2026 ICD-10-CM Diagnosis Code T38.1X5AAdverse effect of thyroid hormones and substitutes, initial encounter
T38.1X5A is a billable ICD-10-CM diagnosis code for adverse effect 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) and groups to MS-DRG 917 through 918. The code is not accepted as a principal diagnosis by the Medicare Code Editor. Coders also document this condition as adverse reaction to thyroglobulin. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Adverse effects of drugs and medicaments, initial encounter.
Code Identity
Code Classification
Code EditsBilling
Medicare Code Editor checks that affect claim validity for T38.1X5A.
Approximate SynonymsGuidance
Alternate terms and clinical phrases that map to this code.
- Adverse reaction to thyroglobulin
- Adverse reaction to thyroid hormone
- Dextrothyroxine sodium adverse reaction
- Hypothyroidism caused by iodide excess
- Liothyronine adverse reaction
- Thyrotoxicosis on thyroxine therapy
- Thyroxine adverse reaction
Coding GuidelinesGuidance
When coding an adverse effect of a drug that has been correctly prescribed and properly administered, assign the appropriate code for the nature of the adverse effect followed by the appropriate code for the adverse effect of the drug.
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
Drug Reactions
Most of the time, medicines make our lives better. They reduce aches and pains, fight infections, and control problems such as high blood pressure or diabetes. But medicines can also cause unwanted reactions, such as drug interactions, side effects, and allergies.
The full article covers:
- What is a drug interaction?
- What are side effects?
- What are drug allergies?
- How can I stay safe when taking medicines?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Convert T38.1X5A to ICD-9-CMHistory
The closest ICD-9-CM equivalents under the General Equivalence Mappings.
Code HistoryHistory
Questions About T38.1X5AOverview
Is T38.1X5A (Adverse effect of thyroid hormones and substitutes) a billable code?
Yes. This is a billable ICD-10-CM code, specific enough to report adverse effect 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 T38.1X5A mean?
The final character A marks the initial encounter: use it while the patient is receiving active treatment for adverse effect of thyroid hormones and substitutes, such as an emergency visit or first evaluation.
What MS-DRG does T38.1X5A group to?
On inpatient claims, adverse effect of thyroid hormones and substitutes, initial encounter maps to MS-DRG 917, 918, with relative weights from 0.8571 to 1.5684 depending on complications. Higher weights mean higher Medicare reimbursement.
Can T38.1X5A be a principal diagnosis?
No. The Medicare Code Editor rejects this code as a principal diagnosis because adverse effect 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.
What is the ICD-9 equivalent of T38.1X5A?
Under the General Equivalence Mappings, adverse effect of thyroid hormones and substitutes, initial encounter converts to ICD-9-CM 995.29 (adv eff med/biol NEC/NOS) and E932.7 (adv eff thyroid & deriv). The mapping is approximate, so confirm the match fits the documentation.
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.
