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T66 ICD-10-CM Code: Radiation sickness, unspecified

Billing Status: NO. This is a clinician non-billable / parent hierarchy grouping in the ICD-10-CM system.

Coding at a Glance

7th character
Required — characters A · D · S
Tabular directives
7 Excludes1 · 6 code-also instructions

Coding instructions

Official Tabular Instructional Notes: the inclusion, exclusion and sequencing notes published for T66 in the official ICD-10-CM tabular list, quoted as published.

Source: CMS/CDC — ICD-10-CM Tabular ListRelease: FY2026Effective: October 1, 2025

Notes without a marker are published on T66 itself; “inherited from” names the category or block whose note applies here.

Excludes1 — Not Coded Here

Conditions generally not reported together with this code (Excludes1) -- an error unless the two conditions are documented as unrelated to each other (ICD-10-CM Official Guidelines, Section I.A.12.a).

Code Also

Additional codes that may be required to fully describe the encounter.

  • Code also, if applicable, place of occurrence, such as:
  • highway rest stop (Y92.523)
  • school (private) (public) (state) as the place of occurrence of the external cause (Y92.21-)
  • service areas as the place of occurrence of the external cause (Y92.52-)
  • single-family non-institutional (private) house as the place of occurrence of the external cause (Y92.01-)
  • transport vehicle as the place of occurrence of the external cause (Y92.81-)

Source: inherited from T66-T78

7th Character Guide

"The appropriate 7th character is to be added to code T66"

  • A — initial encounter
  • D — subsequent encounter
  • S — sequela

How the encounter character is assigned. “Initial encounter” means the patient is receiving active treatment for the condition — it is not limited to the first visit, and a different or new provider giving active treatment still assigns it. “Subsequent encounter” covers care during the healing or recovery phase after active treatment. “Sequela” is for complications or conditions that arise as a direct result of the original condition.

ICD-10-CM Official Guidelines for Coding and Reporting, Section I.C.19.a (paraphrased by MedCoder).

Variant codes in this family

T66-T78, T66.XXXA, T66.XXXD, T66.XXXS

Character meanings are CMS's official 7th-character extensions for this family, as carried in each variant code's official description; variant codes are registry rows. The explanation of how the encounter character is assigned is MedCoder editorial, distinct from the official content above it.

Coder workflow for T66

MedCoder structured workflow — derived from this code’s own official record

Before you code T66

  1. T66 is not reportable as written. Select the more specific code beneath it that the documentation supports. Codes are reported to the highest level of specificity the classification provides (Guidelines I.B.2).

    ReviewT66.XXXA, T66.XXXD, T66.XXXS

    See the relationships section · Guide: How to choose an ICD-10-CM code →

  2. Unspecified does not mean incorrect. When the record gives no greater specificity, T66 may be the appropriate code. Check the record for detail that supports a more specific sibling. An unspecified code is for records that do not provide the detail a more specific code needs; a query, not an assumption, is the route to specificity (Guidelines I.A.9.b, I.B.18).

    See the relationships section · Guide: Other vs unspecified (NEC vs NOS) →

  3. A 7th character is required in this family. Confirm the documented encounter: initial (active treatment), subsequent (healing or recovery phase), or sequela. T66 is not valid without one. “Initial” means active treatment, not the first visit (Guidelines I.C.19.a).

    See the 7th Character Guide · Guide: 7th characters: initial, subsequent, sequela →

  4. Check the Excludes1 note: if the documentation supports a condition named there, do not simply proceed with T66. Excludes1 conditions are not reported together with this code unless the record shows they are unrelated (Guidelines I.A.12.a).

    See the official tabular notes

Choose the right path

  1. Does the documentation support one of the more specific codes beneath T66?
    Yes → Select that code and continue the checks below on its own page.
    No → T66 cannot be reported as written; query for the specificity its subcategory needs.

    ReviewT66.XXXA, T66.XXXD, T66.XXXS

  2. Does the record document the detail a more specific sibling code needs?
    Yes → Review the specific siblings in this subcategory.
    No → Continue — T66 is appropriate when the documentation goes no further.
  3. Does the documentation support a condition named in T66’s Excludes1 note?
    Yes → Do not simply proceed: review the excluded code. Both are reported only when the record shows the two conditions are unrelated.
    No → Continue.

    ReviewK52.0, J70.0, L55

  4. Is the encounter phase documented — active treatment, healing or recovery, or a sequela?
    Yes → Assign the matching 7th character; the variant codes are in the 7th Character Guide.
    No → The code is invalid without its 7th character — query for the missing element.

Consider T66. Then review the Code Also note, and confirm the code is valid for the date of service in the Verify section.

