2026 ICD-10-CM Diagnosis Code T38.2X6AUnderdosing of antithyroid drugs, initial encounter

ICD-10-CM Codes›S00–T88›T36-T50›T38

ICD-10-CM T38.2X6A
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

T38.2X6A is a billable ICD-10-CM diagnosis code for underdosing of antithyroid drugs, 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

ICD-10-CM Code
T38.2X6A
Billable Status
Yes — Valid for Submission
Code Describes
Underdosing of antithyroid drugs, initial encounter
Short Description
Underdosing of antithyroid drugs, initial encounter
Same as the full description in the CMS dataset.
Parent Code
T38.2X6
Underdosing of antithyroid drugs

Code Classification

ChapterS00–T88Injury, poisoning and certain other consequences of external causes
SectionT36-T50Poisoning by, adverse effect of and underdosing of drugs, medicaments and biological substances
CategoryT38Poisoning by, adverse effect of and underdosing of hormones and their synthetic substitutes and antagonists, not elsewhere classified
This CodeT38.2X6AUnderdosing of antithyroid drugs, initial encounter

Code EditsBilling

Medicare Code Editor checks that affect claim validity for T38.2X6A.

Unacceptable principal diagnosis
There are selected codes that describe a circumstance which influences an individual's health status but not a current illness or injury, or codes that are not specific manifestations but may be due to an underlying cause. These codes are considered unacceptable as a principal diagnosis.

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 hormones and their synthetic substitutes and antagonists, not elsewhere classified (T38). 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.

CCSR INJ029
Underdosing of drugs and medicaments, initial encounter
Default principal diagnosis: inpatient No · outpatient No

Clinical InformationClinical

  • Carbimazole

    an imidazole antithyroid agent. carbimazole is metabolized to methimazole, which is responsible for the antithyroid activity.

  • Diiodotyrosine

    a product from the iodination of monoiodotyrosine. in the biosynthesis of thyroid hormones, diiodotyrosine residues are coupled with other monoiodotyrosine or diiodotyrosine residues to form t4 or t3 thyroid hormones (thyroxine and triiodothyronine).

  • Receptors, Thyroid Hormone

    specific high affinity binding proteins for thyroid hormones in target cells. they are usually found in the nucleus and regulate dna transcription. these receptors are activated by hormones that leads to transcription, cell differentiation, and growth suppression. thyroid hormone receptors are encoded by two genes (genes, erba): erba-alpha and erba-beta for alpha and beta thyroid hormone receptors, respectively.

  • Methimazole

    a thioureylene antithyroid agent that inhibits the formation of thyroid hormones by interfering with the incorporation of iodine into tyrosyl residues of thyroglobulin. this is done by interfering with the oxidation of iodide ion and iodotyrosyl groups through inhibition of the peroxidase enzyme.

  • Methylthiouracil

    a thiourea antithyroid agent that inhibits the synthesis of thyroid hormone. it is used in the treatment of hyperthyroidism.

  • Propylthiouracil

    a thiourea antithyroid agent. propythiouracil inhibits the synthesis of thyroxine and inhibits the peripheral conversion of throxine to tri-iodothyronine. it is used in the treatment of hyperthyroidism. (from martindale, the extra pharmacopeoia, 30th ed, p534)

  • Thiourea

    a photographic fixative used also in the manufacture of resins. according to the fourth annual report on carcinogens (ntp 85-002, 1985), this substance may reasonably be anticipated to be a carcinogen (merck index, 9th ed). many of its derivatives are antithyroid agents and/or free radical scavengers.

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.

SubstanceAccidentalSelf-harmAssaultUndeter­minedAdverse
Effect		Under­dosing
Antithyroid drug NECT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
BenzylthiouracilT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
CarbimazoleT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
DiiodotyrosineT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
IothiouracilT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
MethiacilT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
MethimazoleT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
MethylthiouracilT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
PropylthiouracilT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
TapazoleT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
ThiamazoleT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
ThiocarbamideT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
Thiouracil (benzyl) (methyl) (propyl)T38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6
ThioureaT38.2X1T38.2X2T38.2X3T38.2X4T38.2X5T38.2X6

Patient EducationClinical

Medication Errors

Medicines treat infectious diseases, prevent problems from chronic diseases, and ease pain. But medicines can also cause harmful reactions if not used correctly. Errors can happen in the hospital, at the health care provider's office, at the pharmacy, or at home. You can help prevent errors by:

Read the full article at MedlinePlus

Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.

Convert T38.2X6A to ICD-9-CMHistory

The closest ICD-9-CM equivalents under the General Equivalence Mappings.

ICD-9-CM
No Map There is no ICD-9 equivalent for this code.

Code HistoryHistory

FY 2016AddedAdded to the ICD-10-CM code setEffective October 1, 2015, the first year of ICD-10-CM.
FY 2017–2025No changes
FY 2026CurrentCurrent code set, no changesEffective October 1, 2025 through September 30, 2026.

Questions About T38.2X6AOverview

Is T38.2X6A (Underdosing of antithyroid drugs) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of antithyroid drugs, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What does the 7th character A in T38.2X6A mean?

The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of antithyroid drugs, such as an emergency visit or first evaluation.

Can T38.2X6A be a principal diagnosis?

No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of antithyroid drugs, 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.