2026 ICD-10-CM Diagnosis Code T66Radiation sickness, unspecified
T66 is a non-billable ICD-10-CM category code for radiation sickness, unspecified, so it cannot be submitted on claims. Use a more specific code from this category instead, such as T66.XXXA, T66.XXXD, and T66.XXXS.
Code Identity
Code Classification
Coding GuidelinesGuidance
The appropriate 7th character is to be added to each code from block Radiation sickness, unspecified (T66). 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.
Tabular List NotesGuidance
Coding notes and annotation back-references applicable to this code.
Type 1 Excludes
- specified adverse effects of radiation, such as:
- burns T20 T31
- leukemia C91 C95
- radiation gastroenteritis and colitis K52.0
- radiation pneumonitis J70.0
- radiation related disorders of the skin and subcutaneous tissue L55 L59
- sunburn L55
7th Character Note
- The appropriate 7th character is to be added to code T66
7th Character
- A - initial encounter
- D - subsequent encounter
- S - sequela
A type 1 excludes note is a pure excludes note. It means "NOT CODED HERE!" An Excludes1 note indicates that the code excluded should never be used at the same time as the code above the Excludes1 note. An Excludes1 is used when two conditions cannot occur together, such as a congenital form versus an acquired form of the same condition.
Certain ICD-10-CM categories have applicable 7th characters. The applicable 7th character is required for all codes within the category, or as the notes in the Tabular List instruct. The 7th character must always be the 7th character in the data field. If a code that requires a 7th character is not 6 characters, a placeholder X must be used to fill in the empty characters.
Indicates that a seventh character is to be assigned to codes in a subcategory.
Index to Diseases and InjuriesGuidance
Alphabetical index entries that point to this code.
- Irradiation effects, adverse - T66
- effects NOS - T66
- sickness NOS - T66
External Cause of Injuries IndexGuidance
References for this code in the External Cause of Injuries Index.
- Effect, adverse
- infrared (radiation) (rays) NOS
- Effect, adverse
- radiation (diagnostic) (infrared) (natural source) (therapeutic) (ultraviolet) (X-ray) NOS
- Effect, adverse
- ultraviolet (radiation) (rays) NOS
- Irradiation effects, adverse
- Radiation
- effects NOS
- Radiation
- sickness NOS
- Sickness
- radiation
Patient EducationClinical
Radiation Exposure
Radiation is energy. It travels in the form of energy waves or high-speed particles. Radiation can occur naturally or be man-made. There are two types:
The full article covers:
- What is radiation?
- What are the sources of radiation exposure?
- What are the health effects of radiation exposure?
- What are the treatments for acute radiation sickness?
- How can radiation exposure be prevented?
Read the full article at MedlinePlus
Courtesy of MedlinePlus, a service of the U.S. National Library of Medicine.
Code HistoryHistory
Questions About T66Overview
Is T66 (Radiation sickness, unspecified) a billable code?
No. This is a category header that groups the codes for radiation sickness, unspecified, and headers cannot be submitted on claims. Claims for radiation sickness, unspecified need a more specific code from this category, such as T66.XXXA, T66.XXXD, and T66.XXXS.