Documentation check

The provider’s diagnostic statement
Codes are assigned from the provider’s documented diagnosis, not from clinical criteria, test values or a medication list (Guidelines I.A.19).
Encounter type
Active treatment, healing or recovery phase, or sequela — the basis of the 7th character (Guidelines I.C.19.a).
Any detail beyond this code’s title
What the record states that a more specific sibling code would capture — or its absence, which itself supports the unspecified code.

Official instructions as workflow

  • Excludes1 — check before selecting T66(7 notes)

    Coding workflow: If the documentation supports a condition named in this note, do not simply proceed with T66: the two are not reported together. The one exception is when the record shows the two conditions are unrelated to each other.

    CompareK52.0, J70.0, L55

    See the official tabular notes · Guidelines I.A.12.a

  • Code Also — related condition(6 notes)

    Coding workflow: Review the related condition when both are documented and the instruction applies. A Code Also note does not fix sequencing; the order follows the circumstances of the encounter.

    ReviewY92.523, Y92.21, Y92.52, Y92.01, Y92.81

    See the official tabular notes · Guidelines I.A.17

Coding decision scenarios

Pattern scenarios for this code’s structure — decision rules, not clinical cases

Documentation: The provider documents the condition in the terms of this code’s title and records no further detail.

Coding question: Is a more specific sibling code supportable?

Path: Review the subcategory for a sibling that names the missing detail.

Reason: A more specific code needs documentation of the distinguishing element; without it the unspecified code is appropriate, and a provider query is the route to specificity (Guidelines I.A.9.b, I.B.18).

Documentation: Both the condition T66 describes and a condition named in its Excludes1 note are documented for the same encounter.

Coding question: Can both codes be reported?

Path: Review the Excludes1 note and the excluded code, and look for a provider statement on whether the two conditions are related.

Reason: Excludes1 means the two are not coded together; the exception is when the record shows the conditions are unrelated to each other (Guidelines I.A.12.a).

ReviewK52.0, J70.0, L55

Documentation: The patient returns during the healing or recovery phase after active treatment of the same condition.

Coding question: Which 7th character applies?

Path: Review the official 7th-character definitions for this family in the 7th Character Guide.

Reason: “Initial encounter” means active treatment, not the first visit; care during recovery takes “subsequent encounter”, and a complication arising from the original condition takes “sequela” (Guidelines I.C.19.a).

Every row is derived from this code’s own record — its title, tabular notes, 7th-character family and same-category siblings — with fixed MedCoder wording; nothing is inferred about a patient. The official notes and guideline text are in the sections each row links to, and they control.

Code Overview

Radiation sickness, unspecified is a non-billable ICD-10-CM category code (T66). A more specific billable subcode must be selected for claims submission.

MedCoder summary Summary composed by MedCoder from this code's official ICD-10-CM record. The tabular instructional notes themselves appear verbatim below.

Indexed Clinical Terms (7)

Official source data — entries quoted as published, in the Index’s own lookup phrasing

Clinical term phrases from the official ICD-10-CM Index to Diseases and Injuries that map to this code. These are alphabetic-index entries shown as the Index writes them — lookup phrasing, not necessarily the wording of a final diagnosis.

Decision Points

The directives on this code's own record, as a pre-claim checklist.

  1. A 7th character applies in this family. Confirm the documented encounter matches the character assigned. See the official 7th-character definitions and this code’s variants
  2. 6 Code Also notes — a second code may apply; the guidelines leave its sequencing to the circumstances of the encounter. See the Code Also notes
  3. 7 Excludes1 entries — codes named there are generally not reported together with this code (Guidelines I.A.12.a). See the Excludes1 notes

Checklist rows are derived from this code's own official directives; the wording of each check is MedCoder editorial. The official notes themselves are in the sections each row links to.

Verify Before Coding

From the code registry, the Medicare Code Editor, and the MS-DRG Definitions Manual. Check it against a full claim in Claim Check.

Relationships & Classification

MedCoder structured relationships — computed from published CMS and AHRQ datasets

Other codes that name T66 or its code family, from the CMS ICD-10-CM tabular instructional notes. Tabular-note edges are stored at the code family level that carries each note.

Referenced by 1 Excludes2 note: Z77.123 — Contact with and (suspected) exposure to radon and other naturally occurring radiation.

These codes’ tabular lists name this diagnosis as distinct — both may be reported when both are documented.

Related Codes

Same Index main term, other category

The ICD-10-CM Index to Diseases and Injuries files this code under the main terms “Effect, adverse”, “Sickness”; these codes share that main term but sit in a different category of the Tabular List.

