2026 ICD-10-CM Diagnosis Code E51.9Thiamine deficiency, unspecified

ICD-10-CM Codes›E00–E89›E50-E64›E51

ICD-10-CM E51.9
CMSSource: CMS FY 2026 ICD-10-CM dataset · Effective Oct 1, 2025 – Sep 30, 2026

E51.9 is a billable ICD-10-CM diagnosis code for thiamine deficiency, unspecified. It is valid on HIPAA claims for fiscal year 2026 (October 1, 2025 through September 30, 2026) and groups to MS-DRG 640 through 641. Coders also document this condition as inadequate intake of thiamine. In AHRQ's Clinical Classifications Software (CCSR), this diagnosis falls under Nutritional deficiencies.

Code Identity

ICD-10-CM Code
E51.9
Billable Status
Yes — Valid for Submission
Code Describes
Thiamine deficiency, unspecified
Short Description
Thiamine deficiency, unspecified
Same as the full description in the CMS dataset.
Parent Code
E51
Thiamine deficiency

Code Classification

ChapterE00–E89Endocrine, nutritional and metabolic diseases
SectionE50-E64Other nutritional deficiencies
CategoryE51Thiamine deficiency
This CodeE51.9Thiamine deficiency, unspecified

Approximate SynonymsGuidance

Alternate terms and clinical phrases that map to this code.

  • Inadequate intake of thiamine
  • Niacin deficiency
  • Thiamine and niacin deficiency
  • Thiamine deficiency

Index to Diseases and InjuriesGuidance

Alphabetical index entries that point to this code.

External Cause of Injuries IndexGuidance

References for this code in the External Cause of Injuries Index.

    • Aneurin deficiency
    • Deficiency, deficient
      • aneurin
    • Deficiency, deficient
      • thiamin, thiaminic (chloride)
    • Deficiency, deficient
      • vitamin (multiple) NOS
        • B1 NOS
    • Deficiency, deficient
      • vitamin (multiple) NOS
        • thiamin
    • Thiamin deficiency

Clinical ClassificationClinical

AHRQ’s CCSR groups this code into broader clinical categories.

CCSR END007
Nutritional deficiencies
Default principal diagnosis: inpatient Yes · outpatient Yes

Clinical InformationClinical

  • Thiamine Deficiency

    a nutritional condition produced by a deficiency of thiamine in the diet, characterized by anorexia, irritability, and weight loss. later, patients experience weakness, peripheral neuropathy, headache, and tachycardia. in addition to being caused by a poor diet, thiamine deficiency in the united states most commonly occurs as a result of alcoholism, since ethanol interferes with thiamine absorption. in countries relying on polished rice as a dietary staple, beriberi prevalence is very high. (from cecil textbook of medicine, 19th ed, p1171)

Patient EducationClinical

B Vitamins

The B vitamins are:

Read the full article at MedlinePlus

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

Convert E51.9 to ICD-9-CMHistory

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

ICD-9-CM
265.1 Thiamine defic NEC/NOS
Approximate The match is approximate rather than exact.

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 E51.9Overview

Is E51.9 (Thiamine deficiency) a billable code?

Yes. This is a billable ICD-10-CM code, specific enough to report thiamine deficiency, unspecified on HIPAA-covered claims from October 1, 2025 through September 30, 2026.

What MS-DRG does E51.9 group to?

When thiamine deficiency, unspecified is the principal diagnosis on an inpatient stay, it groups to MS-DRG 640, 641, with relative weights from 0.7782 to 1.3356 depending on complications. Higher weights mean higher Medicare reimbursement.

What is the ICD-9 equivalent of E51.9?

Under the General Equivalence Mappings, thiamine deficiency, unspecified converts to ICD-9-CM 265.1 (thiamine defic NEC/NOS). 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.