered not chronic. Some codes designated as not chronic are acute conditions. Other diagnosis codes that indicate a possible chronic condition, but for which the duration of the illness is not specified in the code description (i.e., we do not know the condition has lasted 12 months or longer) also are considered not chronic.