O74.5 — Spinal and epidural anesthesia-induced headache during labor and delivery (anesthesia, spinal and epidural, headache, in labor and delivery), O74.6 — Other complications of spinal and epidural anesthesia during labor and delivery (anesthesia, spinal and epidural, specified NEC, in labor and delivery), O74.8 — Other complications of anesthesia during labor and delivery (anesthesia, specified NEC, in labor and delivery), O74.9 — Complication of anesthesia during labor and delivery, unspecified (anesthesia, in labor and delivery), O89.3 — Toxic reaction to local anesthesia during the puerperium (anesthesia, local, toxic, postpartum, puerperal), O89.4 — Spinal and epidural anesthesia-induced headache during the puerperium (anesthesia, spinal and epidural, headache, postpartum, puerperal), O89.5 — Other complications of spinal and epidural anesthesia during the puerperium (anesthesia, spinal and epidural, specified NEC, postpartum, puerperal), O89.8 — Other complications of anesthesia during the puerperium (anesthesia, specified NEC, postpartum, puerperal), O89.9 — Complication of anesthesia during the puerperium, unspecified (anesthesia, postpartum, puerperal), T59.9 — Toxic effect of unspecified gases, fumes and vapors (gases, fumes, or vapors), T69.1 — Chilblains (cold, chilblains), T69.8 — Other specified effects of reduced temperature (cold, specified effect NEC), T69.9 — Effect of reduced temperature, unspecified (cold), T70.0 — Otitic barotrauma (high altitude NEC, on, ears), T70.1 — Sinus barotrauma (high altitude NEC, on, sinuses), T70.20 — Unspecified effects of high altitude (altitude), T70.29 — Other effects of high altitude (Andes), T70.3 — Caisson disease [decompression sickness] (decompression), T70.4 — Effects of high-pressure fluids (high pressure fluids), T70.8 — Other effects of air pressure and water pressure (air pressure, specified NEC), +32 more

Contextual Map

Every relationship of T66 in one view: hierarchy, official tabular instructions in both directions, clinical classification, risk adjustment, MS-DRG participation, index terms and change history — each edge carrying the CMS source it derives from.

Run T66 with these 14 related codes in Claim Check

Hierarchy

Excludes1 (23)

Code Also

Referenced by Excludes2 notes

Index entries

  • Effect, adverse, infrared NOS (radiation) (rays)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Effect, adverse, radiation NOS (diagnostic) (infrared) (natural source) (therapeutic) (ultraviolet) (X-ray)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Effect, adverse, ultraviolet NOS (radiation) (rays)[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Irradiation effects, adverse[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Radiation, effects NOS[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Radiation, sickness NOS[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026
  • Sickness, radiation[Index term]— CMS ICD-10-CM Index to Diseases and Injuries · icd10cm-fy2026

Nearest codes

Change history

  • FY2016 — In the code set at ICD-10-CM adoption [Change history]— CMS release files (code change ledger) · icd10cm-fy2016

Common coding questions

Can T66 be billed directly?

No. T66 (Radiation sickness, unspecified) is a non-billable ICD-10-CM category code. A more specific billable subcode must be selected based on clinical documentation.

Does T66 require a 7th character?

Yes. The appropriate 7th character is to be added to code T66; A: initial encounter; D: subsequent encounter; S: sequela.

Sources for this page

Codes, titles, notes, index terms and mappings on this page are transcribed from the datasets below. Relationships MedCoder computed and text MedCoder wrote are labelled where they appear.

Code, title, tabular notes and index terms Official source data
CMS/CDC ICD-10-CM FY2026 tabular list, index and tables, effective October 1, 2025 Release, file and checksum · Publisher’s page
Claim edits Official source data
CMS Definitions of Medicare Code Edits — v43.1 (April 2026) Release, file and checksum · Publisher’s page
Change history and date-of-service validity Official source data
CMS ICD-10-CM release addenda, ingested release by release into the change ledger Release, file and checksum · Publisher’s page
Comparisons, relationships and the contextual map MedCoder structured relationship
Computed by MedCoder from the tabular notes and tables above; every derived relationship is marked as derived where it appears
Summary and FAQ answers MedCoder explanation
Written by MedCoder to explain the sources above: drafted with AI assistance, checked by a person against the release files, and labelled as MedCoder text where it appears. Not official text.

Labels on this page: Official source data · MedCoder structured relationship · MedCoder explanation. How to read the labels · All data sources and release dates

Change history

  • FY2016 — October 1, 2015
    In the code set at ICD-10-CM adoption
    Radiation sickness, unspecified

No changes since FY2016 — additions, deletions, description changes and billable-status changes are tracked through FY2027 (effective October 1, 2026), and none are recorded for this code. Note changes are tracked from FY2027 only.

Nearest Codes in This Family

Official ICD-10-CM classifications closest to T66 in its code family, with their registry titles.