2026 ICD-10-CM Diagnosis Code T38.1X6AUnderdosing of thyroid hormones and substitutes, initial encounter
T38.1X6A is a billable ICD-10-CM diagnosis code for underdosing of thyroid hormones and substitutes, 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 T38.1X6A.
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.
Clinical InformationClinical
Correspondence as Topic
works on written communication between persons or between institutions or organizations.Histiocytosis, Langerhans-Cell
a group of disorders resulting from the abnormal proliferation of and tissue infiltration by langerhans cells which can be detected by their characteristic birbeck granules (x bodies), or by monoclonal antibody staining for their surface cd1 antigens. langerhans-cell granulomatosis can involve a single organ, or can be a systemic disorder.Letter
work consisting of written or printed communication between individuals or between persons and representatives of corporate bodies. the correspondence may be personal or professional. in medical and other scientific publications the letter is usually from one or more authors to the editor of the journal or book publishing the item being commented upon or discussed. letter is often accompanied by comment.Thyroglobulin
tumors or cancer of the thymus gland.Iodide Peroxidase
a hemeprotein that catalyzes the oxidation of the iodide radical to iodine with the subsequent iodination of many organic compounds, particularly proteins. ec 1.11.1.8.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.Thyroxine
the major hormone derived from the thyroid gland. thyroxine is synthesized via the iodination of tyrosines (monoiodotyrosine) and the coupling of iodotyrosines (diiodotyrosine) in the thyroglobulin. thyroxine is released from thyroglobulin by proteolysis and secreted into the blood. thyroxine is peripherally deiodinated to form triiodothyronine which exerts a broad spectrum of stimulatory effects on cell metabolism.Thyroxine-Binding Globulin
a thyroid hormone transport protein found in serum. it binds about 75% of circulating thyroxine and 70% of circulating triiodothyronine.Thyroxine-Binding Proteins
blood proteins that bind to thyroid hormones such as thyroxine and transport them throughout the circulatory system.Thyroid Hormone-Binding Proteins
membrane-bound proteins found in the endoplasmic reticulum of cultured cells and neoplasms. they were identified originally based on their capacity to bind thyroid hormone.Triiodothyronine
a t3 thyroid hormone normally synthesized and secreted by the thyroid gland in much smaller quantities than thyroxine (t4). most t3 is derived from peripheral monodeiodination of t4 at the 5' position of the outer ring of the iodothyronine nucleus. the hormone finally delivered and used by the tissues is mainly t3.Triiodothyronine, Reverse
a metabolite of thyroxine, formed by the peripheral enzymatic monodeiodination of t4 at the 5 position of the inner ring of the iodothyronine nucleus.
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
Hormones
Hormones are your body's chemical messengers. They travel in your bloodstream to tissues or organs. They work slowly, over time, and affect many different processes, including:
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T38.1X6A to ICD-9-CMHistory
Code HistoryHistory
Questions About T38.1X6AOverview
Is T38.1X6A (Underdosing of thyroid hormones and substitutes) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report underdosing of thyroid hormones and substitutes, initial encounter on HIPAA-covered claims from October 1, 2025 through September 30, 2026.
What does the 7th character A in T38.1X6A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for underdosing of thyroid hormones and substitutes, such as an emergency visit or first evaluation.
Can T38.1X6A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because underdosing of thyroid hormones and substitutes, 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.
