Radiation sickness, unspecified (T66) ICD-10-CM
The T66 code range covers radiation sickness, unspecified with 4 ICD-10-CM diagnosis codes. 3 of them are billable and valid for claim submission in fiscal year 2026, and the category headers group them but cannot themselves be billed.
Type 1 Excludes
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.
- 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
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.
- The appropriate 7th character is to be added to code T66
7th Character
Indicates that a seventh character is to be assigned to codes in a subcategory.
- A - initial encounter
- D - subsequent encounter
- S - sequela
Codes in the T66 Range 4 codes · 3 billable
Clinical Terms in This Code Range
Definitions from the National Library of Medicine for conditions coded in the T66 range.
Acute Radiation Syndrome
A condition caused by a brief whole body exposure to more than one sievert dose equivalent of radiation. Acute radiation syndrome is initially characterized by ANOREXIA; NAUSEA; VOMITING; but can progress to hematological, gastrointestinal, neurological, pulmonary, and other major organ dysfunction.
About the T66 Code Range
T66 codes cover radiation sickness when the specific type or cause is not detailed, capturing initial, subsequent, or late effects of exposure to harmful radiation. These codes help document symptoms and complications from radiation exposure without pinpointing specialized conditions.
The ICD-10 code for radiation sickness unspecified (T66) is used to classify general radiation illness cases. Its variations—T66.XXXA for initial encounters, T66.XXXD for follow-up visits, and T66.XXXS for sequela or late effects—encompass a wide range of radiation-induced problems. Commonly associated terms include acute radiation syndrome, radiation sickness, and radiation-induced anemia or infertility, reflecting the diverse health issues caused by overexposure to ionizing radiation. These codes are essential when the detailed diagnosis is not available but the patient’s condition results from radiation effects such as nausea, aplastic anemia, or radiation nephritis. Medical coders use these codes to report radiation toxicity and adverse effects, even when the precise subtype is undetermined, making them critical in radiation injury documentation and healthcare tracking.
Questions About This Page
How many billable codes are in the T66 range?
Of the 4 codes in this range, 3 are billable and valid for claim submission from October 1, 2025 through September 30, 2026. Category header codes group them but cannot be reported on claims.
What does the T66 range classify?
The range classifies radiation sickness, unspecified. Each code links to its own reference page with billing status, MS-DRG grouping, coding notes, and clinical information.
Related References
Source: CMS FY 2026 ICD-10-CM Tabular List and order file, effective October 1, 2025 through September 30, 2026.
